Zoölogy: The Science of Animal Life
by Ernest Ingersoll · English · Gutenberg #48196 · 609,117 chars · public domain (US)
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Title: Zoölogy: The Science of Animal Life
Author: Ernest Ingersoll
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[Illustration:
Photo, American Museum of Natural History]
[Illustration: _Giraffes in their native African haunts_]
POPULAR SCIENCE LIBRARY
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN
ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER
ERNEST INGERSOLL WILFRED MASON BARTON
WILLIAM B. SCOTT ERNEST J. STREUBEL
NORMAN TAYLOR DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration]
VOLUME TWELVE
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
BY P. F. COLLIER & SON COMPANY
MANUFACTURED IN U. S. A.
ZOÖLOGY
THE SCIENCE OF ANIMAL LIFE
BY
ERNEST INGERSOLL
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
In this volume, occupying the place in the series assigned to the
subject "Zoölogy," the writer was called upon to survey the whole
range of animal life on the globe, and to keep in view the fact that
these books were to be a library of science. The casual reader, with
no particular interest in natural history, seeks in such a book little
more than stories of animal life thought of mainly as "big game," with
an appetite for the adventurous and wonderful. But beasts and birds
and snakes, although they number in the aggregate thousands of kinds,
are but few compared with the almost innumerable hosts of the lower
orders of animal life that dwell in the wildernesses of the world,
or throng in the seas, or hover about us in the air; yet they are a
part of the zoölogy of the globe, and a most important part. Although
they may rarely have the picturesque interest that attaches to the
vertebrate groups, they exhibit great beauty in many cases, and are
the foundation on which the others rest, for they furnish the food
on which the more highly organized creatures subsist. To the student
this lower half is often more attractive than the upper half; and the
history and philosophy of animal life could not be understood unless it
was fully considered. The author has therefore devoted a proportionate
space to the lower orders, at the expense of detailed descriptions of
birds and beasts, knowing that these are easily accessible elsewhere.
The arrangement of the matter in the volume is according to the latest
results of critics of classification, and it illustrates, as well as
any lineal arrangement can, the principle of the development of the
higher classes from the inferior by a gradual evolution toward more and
more complex forms. Space did not permit of much exposition of methods
of development, as revealed by fossils; and the volume on Paleontology
should be read in connection with this one.
ERNEST INGERSOLL.
CONTENTS
CHAPTER PAGE
I. HOW THE GLOBE WAS STOCKED WITH LIFE 9
II. THE SEA A VAST AQUARIUM 16
III. A CHAPTER OF FOUNDATIONS 25
IV. THE HUMBLEST OF ANIMALS--SPONGES 30
V. FLOWERS OF THE SEA 33
VI. UNINVITED GUESTS 44
VII. DWELLERS BETWEEN TIDE MARKS 46
VIII. BUILDERS OF THE PEARLY SHELLS 57
IX. BUILDERS OF THE PEARLY SHELLS--_Continued_ 64
X. ANIMALS WITH JOINTED FRAMES 81
XI. FROM BUTTERFLIES TO BEETLES AND BEES 99
XII. AT THE DOORWAY OF THE "UPPER CLASSES" 127
XIII. FISHES--THE ARISTOCRACY OF THE WATERS 132
XIV. SHARKS--THE TIGERS OF THE SEA 142
XV. THE FEARFUL DEVILFISH AND OTHER RAYS 148
XVI. BONY FISHES--TELEOSTOMI 151
XVII. MODERN FISHES--TELEOSTEI 154
XVIII. AMPHIBIANS--A CONNECTING LINK 167
XIX. AMPHIBIANS--_Continued_ 174
XX. REPTILES--MONARCHS OF THE MESOZOIC WORLD 182
XXI. SERPENTS, GOOD AND BAD 211
XXII. BIRDS--KINGS OF THE AIR 239
XXIII. SOME NOTABLE WATER BIRDS 250
XXIV. VULTURES, FALCONS, AND GAME BIRDS 260
XXV. FROM GULLS TO KINGFISHERS 264
XXVI. PASSERINE BIRDS 268
XXVII. THE BEASTS OF THE FIELD--SOME PRIMITIVE TYPES 270
XXVIII. THE GNAWERS 285
XXIX. MAMMALS OF THE SEA 297
XXX. THE WORLD'S HERDS AND FLOCKS 300
XXXI. THE WORLD'S HERDS AND FLOCKS--_Continued_ 313
XXXII. SOME SUPREMELY USEFUL ANIMALS 324
XXXIII. BEASTS OF PREY--THE CARNIVORA 332
XXXIV. BEASTS OF PREY--_Continued_ 354
XXXV. INSECTIVORA--HEDGEHOGS, MOLES, AND SHREWS 366
XXXVI. THE BATS--WING-HANDED MAMMALS 369
XXXVII. MAN'S HUMBLE COUSINS 373
LIST OF ILLUSTRATIONS
GIRAFFES, IN THEIR NATIVE AFRICAN HAUNTS _Frontispiece_
FACING PAGE
STARFISH AND OTHER TYPICAL LIFE IN A TIDE POOL 48
CORAL FORMATIONS OF THE GREAT BARRIER REEF, AUSTRALIA 49
CATERPILLAR BEGINNING TO WEAVE ITS COCOON 118
MOTH AND EGGS 119
CATERPILLAR PROTECTED BY FORM AND COLOR RESEMBLING THE TWIGS
OF A TREE 148
SEA HORSE PROTECTED BY FORM AND COLOR RESEMBLING THE MARINE
PLANTS AMONG WHICH IT LIVES 148
GILA MONSTER, FEARED, THOUGH ITS BITE IS NOT ALWAYS DEADLY
TO MAN 149
IGUANA, A REMARKABLE LIZARD OF THE NEW WORLD 149
PELICAN, NOTABLE FOR ITS THROAT POUCH 248
PEACOCK WITH BRILLIANT TAIL SPREAD 249
SACRED PHEASANT 249
OPOSSUM MOTHER AND YOUNG 272
ANTEATER, WHICH LIVES ON INSECTS CAUGHT IN THE STICKY SALIVA
OF ITS LONG TONGUE 273
SLOTH, AN ANIMAL WHICH KEEPS TO TREES AND IS ALMOST HELPLESS
ON THE GROUND 273
WART HOG, ONE OF THE UGLIEST ANIMALS TO SEE 304
MALAY TAPIR, RELATED TO THE PIG AND THE RHINOCEROS 304
MARKHOR, AN ASIATIC WILD GOAT 305
MOUNTAIN SHEEP OR BIGHORN OF THE ROCKY MOUNTAINS 305
THE KUDU, OR STRIPED ANTELOPE, OF AFRICA 312
HEAD OF THE GREATER SABLE ANTELOPE 312
HEAD OF ALASKAN MOOSE 312
AXIS, OR SPOTTED DEER OF THE EAST INDIES 313
AMERICAN DEER WITH HORNS IN VELVET 313
WOLVES IN WESTERN NORTH AMERICA 336
BEAR IN A ROCKY MOUNTAIN FOREST 337
BAT, WITH YOUNG BAT IN EACH POUCH 352
BLACK SPIDER MONKEY 353
RUFFED LEMUR, ATTRACTIVE BY ITS COLORING 353
MANDRILL, MOUTH OPEN TO SNARL 368
GORILLA, SOMEWHAT THOUGHTFUL 368
CHIMPANZEES, IMITATING SOME OF THE WAYS OF MAN 369
CHAPTER I
HOW THE GLOBE WAS STOCKED WITH LIFE
Ever since man began to think in the connected way that follows
self-consciousness, he has pondered, with a mixture of fear, reverence,
and curiosity, on the nature of life and its origin. The world in which
he found himself was a vast mystery which, very crudely at first, he
sought to penetrate. All his paths of thought led him circling back to
himself as the greatest mystery of all. He struggled with the problem
for thousands of years, framing fanciful guessworks, erecting elaborate
structures of logic on foundations of error, emotion, and presumption,
fashioning beautiful fables and theories (and waging wars to compel
other men to accept them), yet found no better solution than that life
must be a gift from some unknown, perhaps unknowable, source. Even
lately, learned philosophers, such as Helmholtz and Kelvin, supposed it
brought to the earth (in germs) by meteorites--fragments of exploded
planets that had borne life before they went to destruction; or, like
Arrhenius, postulated an impalpable dust, or "panspermia," scattered
through all space and borne from the atmosphere of one planet to
another. But all such hypotheses only threw the question of origin one
step further back.
Meanwhile, beginning a few hundred years ago, when greater privilege of
inquiry became possible in a jealous society, naturalists had tried
to attack the problem from a new angle. They asked themselves whether
they might not, by intensive study of living things, find the quality
of life itself, hoping that if that could be done the source of it
might be disclosed. In their earnest work they constantly improved
their methods and their instruments, and so penetrated deeper and
deeper into the constitution of plants and animals, until at last they
found the ultimate particle in the cell and discovered living things so
simple that they consisted of one cell alone; but why that microscopic
particle was _alive_, while the grain of crystal beside it, or the drop
of water in which it swam, was _not_ alive, remained unexplained.
Thereupon some of the naturalists fell back into the ranks of the
speculative and religious persons who were content to believe the
endowment of the world with life an act of a Divine Creator--something
above and outside of nature as otherwise manifested; others asserted
an equivalent but more materialistic doctrine that they styled
"spontaneous generation," which presently was shown to be untrue, at
least in the way they formulated it; and a third group confessed that
they did not know whence life came, nor were they much concerned to
know.
LIFE CONSTANTLY ORIGINATED BY NATURE'S PRODUCTIVE ENERGY
This quest having failed--although it had taught much by the way--the
chemists, who had been making marvelous discoveries in the inorganic
lifeless half of nature, undertook a far more serious exploration of
the organic living half. You have interpreted very fully, they told
the naturalists, the forms, and structure, and functions of organisms,
but can get no further; now let us chemists try whether we cannot find
the principle of life by analyzing the _substance_ of living things.
Profiting by their experience, they turned to the colloids in hope of a
clue. A colloid is a substance that shows no power of crystallization,
and is composed of molecules united by their own affinity, and not
by atomic affinity. They have a gelatinlike nature or composition,
although varying greatly in chemical composition and general character.
They differ widely in stability, for instance, some being easily upset
by a change in conditions; and this peculiarity is of great importance
in relation to the phenomena of life, for colloids enter largely into
the composition of all living bodies, but always in a delicately
balanced union with crystalloids. "The colloid is in fact," declared
Thomas Graham, who first investigated its properties, "a dynamical
state of matter; the crystalloid being the statical condition. The
colloid possesses Energia. It may be looked upon as the probable
primary source of the force appearing in the phenomena of vitality."
Now, many of the properties of inorganic colloids approximate those
found in living structures, which appear to be "alive" by reason of the
conversion of the energy of the sunlight into the chemical energy of
their constituent (organic) colloids. The agent in this conversion is
the green substance chlorophyll in the cell or cells of the plant; and,
directly or indirectly, all the energy in living things arises from
this one source, transmuted by this one transformer. Yet chlorophyll
is far too complex a substance to arise as a first step from inorganic
matter, even where conditions are suitable for life to appear; and the
spontaneous production of such a thing as a bacterium would not solve
the problem, for the new-born cell would have no organic food, and
must at once perish. In an utterly lifeless planet inorganic colloids
must first develop, and in time one of these must begin to evolve not
a living cell, or anything so complex as a bacillus, but something in
the way of a molecule holding a higher store of chemical energy than
anything before it. Later such colloids, perhaps uniting with others,
would begin to condense and form more complex organic molecules, and
finally effect unions with crystalloids. Thus would organic complexity
gradually be led up to, chlorophyll brought into being, and life
appear. One of the foremost of the biochemists, Prof. Benjamin Moore,
of the University of Liverpool, has summed this up picturesquely:
"It was no fortuitous combination of chances, and no cosmic dust, which
brought life to the womb of our ancient Mother Earth in the far-distant
Paleozoic ages, but a well-regulated orderly development, which comes
to every mother earth in the universe in the maturity of her creation
when the conditions arrive within the suitable limits. Given the
presence of matter and energy forms under the proper conditions, life
must come inevitably.... If this view be the true one, there must
exist a whole world of living creatures which the microscope has never
shown us, leading up to bacteria and the protozoa. The brink of life
lies ... away down among the colloids, and the beginning of life was
not a fortuitous event occurring millions of years ago and never again
repeated, but one which in its primordial stages keeps on repeating
itself all the time and in our generation. So that, if all intelligent
creatures were by some holocaust destroyed, up out of the depths in
process of millions of years intelligent beings would once more emerge."
That is to say, life arose through a recombination of forces
preexisting in the cosmos, and the fact was but a step in the
evolutionary process. "Such evolution," the American biologist, Henry
Fairfield Osborn, declares with emphasis, "is essentially constructive,
and ... is continually giving birth to an infinite variety of new forms
and functions which never appeared in the universe before. It is a
continuous creation or creative evolution. Although this creative power
is something new derived from the old, it presents the first of the
numerous contrasts between the living and the lifeless world."
LIFE'S BORDERLAND
Although in some respects a deceptive resemblance may appear between
the living and the nonliving, the distinction is definite. Living
bodies, plant or animal, are made up of protoplasm, which, although
mineral in substance, consists of a combination never found in the
mineral kingdom. It gives to the body containing it the power of
growth, and this growth is by additions from within. Minerals may
increase in size, but only by additions from without. The prime
characteristics of living organisms is that they reproduce their kind,
given favorable conditions. Minerals never do so. A correlative of
life and growth is death, but minerals never die. In the course of its
career every animal or plant, in proportion to its need or the degree
of complexity of its organs, develops within itself characteristic
compounds, such as albumin, gluten, starch, cellulose, fat and other
chemical results, not a trace of any of which is to be found in rocks
or soil, or in the water or in the air. No distinction in nature is
so absolute as that between the inorganic and the organic realms, the
nonliving and living things, so far as our senses can perceive them.
When, however, we consider the two prime divisions of the living
world--animal and vegetable--so diverse in their higher developments,
we find them springing from the same base in a single cell of almost
structureless protoplasm, and so alike in this simplest form as to
be in some cases indistinguishable--mere drops of living matter
whose functions are so limited that they present no discriminative
characteristics. Indeed, marking a definite boundary between animals
and plants may be difficult in cases much higher in the scale than
these primitive globules of protoplasm.
A fundamental distinction between plants and animals as we now know
them is the exclusive possession by plants of the green substance
chlorophyll, by the presence of which their food is transformed under
the influence of sunlight into vital energy in a manner essentially
different from that by which animals assimilate their substance.
Chlorophyll is a complex, nitrogenous, colloidal substance, produced
by and always associated with, protoplasm, and related to the coloring
matter of the blood of animals. It is restricted to plants, and usually
resides only in definite portions of the cell; yet we have good reason
for believing, as Prof. William F. Ganong tells us, that our present
green plants were preceded in time by a colorless kind of the utmost
simplicity, and without chlorophyll, which yet could make their own
food from carbon dioxide and water by using the energy of chemical
oxidation of soil-minerals in place of sunlight. "We have precisely
such chemosynthetic organisms, a kind of soil bacteria, still living on
the earth at this day; and they are doubtless the lineal descendants
of the ancient forms, which probably lived in the mud of shallow seas
that may be full of them yet." These ancient chemosynthetic organisms
were neither animal nor plant, but both and between. They must have
expanded, varied, evolved, thus originating a great many branches, most
of which perished.
Now, from this biochemical borderland of life, let us turn our
attention to the living world as we know it to-day, or as preserved
for us in the "record of the rocks," pausing only to fix well in our
minds the main distinctions between animals and plants. Plants have no
special organs for digestion or circulation, nor any nervous system.
Most plants absorb inorganic food, such as water, carbonic acid gas,
nitrate of ammonia, phosphates, silica, etc. No animal swallows any
of these minerals as food. On the other hand, plants manufacture from
such materials the food on which animals exist, by the production and
storage in their tissues of starch, sugar, and nitrogenous substances.
The two kingdoms supplement one another. They are mutually dependent,
and probably originated simultaneously.
CHAPTER II
THE SEA A VAST AQUARIUM
No results of investigation in natural history have been more amazing
than those that show the marvelous richness of the sea in plant and
animal life--not merely at its warm margin, but far out in the centre
of what the ancients used to call "the desert of waters"; not only at
its surface, but in its profoundest depths, and under the polar ice as
well as amid the tropics. Sea populations differ somewhat according
to situation, those of the shallow shore lines, which are of the
"littoral" fauna, differ largely from those living in the open sea
and belonging to the "pelagic" fauna, and there are surface swimmers,
and others confined to the abysses; but virtually every class and
subdivision in the animal kingdom is represented in greater or less
variety in the zoölogy of the ocean. The list stretches from the merest
monads to the huge sharks and still bigger whales.
This multitude and diversity of animal life is possible in the sea
because of an even greater plenitude of plants there, which furnish
a never-failing food resource. Bacteria and blue-green algæ are at
the base of this. Bacteria exist in all seas, as in all soils, and
the fertility of nature above ground and under water depends on these
microscopic organisms, whose numbers in the ocean are as incalculable
as the grains of sand on its brink. In equal multitude are the
diatoms, unicellular algæ with flinty cases, by which the waves are
sometimes discolored over broad areas; and millions of other green
plants, living alone, or in chains, minute in size, but each a chemical
laboratory converting the salt water they absorb into meals for the
animals that swallow them--animals in most cases almost as small and
simple as the things they eat, and themselves destined to be sucked
into the mouth of something a little bigger, to be in turn a tidbit for
a third hungry mouth, and so on to the broiled mackerel for our own
breakfast.
THE PLANKTON AND ITS PHOSPHORESCENCE
The assemblage of plants and animals that together float or swim at or
near the surface of the ocean (or other water), say within a layer of
water one hundred fathoms thick, is scientifically called _plankton_
of the sea. In the open ocean, the pelagic plankton is much alike all
round the world of waters, although it varies a little in composition,
and still more in relative abundance, being denser in temperate than
in either tropical or polar latitudes; but nowhere is it absent. The
"waste of waters" teems with life. The plankton of the shallow waters
near continental shores, however, presents a decidedly different
assemblage from the pelagic plankton.
In the pelagic plankton, single-celled animals of the groups called
foraminifers and radiolarians are exceedingly prominent, and play an
enormous part in the economy of the sea, although almost or quite
microscopic in size. They are incased in chambered shells of lime or
flint; and over vast areas in warm latitudes the ocean floor is so
thickly covered with the dead shells of one kind that the mud is
called globigerina ooze. They are the eaters of the microscopic plants,
and themselves are food for a wide variety of hydroids and jellyfish,
large and small, whose silvery forms are often visible to the voyager,
and which are mostly responsible for the pale stars of phosphorescence
that shine about his prow and glorify his wake in dark nights. The
queen of these far swimmers is the radiant Portuguese man-of-war. In
the night a dragging fine-meshed net will capture more than by day of
the plankton, because many little creatures that in daylight sink to
considerable depths come to the surface at night.
Rising a step to the worms, we find them comparatively rare, but one
kind of marine flatworm that abounds in midocean is rose-red and
several inches long. Much more numerous is another flatworm, Sagitta,
"which along with copepoda, salpæ, pteropoda and radiolaria, everywhere
constitute the bulk of the small pelagic organisms" captured by towing
nets. Like almost all of these usually defenseless creatures they
are perfectly transparent, but some of them depart from the rule of
pale blue in tint and shine in bright red. A longer step takes us to
the Crustacea, represented in the pelagic plankton by queer little
shrimplike forms that in countless hosts of individuals play a part in
the ocean comparable to that of insects on land. The copepods are the
most numerous probably--little things only a fraction of an inch in
length, but amazingly abundant, and the principal users of plant food.
Their relatives, the little ostracods, have similar habits, and are
noted for their intense phosphorescence. Haeckel relates that on his
way to Ceylon he saw the entire sea like a twinkling ocean of light,
and his microscope showed him that it was made by throngs of ostracods,
with some jellyfishes, salpæ and worms. Crustaceans of higher rank
abound also. In northern waters species of Schizopoda, small,
transparent prawns with red spots around the mouth and big, black eyes,
swarm in enormous numbers, and are known to the fishermen as "kril."
An important part of the pelagic plankton consists of certain small
mollusks; and "as regards abundance of individuals few groups of
pelagic animals can compare with the winged snails, or Pteropoda."
These are minute, rapidly swimming creatures with thin, glassy shells,
and in some parts of the warmer oceans these discarded shells are so
numerous on the bottom that they give the name pteropod ooze to the
mud. One kind (Limacina), with a coiled shell about the size of a
pinhead, which abounds in the north Atlantic, is much feared by the
Norwegian fishermen because they very often spoil the herring that
feed on them. Another kind (Clione), looking somewhat like a reddish
butterfly an inch or so long, swims in shoals in the icy seas of the
far North, and is known as "whales' food." Some larger mollusks, of
which the beautiful purple Ianthina is most conspicuous, live among the
vast patches of floating seaweed in the Sargasso Sea.
Great numbers and variety of tunicates or ascidians and their larvæ
are taken in the surface nets of the sea naturalists, among them the
salpæ--free-swimming, barrel-shaped, transparent animals well known
to all seafaring people, and often seen crowding the surface of the
ocean. One genus of them is Pyrosoma, which has from the earliest
days excited the interest of mankind, mainly on account of the strong
phosphorescent light emitted, the name, indeed, meaning "fire animal."
These salpæ aggregate into colonies often several yards in length which
glow like fiery serpents as they move sinuously on their way.
This property of luminosity, so widely possessed by marine animals, is
one of the unsolved mysteries. It is called "phosphorescence," because
it resembles the cold light given by phosphorus when undergoing slow
oxidation, but phosphorus has nothing to do with the manifestation
here, or in such insects as the firefly; nor is it owing to bacteria,
as in the case of shining wood or decaying fish. What it really is no
one knows, but it has, at least, been learned that in animals the power
of emitting light is always attributable to certain structures of a
glandular nature that secrete a slimy, luminous substance, or, rather,
two substances, one luciferin and the other luciferase. When both
together are exposed to seawater phosphorescent light results.
As a rule, the light organ is surrounded by a layer of black pigment
that acts as a reflector, and often the light is projected through a
transparent lens; and there is reason to believe that in the case of
the higher animals, such as deep-sea fishes and squids, the rays may
be thrown when and where the creature desires, as a man handles an
electric flashlight. But for what purpose? Is it to illuminate the
surrounding water so as to perceive, or to attract prey, or is it to
avoid foes? A learned oceanographer replies that no one certainly
knows. "At all events," he concludes, "the answers would probably tend
to show that the many different kinds of light organs serve different
purposes."
PRISONERS IN THE DARK AND ICY DEPTHS
So much for the surface population of the ocean--the plankton layer is
regarded as a hundred fathoms thick. We have considered only that over
the mid-oceanic depths, but that of the shallow margins is different
simply in the absence of some purely pelagic creatures, and in the
presence of vast hordes of eggs and larvæ of the animals rooted in the
sand or attached to the rocks and weeds from high-water mark down to a
comparatively short distance below low-water mark. These I shall speak
of more completely hereafter.
Before that, however, I want to say a few words in regard to the
extraordinary inhabitants of the ocean's depths--depths which in some
places exceed the elevation of the highest mountains on the land.
The conditions under which animal life exists there are vastly
different from those at the surface, and it is not surprising to find
these creatures of an extraordinary character. The pressure exerted
by water on anything lowered into it increases at a rapid rate as the
object sinks, so that at a depth of only 500 fathoms it equals about
100 times the pressure at the surface. This contributes to the density
of underlying waters; the saltiness of the sea also adds to the water's
density, but this decreases slightly from the surface downward. More
important than density in its effect on living things is temperature.
In the Sargasso Sea in summer the water at the surface will indicate
about 52 degrees F., and at 100 fathoms of depth 48 degrees, below
which it diminishes slowly to a little below the freezing point--32
degrees F. The water below a few hundred fathoms may therefore be
regarded as a series of layers measured by degrees of density,
temperature, etc., and this means a series of biological strata in
each of which the denizens are more or less limited by unfavorable
conditions above and below them.
A fourth factor conditioning deep-sea life is that of light. The
sunlight penetrates to a much greater distance than was formerly
believed; and experiments with photographic plates show that the blue
rays may sink as far as 800 fathoms, but the red rays go much less
down. Below that glimmer is absolute darkness, illuminated only by the
phosphorescent glow of the lanterns carried by the animals moving about
in that Stygian and icy abode--which would seem to us the most dreadful
fate to which any creature on the globe is born.
It has been said that the ocean depths seem to be divided into
horizontal zones, certain groups of animals being confined, when
adults, within limits of depth determined by conditions suitable
to them, one zone above the other. Practically, however, these
intermediate life-zones can hardly be defined, and vary in different
seas, and under changing conditions, as of season, and so forth.
Animals taken only by deep hauls of the nets within the tropics,
for instance, may be captured in cooler latitudes near the surface;
furthermore, the vertical distribution of fishes, as a class, may
differ from that of crustaceans as a class. Nevertheless it is true
in general that many sorts of pelagic animals dwell at intermediate
depths, from which, when they have become mature, they cannot either
rise or descend any great distance. Among them are representatives of
all the classes of marine life.
Let us now consider the creatures of the lowest level--those abysmal
depths where eternal cold, stillness, darkness, and equability unite
to make an environment so forbidding that human imagination would
refuse to people it with living beings; yet where life and strife do
actually exist, although by no means uniformly distributed. We know
most about it as it exists in the bed of the north Atlantic.
The real bottom animals are mainly fixed--sponges, hydroids,
sea anemones, bryozoans, brittle-stars, crinoids, brachiopods,
holothurians, worms and mollusks. They are nowhere numerous remote
from a shore, and below 2,500 fathoms are very scarce, to judge by
the results of dredging. Their food comes wholly from the surface,
apparently, some catching it as it falls and others sucking it out of
the ooze. Moving about among these, and feeding on them, is a scanty
population of snails, squids, crabs, and fishes, making their living
upon or close to the bottom; and a larger and more varied company of
relatives swim in the water above them up to, say, the 2,000-fathoms
line. All these are of forms different in many respects from kindred
species at or near the surface; and some brought up by the deep-sea
dredge can hardly be distinguished from fossils entombed in the oldest
fossiliferous rocks--so unchangeable is the environment in which their
race has been propagated for perhaps fifty millions of years.
Through these dark abysses swim fishes with extraordinary and grotesque
adaptations to their conditions. All are small, rarely six inches long,
often less than an inch, yet armed to the teeth. This is especially
true of the families Stomiatidæ and Sternoptychidæ, in which one finds
fishes of the queerest shape, with big heads and a savage array of long
sharp teeth. All are voracious, for food is scant and must be fought
for; and some, as Chiasmodus, have mouths so capacious that they often
swallow fishes larger than themselves, when their stretched stomachs
hang beneath their slender bodies like the yolk sacs of newly born
trout. All are dark in color, brown, blue or violet marking the abyssal
species. Some of them have light-giving organs; and this was formerly
regarded as a peculiar possession of deep-sea fishes, enabling them to
see their prey in the gloom of their habitat, but it is now known that
light-giving organs are especially characteristic of pelagic fishes of
the region between the surface and 250 fathoms of depth. It must be
remembered, however, that the sedentary invertebrates of the bottom
glow with phosphorescence.
This outline of a vast body of information shows that the waters of the
oceans are everywhere inhabited, to their uttermost deeps, by living
beings; that these are adapted to various circumstances, and so form
faunas of local extent and character; and that probably the sea derived
its wealth of population--at least all that part superior to the
monads--from the land, beginning with the earliest dawn of life on the
globe.
CHAPTER III
A CHAPTER OF FOUNDATIONS
THE NATURAL BASIS OF CLASSIFICATION
I mentioned in my introductory chapter that the simplest form of animal
was one whose whole being was contained within a single envelope, or
"skin," called a cell. Such a cell contains nothing but that strange
primitive life-substance named protoplasm, condensed at one point into
a nucleus, and it is precisely of such cells that the bodies of all the
animals we commonly know are made up; nevertheless an immense variety
of creatures still exists, especially in the plankton of the sea, that,
like those at the dawn of life, consist of one cell alone. Here then we
stand at the first grand division of the animal kingdom:
A. Animals consisting of a single cell--_Protozoa_.
B. Animals composed of an aggregation of cells--_Metazoa_.
This distinction, you see, is one of structure, as must be all the
subdivisions that follow, if they are to be natural; and it is the
clearest possible illustration of what we mean in zoölogy when we
speak of "lower" and "higher" rank, for it is evident that it is a
step upward, an advance from utter simplicity to greater and greater
complexity, to proceed from a single-celled, all but helpless
animalcule to one composed of many cells, with so vast a division of
labor and extensive power of action as belong to such a combination of
forces.
I do not propose to describe the Protozoa, because both of lack of
space and lack of popular interest; anyone may learn about them in any
good zoölogical textbook. But I do want to mention one very important
point, on account of its bearing on the history of the higher animals.
The protozoans reproduce their kind by simply splitting into two
individuals, and these again split into another two, and so on; the
process is called "fission." There comes a time, however, when the
ability to do this ceases, and the protozoans of this strain will die
out unless one or more of them meets with the same kind of animalcule,
and the two "conjugate," or merge into one another, thus renewing their
power to go on dividing.
Turning now to the Metazoa, or animals in general, we may say that
they are flexible and usually motile beings, needing a supply of solid
food which they convert by digestion into a fluid form, and then
diffuse through their tissues. This accounts for the fact that all
animals consist essentially of a tube, which in the simpler forms is
very apparent. This typical tube consists of at least two layers--an
outer, protective, and sensitive coat (ectoderm), and an inner,
digestive one (endoderm). This two-layered condition is the limit for
a few fresh-water and a vast number of marine animals therefore called
"coelenterata," of which the jellyfish and corals are examples. The two
coats are separated, and at the same time connected, by a greater or
less amount of a jellylike filling called the "mesenchyme." Into this
intermediate mesenchyme both ectoderm and endoderm bud off cells which
have certain functions--that is, they circulate the digested food,
perform the creeping movements when such occur, expel the waste of the
body, and most important of all, provide the germ cells by which the
race is perpetuated.
Now in animals superior to the jellyfishes and the flatworms, the
mesenchyme is replaced by a definite hollow tissue that produces a more
efficient system of muscular, excretory, and reproductive organs. This
hollow tissue is the "coelom," and in the most advanced animals, such
as the chordates, "the coelom and its products are of the greatest
importance, for they give rise to the vertebræ and the muscles, and
in so doing mold the shape of the fish, amphibian, reptile, bird, and
mammal."
In this brief sketch of some broad distinctions among the masses of
animals we have a hint of the basis of their classification.
ANIMAL LIFE IN ORDERLY ARRANGEMENT
Classification is really only a sorting out of things into groups of
the same kind. It may be artificial, according to fancy or convenience,
or it may be by discovery of nature's inevitable development. It has
been done crudely ever since men began to show curiosity about the
things around them. They spoke of animals of the land, of the water,
and of the air; of those that lived on vegetable fare as different
from the flesh eaters; and in a more particular way they recognized
various obviously like and unlike groups within the larger ones. All
these distinctions were made on external appearance or behavior, and
closer observation presently showed bad combinations, such as placing
bats with birds simply because both flew, or whales with fish because
both lived in water. Slowly it became evident that the only proper way
to classify animals was by putting together those of like structure,
and this could be accomplished only by intense comparative study of
the interior anatomy of their bodies. Even here, however, progress
was limited until the great light from the idea of organic evolution
fell on biological science, by which it was perceived that the true
criterion by which the proper place of any animal could be determined
was its line of descent--a matter wherein the student of fossils
could render, and has rendered, vast assistance. In other words a
real, natural classification is according to ancestry, just as human
relatives are grouped into families according to their known descent
from the same forefather.
In this evolutionary light zoölogists have now perfected, at least in
respect to its larger divisions, a classification of the animal kingdom
which is generally accepted, and is followed in this book. It proceeds,
reading downward, from the simpler and older forms of animal life to
the more complex and more recent forms.
As to the names and relative order, or rank, of the subdivisions that
we shall have occasion to mention, a few words are desirable. The only
real fact is the individual animal. A collection of these so similar
that they cannot be divided, and which will interbreed, but usually are
sterile as to other animals, is termed a _species_. A number of species
closely similar are bracketed together as a _genus_ (plural _genera_),
and this done, every individual is given a double name, as _Felis
leo_ to the lion, the first part of which indicates its genus, and is
called its "generic" name, and the second indicates its species, and
is called its "specific" name. This "scientific name" is given in Latin
(or Latinized Greek) so that it may be unmistakably understood in all
parts of the world, for a local name in one language would mean nothing
to a student speaking some other language, or perhaps speaking the same
language in another country; thus the name "robin" is applied to half
a dozen very different birds in separate parts of the English-speaking
world, and endless confusion would result were not each animal labeled
in a language understood by everybody; and this must be a dead
language, so that the significance of the terms applied shall not vary
in place or time.
Several similar genera may form a _family_; families that agree in
essential characteristics are united as _orders_; orders are grouped
into _classes_; and finally like classes are assembled into a _phylum_
(Greek, "a leaf": plural _phyla_), which is the largest division except
the primary distinction of Protozoa and Metazoa.
CHAPTER IV
THE HUMBLEST OF ANIMALS--SPONGES
At the foot of the arrangement of phyla in the metazoa stand the
Porifera, or sponges, fixed, plantlike, queerly shaped beings living
in the sea, except one family in fresh waters, and abundant in all
the warmer parts of the world on rocky bottoms. Whatever its size or
shape, a live sponge (of which the commercial article is the more or
less perfect skeleton) is coated with a thin fleshy membrane perforated
by minute "inhalant pores" and larger holes termed "oscula," or
mouths. Through the inhalant pores the sea water, with its burden of
microscopic food, enters one of many spaces beneath the surface from
which incurrent canals penetrate the interior of the sponge, constantly
branching and growing smaller until lost to sight. The fine tips
communicate with small cavities lined with cells that are fitted to
seize and assimilate the nourishment brought them by the water. From
these rudimentary stomachs go similar excurrent ducts that unite near
the surface into trunk canals that carry out the used water and waste
products. This system of circulation, bringing nutrient water strained
through the pores, and expelling it forcibly after it has been cleared
of food value, is kept in motion, with occasional periods of rest,
by the action of "flagellate cells" that line certain tracts in the
canals. These are elongated cells from which project whiplashlike
filaments, one to each cell, whose movements in concert "resemble
those which a very supple fishing rod is made to undergo in the act
of casting a long line"--the movement being much swifter from without
inward.
Beneath the outer skin, and all among the canals and cavities, is a
filling of gelatinous materials, largely protoplasm, in which are
formed great numbers of variously branched and strengthening spicules,
of limy material in one group, and in others of a flinty or glassy
nature, or in the absence of these, a network of "spongin," such
as forms the skeleton of our common washing sponges. Spongin is a
substance allied to silk in chemical composition, and the threads are
felted together in such a way as to form a firm, yet elastic structure.
"In some Noncalcarea, which are devoid of spicules, the place of these
is taken by foreign bodies--shells of Radiolaria, grains of sand, or
spicules from other sponges. In others again, such as the Venus's
flower basket (Euplectella), the glass-rope sponge (Hyalonema), and
others, the skeleton consists throughout of siliceous spicules bound
together by a siliceous cement."
Sponges are reproduced both by budding in some form, which is an
asexual way, and by the sexual method of eggs and male cells; these are
formed in the same sponge, but rarely at the same time, and the early
stages of development are passed in a brood-cell within the body of the
parent sponge. Finally, the embryo escapes through one of the outgoing
canals, swims about awhile, becomes thimble-shaped, and settling down,
fastens itself by the closed end to some patch of mud, a rock, dead
shell or seaweed, closes the open end of the "thimble," and proceeds
to grow.
Sponges do not appear to be eaten by fishes or anything else. Countless
lower animals, such as marine worms, mollusks, and so forth, burrow
into them, however, in search of shelter; and in reversal, certain
small sponges, such as the cliona of our shores, burrow into the shells
of mollusks, which explains the honeycombed appearance of many of the
shells picked up on the beach. Sponges have a large part in that very
interesting and widespread phase of marine life called "commensalism,"
in which two animals become intimately associated in a mutually
beneficial way, and are thus spoken of as messmates. Some kinds of
sponge are never found growing except on the backs or legs of certain
crabs; the sponge conceals and protects the crab, while itself benefits
by being carried from place to place, with constantly new changes of
fresh water and food. This sort of partnership occurs in many different
groups of marine animals.
The capture and preparation of sponges for market employ thousands
of men and boats in the eastern Mediterranean, whence the best are
derived, and in the West Indies and Gulf of Mexico, where the sponges
are of a coarser kind, and are gathered and prepared by rougher
methods. They are taken commercially also in other seas, and frequently
dredged from vast depths.
CHAPTER V
FLOWERS OF THE SEA
JELLYFISHES, NAMESAKES OF THE FABLED MEDUSA
The type and simplest form of that great division of aquatic,
and almost exclusively marine, animals constituting the phylum
Coelenterata, is the polyp. It consists of a soft-skinned body,
typically cup-shaped, containing a baglike digestive cavity, or
primitive stomach, open at the top, and surrounded by the soft
mesenchyme. The open upper end is the mouth, which is usually encircled
by few or many tentacles--hollow outgrowths from the wall of the
tubular gullet. Currents of water are drawn in by waving cilia at one
end of the slitlike mouth, and pass out as waste at the other side;
they bring food and oxygen from which nourishment is absorbed by the
cells of the wall of the stomach (endoderm). Certain outgrowths within
the mesenchyme act as feeble muscles for lengthening and shortening the
body and tentacles; but there are no blood vessels or excretory organs.
Most polyps are fixed on some support, but in many the young pass
through a free, swimming stage before settling down for life. All
coelenterates, and these only, are provided with "stinging cells," the
nature and importance of which will be explained presently.
The simplest class is that of the hydroids (Hydroida), the type of
which is the fresh-water hydra, so-called because, like the Hydra
of ancient myth, when it is cut to pieces each part will grow into a
new animal. It lives in ponds and pools of stagnant water, and is so
small that a magnifying glass is necessary to study it, especially
in the case of the green one of our two common American species--the
other is brown. Indeed, similar hydroids of salt water are often taken
and dried by unscientific collectors under the impression that they
are feathery seaweeds. It is stalklike in shape, has long tentacles
which always turn toward the greatest light, influenced like certain
plants by heliotropism, and feeds on minute crustaceans and other
minute organisms. Sometimes hydras are so abundant as to form a velvety
surface in warm pools. The sexes are combined in the same individual,
and the embryo forms within the body, then protrudes as a bud, which
finally breaks away and after a time sinks, attaches itself at the base
to some support, and grows into a perfect hydra. When quiescent or
alarmed the tentacles are withdrawn, and the whole animal shrinks into
a little lump.
Such is the general natural history of the group; but the oceanic
hydroids have developed a vast variety of forms, and, with increased
breadth of life, have added many interesting features and habits.
Many of them are single, rooted in mud, or upon seaweeds, rocks or
shellfish both dead and alive, and look like flowers of lovely tints;
and they reproduce by putting forth separate reproductive parts, called
"zooids," of various kinds. Others are in colonies that spread by
extensions of the base from which arise other hydroids until a bunch
of them are growing side by side; but these groups consist of hydroids
differentiated into separate functions, for some devote themselves
to capturing food which nourishes all, through the common base, while
others produce the buds and eggs by which the colony is increased.
[Illustration: JELLYFISHES
(_Medusa aurita._ _Rhizostoma cuvieri._ _Cyanea capillata._)]
The most remarkable of these processes of reproduction is that which
is represented by the jellyfishes so abundant in all seas, and so
beautiful either when seen floating along just at the surface of the
summer sea, or when at night they glow with phosphorescence like
silvery, greenish rockets in the dark waves. Sometimes they occur in
enormous "schools"--as we say of fish--all of one kind, filling the
water thickly as far as one can see, and now and then in late summer
are cast on the beach in long windrows. They range in size from a
pinhead to ten or twelve feet in diameter. So big a Cyanea would
probably weigh fifty pounds, but after a thorough drying would yield
only a few ounces of semisolid matter, 99 per cent of the creature
being water absorbed in its spongy tissues. Some are egg-shaped, others
like a bell with a long clapper, but the ordinary form is that of an
open umbrella, usually fringed about the edge with tentacles, sometimes
short and fine, sometimes few and long, again a crowded circle of
long snaky appendages. These elastic hanging tentacles are the means
by which the medusa (as such a jellyfish is appropriately termed in
science) captures its food, which consists not only of the minute
things swarming in the plankton, but of other coelenterates, small
crustacea, fishes, anything in fact that it can entangle in its sticky
net and sting to death. Every one of the filmy tentacles is thickly
studded with microscopic cells (cnidocells) covered by a mere film,
and having a spinelike trigger projecting from it. If this trigger is
touched, or the film broken, out springs the coiled thread dart which
is barbed and carries into the wound it makes a poison that benumbs.
Thousands of these microscopic darts may prick the skin of a captive,
and paralyze its strength--as it does that of a man who gets caught
naked in the trailing net of one of the great northern medusæ. Being
thus captured, the prey is drawn up to the mouth, which opens in the
center of the under side of the umbrella float.
At intervals around the margin of the umbrella are small organs by
which, it is believed, the creature maintains a sense of balance and
direction, and perhaps of temperature or light, or both; for many
medusæ sink out of sight by day and come to the surface at night; and
when the sea is rough they descend to quiet depths. Thus they have
the power not only to move ahead by the alternate contraction and
dilatation of the disk, but to so alter their specific gravity as to
sink or rise at will. They thus show the rudiments of both a muscular
and a nervous system.
Very interesting, and often of great beauty, are the free-swimming,
colonial, hydroid polyps called siphonophores. On a long stem or
string are arranged, at the top, a bulb filled with gas or air, as
a float, then a series of swimming bells whose pulsations carry the
colony about, beneath which are various polyps and tentaclelike
appendages, some to gather food, whose digested products circulate
through the whole colony, others performing reproductive functions.
The variety of form is considerable; and one of the most peculiar,
and the only siphonophore familiar to most persons, is the exquisite
Portuguese man-of-war, whose prismatically tinted bulb, as big as
one's fist, is commonly met with in the Gulf Stream in the North
Atlantic, and often is seen in great flocks in the tropics, bobbing on
the surface of the waves in calm weather. Beneath that bulb trails a
long tuft of tentacles and zooids, performing various functions, and
so foreshadowing the division of labor that in the higher animals is
effected by the different limbs and organs.
SEA ANEMONES, CORALS, AND SEA FANS
Sea anemones are simply large polyps of more complicated structure
than the hydroid polyps. Instead of a simple, baglike, enteric cavity,
the slitlike mouth admits food into a flattened gullet which leads
to an enlarged digestive cavity. The gullet does not hang free, but
is joined to the outer wall of the body by a series of radiating
partitions, between which shorter ones extend from the inner surface
of the ectoderm; and below the gullet the stomach wall extends in lobes
between these partitions, through which holes permit the nutritive
juices to circulate throughout the whole body. The whole upper surface
of the polyp is covered by short tentacles arranged in circles. A
current of water, induced by waving cilia, is constantly flowing in
at one corner of the mouth and out at the other, supplying the animal
with oxygen and a certain amount of minute food, and carrying off
waste; but the anemones capture by means of their tentacles small
fishes, mollusks and everything that can be caught and swallowed. As
some anemones exceed a foot in diameter, large and powerful prey may
sometimes be taken. It is interesting to note that anemones distinguish
very quickly between what is good to eat and what is not. Most of them
are sitting near shore on rocks or in tide pools, or are clinging to
the larger seaweeds or clustered on the supports of wharves where the
waves and tidal currents are continually washing about them, often with
much violence, and dashing against them strands of weed or the small
wreckage always floating in such a place. None of this is seized, or at
least is not swallowed; but whether we are to conclude that this choice
is made by intelligence, or only by chemical perception is a matter for
study. When harm threatens, or when they crave rest, they withdraw all
their gorgeous tentacles, infold them within their mouth, and shrink
down into roundish gray lumps that attract neither the eye nor the
appetite of any marauder.
[Illustration: CORALS
Tree coral (_Dendrophyllia nigrescens_). Tuft coral (_Lophophyllia
prolifera_). Fan coral (_Euphyllia pavonia_). Cup corals and skeleton
(_Carophyllia smithii_).]
The coral polyps differ from anemones only in details of structure that
we need not consider, except to note the striking difference that here
the base and the radiating partitions instead of being membranous
secrete a firm skeleton either of lime or of the horny material termed
chitin. The flesh overflows the walls, folding down from the top, so
that the skeleton becomes really internal, although naked at the broad
base. Some of the tropical stony corals are like big anemones, several
inches across; and it is only when they infold all their richly colored
tentacles and become a dull and shapeless lump that their stony cup is
revealed. These are solitary, and form loosely lying corals, like that
called the "mushroom." New ones are produced by the parent throwing
off buds which for a time remain attached by a stalk, but finally fall
off and settle down to grow--a process that may go on for a score of
years. In the case of the huge coral masses called madrepores the buds
remain attached to the parent. If they spread out naturally, W. Saville
Kent explains, they build up by accumulation the large rounded masses
known as "brain" corals and "star" corals, which are most numerous
on coastline reefs, or form the base of the outer barrier reef. On
the other hand, where the budding is terminal, or oblique, branching,
treelike growths result in "staghorn" and similar forms.
The coral animals do not alone construct the reefs. Stony hydroids
(millepores), shells of all sorts of mollusks, limy sea mosses
(Bryozoa), animalcules and diatoms and various algæ stiffened or cased
with lime or flint, and blown sand, contribute to build them up,
especially when they near the surface of the sea.
The distribution of reef-building corals is interesting. At present
they are limited to about 35 degrees each side of the equator, but are
irregularly distributed, owing mainly to differences of temperature in
the water, which must not be colder than 68 degrees F. Hence they exist
farther away from the equator in the path of warm ocean currents. The
Gulf Stream accounts for the coral islands along the coast of Florida
and in the Bermudas, which is their farthest point on the American
coast; and the warmth of the water accounts for their extensive
presence along the eastern coasts of Australia and Africa, when few
exist on the western sides of these continents; similarly the western
coasts of South and Central America are nearly free of coral banks.
Other causes of limitation exist. For example, the noticeable absence
of coral growth along the coast of South America is largely, if not
altogether, owing to the fresh water and silt brought down by the great
rivers there--both prejudicial to coral life.
Coral colonies increase and ultimately form banks wherever warm,
pure sea water is constantly present, and not more than about 125
feet deep. Here, spreading and continually rising on the skeletons
of dead generations, they form a long line close to the land called
a "fringing" reef; and outside of this, beyond a space swept by the
currents, may arise a second, still more flourishing bank, termed
"barrier" reef. The great barrier reef that extends for 1,200 miles
along the eastern coast of Australia--a vast chain of banks and
islands--is an amazing example of what these minute animals can
accomplish, given time; and geology can point to still more stupendous
results of their work in the early history of the globe.
Very characteristic, in the great coral-growing region of the South Sea
archipelago, is the ring-shaped island or "atoll," which incloses a
quiet lagoon, usually with an open entrance. The reason for such a form
has excited much discussion, one explanation being that its origin was
about a small island that slowly subsided, the coral keeping pace in
rising as the island sank, until finally the land disappeared; another
that the circular reef arose from a submerged elevation, and when it
came near the surface ceased to grow except on its outer border because
it ceased to get suitable water and food, until after a time the
central part died out, leaving a ring. Both explanations may be true of
different situations.
When a reef comes near to the surface the branching coral is knocked to
pieces by the waves, and there are added to this breakage shells and
bones, calcareous seaweeds, and what not; and all this is ground into
sand by the surf, washed high on the top of the ridge and manured by
dead plants and animals, and by the droppings of birds, until finally
a soil forms beyond the reach of the tides. Then, if it is in the far
southern seas, a drifting coconut may lodge there and be rolled high
enough to be left to strike its roots into the sand and begin the
grove that by and by will make the islet attractive to men. The thick
husk of the coconut resists harm from sea water, near which this palm
prefers to grow in just such a sandy, shelly soil as the uprising reef
affords. The nuts that so often fall into the surf or are carried
out by rivers make long voyages without losing their vitality. Here,
again, the situation of most coral islets in the course of currents
is advantageous, for thus not only these nuts but other useful seeds
and colonizing elements drift directly to their doors, as it were.
Birds, wandering widely over the waters, espy the bit of land, and
aid by their visits to increase its fertility and often add to its
flora. Reefs near shore, especially in Florida and southward, become
jungles of mangroves, which not only spring from floating seeds but
send down from their branches sprouts that become rooted in the mud and
spread the growth interminably. Such a "mangrove key" soon attracts
an extensive population of plants and animals and speedily becomes a
considerable island.
A great variety of corals, however, are not reef builders, and some
species secrete little if any lime; these solitary relatives are found
scattered all over the oceans, in deep water as well as shallow,
wherever the bottom is suitable, and an immense amount of interesting
information about them is to be found in books devoted to this
beautiful group of animals.
The class includes two or three other orders of coral--polyps that
grow in a solitary way or in groups, forming those elegant objects
called sea fans, sea pens, and so forth, which can be referred to
only briefly. One of these is the order Alcyonaria, in which some are
soft-bodied, others are strengthened by a network of spicules. A very
beautiful one is the "sea pen," which takes the shape of an ostrich
plume; another is the strange mass of parallel tubes called organ-pipe
coral; and some of them are very large, the great tree coral of the
eastern Atlantic depths being sometimes as tall as a man, while it
looks like a sturdy, leafless tree. As in all the others, however,
it is covered by a living fleshy coat of protoplasmic substance
studded with polyps whose gay colors and waving tentacles give it the
appearance of being clothed with minute sessile blossoms. The best
known of this group, probably, is the red coral of commerce, which
is the scarlet, ivorylike interior stem of a branching alcyonarian
colony. This coral has from the earliest time been cut into cameos
by lapidaries, as well as used for making necklaces and other toilet
ornaments.
CHAPTER VI
UNINVITED GUESTS
FLATWORMS, FLUKES AND TAPEWORMS
The phylum Platyhelminthes follows the coelenterates in the ascending
series of zoölogical classification, and includes a baneful company of
creatures badly called "worms," which show none of the segmented or
ringlike form of body that characterizes the true worms of the phylum
Annulata to which we shall come presently. On the contrary, they are a
group of small, soft-bodied, flattened animals, which first show that
two-sided character, or bilateral symmetry, which has apparently been
absent from all the groups we have studied hitherto, whose members are
circular or globular in shape, and whose organs, in the adult, are
arranged radiately.
The simplest are the planarians (Turbellaria), which live a free
life, as a rule, although some are parasitic. They are little, thin,
leaf-shaped creatures that creep on the bottom of ponds and even of
deep lakes, or swim in the sea, and feed upon algæ and minute animals.
Similar to them in appearance are the flukes (Trematoda), of which the
best known of a large variety is that which infests sheep. Most of the
trematodes are parasitic.
The third class of flatworms is the Cestoda, the members of which are
universally parasitic, and are known principally as "tapeworms" in
reference to their form.
The phylum Nematothelminthes contains an assemblage of related worms,
some marine, but mostly living in fresh waters or on land, which are
eellike in form, very slender, and often have amazing length. The first
and lowest class is that of the nematodes, of which the minute "vinegar
eels" and "paste eels" are familiar examples. The remainder of the
nematodes are parasitic, and many of them are dangerous parasites.
In an allied family and genus (Trichina) is placed one of the most
dangerous of human parasites, the _Trichina spiralis_.
Here, too, comes that "hairworm" (Gordius), which most country folks
call "hair eel" or "hair snake." Many assert with the most positive
faith that if you will soak a horsehair in water it will "turn into a
snake," and will show you this long threadworm in a horse trough to
prove it. I never knew a cautiously made experiment in that direction
to succeed; nevertheless the fanciful error survives. The gordius,
which does look like a hair from a gray mare's tail, is somewhat
aquatic in its habits.
CHAPTER VII
DWELLERS BETWEEN TIDE MARKS
THE COLONIAL MOSS ANIMALS
Seaweeds and rocks at and below the limit of the ebbing tide are often
covered with small bushy growths, or with lacelike incrustations that
are alive. These are moss animals, representing the class Polyzoa of
the phylum Molluscoida. They are minute, soft creatures that live in
colonies formed by the repeated budding of the members, all connected
by a fleshy base so that each contributes to the nourishment of all.
"Each little animal occupies a separate stony or horny capsule, into
which it may withdraw and even close the opening with a lid.... The
mouth is surrounded by tentacles that in many species arise from a
horseshoe-shaped or disklike base. These tentacles are always beset
with hairlike bristles which by their movements serve to set up
currents, and thus to drive minute organisms into the mouth."
A typical example of these polyzoans (or bryozoans) is _Bugula
turrita_, so abundant wherever our northeastern coast is rocky that
the rocks below tide level appear covered with its mossy tufts, which
are often ten inches long and profusely branched. The main stems are
orange-yellow, while the terminal branches are yellowish white. The
delicate tracery so frequently seen on the fronds of kelp, and on
shells and stones along both shores of the Atlantic indicate colonies,
or their remains, of the lace coralline (Membranipora); and the dull
red or pinkish crust so common on shells and stones in shaded tidepools
represents successive colonies of the "red-crust" polyzoan (_Escharella
variabilis_), layer crusting over layer. A similar history accounts
for the curious nodules called "false coral" so common in moderately
deep water in Long Island Sound. Similar polyzoans, which exist in
great variety, both modern and fossil, contributed extensively to the
formation of the older strata of sedimentary limestones.
ANCIENT LAMP SHELLS
Associated in structure with these minute colonists is the ancient race
of brachiopods (Brachiopoda, "arm-footed") or lamp shells, although
they much more nearly resemble bivalved mollusks, whence, by the way,
comes the name of the phylum to which both belong--Molluscoida, which
means "mollusklike."
The race of the brachiopods goes back to the beginning of the geologic
record. A few living examples are still found in the ocean, some of
which, as lingula, have changed so little that they can hardly be told
from the most ancient fossils of their family. Certain species are
dredged abundantly on both coasts of the Atlantic from water a few
fathoms deep where the bottom is rocky. They look like small mussels at
first sight, but on examination show a vast difference in structure.
The bivalve shells, instead of growing on the right and left sides of
the animal, as in bivalve mollusks, cover its back and front, and the
head parts are at the gape of the valves. At the hinge end of the shell
the lower valve overlaps (it is the shape of this lower shell, like
that of an old Roman lamp, which suggests their common name, "lamp
shells") and the hinder end of the body projects as a stalk, by which
the animal fastens itself to the rock. "The mouth in the brachiopods is
flanked by two curiously coiled and feathered arms which lie within the
cavity between the shells, and are supported by skeletal rods attached
to the upper shell. These serve as gills, and also to capture the
minute creatures upon which the brachiopod feeds."
Owing to their great abundance, world-wide distribution, and remote
antiquity, as well as their excellent state of preservation,
brachiopods occupy a very conspicuous rank among extinct invertebrates,
and furnish us besides with a large number of important index fossils.
They are to be found in immense variety from the Cambrian to the
present, most numerously in formations from Silurian to Permian times.
[Illustration: STARFISH AND OTHER TYPICAL LIFE IN A TIDE POOL
Photo, American Museum of Natural History]
STARFISHES, SEA URCHINS, AND TREPANGS
We have now arrived at the point (phylum Echinodermata,
"spiny-skinned") where a distinctly new type of interior structure
appears in the possession by animals of a hollow space (coelom) between
the outer skin and the wall of the digestive tube which now becomes
occupied by definite organs instead of by an almost uniform mesenchyme,
as in the sponges and coelenterates. These organs arise from an
interior lining membrane called "mesoderm."
Henceforth, therefore, we shall deal with coelomate animals, among
which the echinoderms are lowest in rank. The simplest of them is the
"sea lily" which lives rooted on the bottom in deep water, and sways
about on a slender, jointed stalk, looking much like the flower after
which it is named. It is of interest chiefly as a survivor of the tribe
of crinoids that were so varied and numerous in early Paleozic times
that massive Devonian limestones are composed largely of their remains;
and the type has changed little through the ages. It consists typically
of a cup, mounted on its stem like the calyx of a flower, and composed
of circles of calcareous plates, definite in form and in relative
position, that contain and protect a well-organized body. Surrounding
the open mouth of the cup is a circle of long, jointed, much-branched
tentacles that sweep the water, capture passing prey, and bring it into
the mouth of the crinoid within the circling base of the arms.
[Illustration: CORAL FORMATIONS OF THE GREAT BARRIER REEF, AUSTRALIA
Photo, American Museum of Natural History]
If now you were to cut off its stalk, lay the crinoid on the sand,
mouth down and arms outspread, beside a brittle star or a basket fish,
which also have many-branched arms, it might be difficult to tell them
apart, yet they represent different orders; and from this, by way of
the naked serpent star, it is but a short transition to the starfish,
where the arms are no longer tentaclelike, but are simply pointed
extensions of a central body; this, in fact, is the case, for now they
are no longer prehensile organs, but are supports, mainly serviceable
in locomotion, and the stomach and ovaries are partly lodged in them.
The main point just now, however, is the fact that here, and in the
successive changes of form to be shown, the pattern of plates that form
a strengthening mosaic in the skin of the central part of the body
remains identical.
All starfishes are not as prettily symmetric as our familiar
five-finger. Some are shorter in the arms, and much broader and
thicker in the body; and if you will examine a collection of preserved
specimens of the echinoderms you will see that you can trace gradation
of form right around to the bun-shaped cake urchin, on whose top the
five-pointed star is printed, and thence to the globular sea egg,
which the French called "sea urchin," using one of their names for the
hedgehog. Furthermore, the five sections of the shell of the urchin,
which represent the five arms of the starfish folded forward and grown
together into a spherical case, are to be traced again, outlined by
appendages, in the elongated and leathery hide of the trepangs and sea
cucumbers of the order Holothuria.
It is as an illustration of homology, that is, the resemblance between
parts that have the same relation to the typical plan of structure,
and as an example of how almost endless variations of form may arise
within a single type, that the echinoderms are of most interest.
Otherwise it may be said that they serve as food for fishes and some
other creatures, including coastwise savages, and as curiosities in
geological museums and in aquaria; and that starfishes are sadly
destructive of cultivated oyster beds. We may therefore dismiss them,
and devote a page or two to the worms.
EARTHWORMS AND BEACHWORMS
Although various parasitic creatures have been described as flat
"worms," round "worms," and so forth, naturalists regard as true worms
only those of higher organization classified in the phylum Annulata, or
annelids, the distinctive characteristic of which is that its members
have elongated bodies divided into ringlike sections. These represent
a division of the internal parts into a series of structural segments
or "matemeres," each supplied with its own set of organs, yet connected
by blood vessels and nerves, and the whole traversed by tubular organs
serviceable to the entire animal. The nervous system consists of a
"brain" in the head, and a double, ventral nerve-cord with a ganglion
in every segment, foreshadowing the nervous system in insects and other
arthropods. The phylum embraces three classes: 1. Chætopoda--earthworms
and marine annelids; 2. Gephyrea--marine worms, otherwise called
sipunculoids; and 3. Hirudinidæ--leeches.
The earthworm or "angleworm" (that is, angler's worm, bait worm)
of the "common garden variety," to use the phrase of old-fashioned
encyclopedias, is a typical example of the first class, whose Latin
name refers to the bristles (setæ) on the flattened lower surface of
the body that serve the worm as "feet." A magnifying glass shows them
in four double rows allowing eight to each of the rings into which the
body is so plainly divided; their extremities are directed rearward,
and by their means the worm pushes itself along, and is able to cling
to and climb not only the walls of its burrow but vertical surfaces
when not too smooth. Thus they are found frequently on roofs and in
other elevated and surprising places, to which they have crawled in
the night, when, as well as in warm, rainy weather, they are likely to
wander a great deal. The long and greatly extensible and elastic body
tapers almost equally at each end, but the head end is that which goes
forward in crawling, and a lens will show a mouth on its lower surface,
beneath a sort of thick lip. A long gullet leads into an expansion
called the crop, and that into a large, tough-walled stomach, beyond
which an intestine leads to the last segment. The thirty-third to
thirty-seventh segments are swollen, forming the "belt" (clitellum),
which denotes maturity, but seems to have no special functions. The
senses are few and dull. No eyes exist, nor sense of hearing, but
the skin is extremely sensitive to vibrations, and to bright light,
as might be expected in a nocturnal animal. The sense of taste is
discriminating. The eggs are extruded in such a way as to form a
glutinous ring about the body, which, when complete, is slipped over
the head of the worm, and left to hatch in warm soil under a stone.
Earthworms live underground in burrows that are sunk well below the
frost line. In digging they work head downward, gnawing--although they
have no hard jaws--and swallowing the earth that is not easily crowded
aside and then throwing it out and perhaps heaping it up as "castings."
The tunnel must be wide enough to let its occupant turn around in it,
and it ends in a deep chamber in which one or more worms may pass the
winter without freezing. These worms naturally seek a loose, damp soil,
not only for ease of working, but because moisture is a necessity,
as they breathe through their skin; hence they abound in meadows and
cultivated soil, and are not found on high, dry plains. During the
day they lie near the surface, often with the head just protruding.
Here they are discovered by sharp-eyed birds and garter snakes, and
sacrificed by thousands, notwithstanding the strength with which they
hang on to their retreats by the tail. When it retires to the depths of
its burrow this worm plugs the mouth of the tunnel with leaves which
it draws always by the base, exhibiting considerable intelligence in
manipulating the various shapes of leaves to that end.
The world-wide distribution of the earthworm is to some extent owing
to man's agency. On our northwestern plains, for example, these worms
originally were absent, but are now widely distributed and flourishing
there, having been carried from the east, as eggs or small worms, in
the soil packed about the roots of trees and shrubs transplanted to
western orchards and gardens. This fact may have something to do with
the recent westward spread of the robin, which, more than any other of
our birds, is a hunter of them. Except where excessively numerous these
worms do far more good than harm in a garden.
The naids (Naidæ) are small transparent worms that creep about on
vegetation in fresh water, and, besides laying large eggs, they
occasionally divide into two at a place in the body that appears
arranged for this purpose, for it consists of a zone of very elementary
tissue. "Gradually," as Minot records, "the tissue of this interpolated
zone transforms itself into muscles, nerves, etc., and, growing
meanwhile, it forms in front a new tailpiece to patch out the anterior
half of the worm, and behind it forms a new head for the posterior
half of the original body. The zone then breaks and there are now two
worms." A relative, the lumbriculus, does the trick in a much more
prosaic way, breaking in two first, and letting the separate halves
acquire head or tail as best they may. This ability to reproduce lost
parts is of much service in the life of the species and often of the
individual, which may still live after some water tiger has bitten it
in two--and these worms are at the base of the food supply of rivers
and ponds, and would soon be exterminated were they not capable of
rapid and profuse multiplication.
Worms of this class dwell in great numbers and variety in the sea and
in salt-water meadows and beaches, and are often beautiful as well as
interesting objects of study for the visitor at the shore. The sea
mouse (Aphrodite), for instance, which is about three inches long and
of oval shape, is covered with hairlike bristles that glisten with
brilliant green, red, and yellow iridescence; it is to be looked for on
the mud just below the low-tide line, and inhabits both coasts of the
North Atlantic. The body of the common "clay worm," dug for bait at low
tide, which is olive in general tone, gleams with pearly iridescence,
while its innumerable feet bear gills that are green and salmon-red.
Another (Lumbriconereis) is known as "opal worm" for good reason; and
our sands abound in slender scarlet worms of the same genus named
"red thread." All these worms bury themselves in the sand, or wander
through it in search of prey, for they are carnivorous, and do not
hesitate to kill and eat each other. Some are fairly sedentary, and
protect themselves against fishes, crabs, mollusks, and bigger annelids
that seek them, by forming tubes by means in some cases of a shelly
secretion, but more usually by cementing bits of shell, stones, and
grains of sand into an irregular tube lining the burrow; the slender,
limy serpentine tubes often seen on stones or dead shells in tide
pools, are, or were, the homes of such protected worms, most commonly
of the "shell worm" (Serpula). "Often a number of these calcareous worm
tubes are seen clustered together. When undisturbed the worm protrudes
its beautiful feathered gills, which resemble a little passion flower
projecting from the mouth of the tube. These gills are variously
colored in different individuals, some being purplish brown, banded
with white and yellow, while others are yellowish green, orange, or
lemon-yellow. At the least disturbance, such as a shock or a shadow,
the gills are instantly withdrawn into the stony tube, and the opening
stopped by a horny disk." In the Gulf of Mexico extensive colonies of
these worms often form, and as the early generations die others erect
their tubes above them; as this goes on sand and shell fragments fill
around and between the tubes, and after a long time the whole mass
becomes a solid reddish, loose-lying rock, composed chiefly of serpula
tubes, which in Florida is dragged up from the beach and used as
building stone.
The third class (Hirudinidæ) of Annulata is that of the leeches, those
ugly, but useful, worms of land and sea. In spite of their sluglike
appearance the leeches are segmented worms, although the wrinkles on
their gray, mottled skins do not indicate the position of the segments
beneath. The mouth on the under side of the head is armed with jaws
and sharp teeth that make three or more cuts through the skin, whence
the blood is sucked; there is also a holding sucker near the tail.
Their attacks cause little pain, and that fact has led physicians to
put them into use when bleeding is required. The eggs of leeches are
laid in moist earth in little packets, and hatch in five or six weeks.
The growth to maturity is slow, and continues during a long life. Many
species abound in ponds and stagnant waters. Asia has terrestrial
leeches, swarming in moist vegetation; and in Ceylon the minute
leeches are a terrible plague in certain regions. Many also are wholly
marine. Some of the larger forms attack fishes directly, and quickly
kill them by sucking their blood away; others are true parasites. On
the other hand the leeches of our lakes are fed on by the whitefish and
similar fishes. They are a great pest to our fresh-water turtles.
CHAPTER VIII
BUILDERS OF THE PEARLY SHELLS
The mollusks, or "shellfish" (phylum Mollusca) are a homogeneous
group of soft-bodied, unsegmented, typically bilateral, elaborately
organized animals, mainly aquatic and marine, whose origin--probably as
a derivative from a wormlike stock--is lost in the mists of geologic
prehistory. In most cases the mollusks secrete from a larval gland
an external shell which serves as skeleton and defensive armor; are
bisexual and produce eggs, or if monoecious are never self-fertilizing.
They possess a heart, and blood circulation (usually colorless);
breathe in the water by means of gills, or, in the air, by a primitive
kind of lung; have a nervous system and senses in some cases of a high
order; the organs are normally paired, and protected by a general
covering integument called the "mantle"; and the creeping species move
by a muscular, elastic, ventral organ styled the "foot," while the
swimmers are provided with a variety of swimming organs. Mollusks vary
in size from all but microscopic minuteness to a bivalve weighing 500
pounds or a squid half as big as a right whale. They occur in all seas
at all depths, abound in fresh waters both swift and stagnant, and are
scattered over the earth wherever vegetation flourishes.
The phylum Mollusca is divided into five classes, as follows, and it
will be noticed that four of the names refer to the locomotive organ
or "foot" (Greek _pous_, "foot"):
I. _Pelecypoda_, the Mussels--mollusks inclosed in a bivalve shell
fastened by a muscular hinge, the adjacent part of the valves being
generally more or less toothed; the foot is as a rule roughly
comparable to the shape of an ax head.
II. _Amphineura_, the Chitons--flattened, bisymmetrical mollusks
whose shell consists of eight crosswise, overlapping plates.
III. _Gastropoda_, Snails, whelks, etc.--mollusks that crawl on the
flat undersurface of the body, or distensible foot.
IV. _Scaphopoda_, Tusk shells--mollusks that possess a long tubular
shell open at both ends; with their small and elongated foot they
are supposed to _dig_ into the mud in which they live.
V. _Cephalopoda_, Cuttlefishes, and Octopods--mollusks with
tentaclelike "arms" arranged about the mouth, and either an external
or internal shell. These are the highest in rank.
THE OYSTER AND ITS RELATIVES
The lowest in rank of these classes is the Pelecypoda, containing
the "bivalves"--mussels, clams, oysters, and the like, in which the
shell is in two parts or valves hinged together over the "back" of
the animal, and attached to it on each side by a powerful muscle,
the "adductor," by the contraction of which the shell may be tightly
shut. Within the shell the body is enveloped in a "mantle," or fleshy
membrane falling like a cloak on each side; and from it is secreted
the outer shell, which grows by additions to its ventral margin. These
additions are in a general way annual, so that the concentric lines of
growth on its exterior are an indication of the years of the mollusk's
life, which is slow in growth, and long-lived. The interior of the
shell is usually pearly, and marked with microscopic rugosities, which,
by breaking up the light, as if by innumerable prisms, gives the
iridescence so beautiful in the pearl oyster, the fresh-water unios and
many others. These pearly layers are called "nacre."
Bivalves were formerly classified in conchology as Acephala, because
they have no proper head, but at the posterior end are two openings of
tubes, provided with cilia. In one, the cilia induce a constant current
of water which after leaving the gills brings into the animal's stomach
floating microscopic food, both plants and animals, including eggs and
larvæ, where it is captured and assimilated while water is ejected
through the other (dorsal) pipe. This food includes bacteria, and if
the mollusk lives and feeds in water polluted by sewage, or otherwise
containing germs of disease, it becomes dangerous as human food; hence
oysters and clams exposed to such bad conditions ought never to be sent
to market because of the disease germs remaining in them.
In bivalves such as the oysters, horse mussels, piddocks, and others
that are sedentary, and often fixed in place, or that, like river
mussels, scallops, etc., move about freely, the mouth tubes are short;
but many bivalves, as the clams, pinnas, razor fish and so forth,
bury themselves in the sand of the bottom, by means of the strong
distensible foot protruding from the forward end of the shell. These
are provided with a double-barreled tube, called the "siphon," which
may be contracted within the protection of the closed shell, or may
be stretched out several inches; the animal may thus sink its body
deep in the sand while its siphon reaches to the surface and inhales
food-bearing water. The little squirts of water often seen jetting
out of the beach at low tide as one walks along it are from clams
so buried, and which, alarmed by the vibration of one's footsteps,
hastily eject the water and withdraw their siphons.
The old name for this class, Lamellibranchiata, referred to the gills,
two of which, on each side, hang like curtains inside the mantle and
between it and the saclike body containing the viscera; when the shell
is open they are laved by the water, and extract from it, by some
quality hardly understood, the oxygen necessary to regenerate the blood
that flows through them; and, in addition, respiration is carried on
through the skin.
The nervous system is very primitive, and the sense organs consist of
an otocyst (a minute sac in which a hard particle floats in a liquid)
in the foot, by which, it is believed, a sense of direction is had,
and which also serves the purpose of an ear; an organ that tests the
water; and in some, as the scallop, rudiments of eyes are situated on
the margin of the mantle. Most pelecypods are of two sexes, but some,
such as our American oysters, are hermaphrodite. Eggs in vast number,
and a cloud of spermatozoa, are thrown out in midsummer, and a little
of the latter succeeds in reaching and so fertilizing fortunate eggs,
but almost all merely serve as food for the host of mollusks, worms,
sea anemones and what not that subsist on such provender. The few
fertilized larvæ drift about and happily escaping multiplied perils,
presently settle to the bottom to attach themselves to some fixed
object, or otherwise get a chance to grow big enough to defy ordinary
enemies. Some interesting variations in this rather commonplace larval
history occur, however, in certain families.
It will be possible to name only a few of the most useful or otherwise
conspicuous bivalves, beginning with the oyster, concerning which
an immense amount of detailed information is accessible to the
reader in the reports of the United States Government (Tenth Census,
and documents issued by the Fisheries authorities) and in those of
States, like Connecticut, New York, and Maryland, where oyster culture
is an extensive industry, said to be worth in the aggregate about
$20,000,000. The oyster of the eastern American coast is to be found
in the Gulf of St. Lawrence, but not in considerable numbers between
there and western Maine, whence it is present southward to the Gulf of
Mexico, except on the shifting sands of the outer beaches. It seeks
protected waters and a rocky or weedy bottom furnishing objects to
which it may, when young, attach itself, and later will not be torn
adrift by storms, for where an oyster establishes itself in infancy it
means to stay all its life. Hence the sheltered waters of Buzzards and
Narragansett Bays, Long Island Sound, and the lagoons and inlets that
lie behind the outer line of sandy beaches from Long Island to Florida
are the sources of our supply--especially Chesapeake Bay.
A full-grown oyster will produce about 9,000,000 eggs, each being about
one five-hundredth of an inch in diameter. When the little oyster
(spat) is about one-eighth inch wide shells begin to form on its
sides, and it settles to the bottom with its left side down, usually
where other oysters are; and hence extensive colonies, or "reefs," of
these mollusks form, and "rise on their dead selves" to a level where
they may be reached by the oysterman's rake. Many years ago, however,
it was discovered that large, marketable oysters were becoming very
scarce. Oystermen therefore sought favorable places, and raking the
natural beds transplanted their catch, little and big, to new ground,
where they were left to mature. This crude method was next improved
on by sowing thickly over the new ground, just before spawning time
in midsummer, a great quantity of empty oyster and other shells.
These were favorable to the catching of "spat," and would result in a
new bed that in about four years would furnish salable oysters; and
annual plantings produced, after a time, an annual crop. These are the
essential facts of oyster culture everywhere, although methods differ
somewhat in other parts of the world--in France, for example, fascines
of twigs are spread over tidal flats to catch the spat, instead of
shells.
Our eastern American oysters are undoubtedly the largest and finest for
the table of the many species that exist all round the globe. Those of
the Pacific coast of the United States are excellent, but small; and
the same is true of the European species; nor is the use of oysters
abroad so general and extensive as in the United States.
The pearl-bearing oysters are somewhat distant relatives of the edible
oyster (Ostræa), the thorny oysters (Spondylus), the hammer shell, the
windowglass shell (Placuna) and others. The pearl oyster of commerce is
named _Meleagrina margaritifera_ and is found in scattered localities
within the tropics on both continents. The chief fisheries are in the
Persian Gulf, around Ceylon, in Australia, among the Sulu Islands
and on the west coast of Panama. The Pearl Islands, south of Panama,
yielded to the early Spanish adventurers riches in gems that rivaled
those their competitors obtained from gold mines; but now they are a
field of small importance. In fact, the pearl fishery is carried on
now far less in hope of a profitable collection of gems than for the
profit in the shells, which have a nacreous interior of remarkable
beauty--the mother-of-pearl--and the great advantage of offering this
in almost flat surfaces, sometimes eight or nine inches broad, making
it useful in the arts as well as in the more practical line of buttons,
knife handles, etc. Sometimes the whole surface of a fine shell has
been carved, cameowise, with cunning art and an exquisite effect.
CHAPTER IX
BUILDERS OF THE PEARLY SHELLS--_Continued_
MUSSELS, SCALLOPS AND CHITONS
The familiar marine mussels of the family Mytilidæ will some day become
of great importance in this country as a food supply, as now they are
useful in resisting encroachment by the sea on certain parts of the
coast. They exist in vast numbers on both our coasts, and elsewhere in
the world, in two genera, Mytilus and Modiolus, which differ a little
in form, but not in habits. They have acquired the stationary habit,
and in place of a "foot" of serviceable size have developed a gland
that secretes an exceedingly tough, fibrous bunch of threads known as
a "byssus," by means of which the animal may not only attach itself
firmly to any sort of object, but may actually move about. The common
species of Modiolus, the "horse mussel," lives in great numbers north
of Cape Hatteras at and below the line of low water, and is much larger
than the edible mussel just described. A smaller species of Modiolus
is extremely numerous on the New England coast, and down to the
Carolinas, forming dense tangled beds on muddy patches as well as among
rocks, and serving to bind the mud and plants together and hold them
from disintegration by stormy waves, in spite of the thin and brittle
character of their shells. A southern species is bright yellow, with
dark rays; and the common modiola of the Pacific coast is dark, glossy
brown. Such mussels are eaten regularly in Europe, and come to us in a
pickled condition as a luxury. There is no reason why we should neglect
to add our own to our long list of sea foods.
The next useful mollusk to be considered is the scallop, one of
the many species of the family Pectinidæ, of which we eat only the
adductor muscle. The commercial species is _Pecten irradians_, the name
referring to the (nineteen) ridges that radiate from the flattened
hinges to the scalloped margin of the shell, which is prettily colored.
This species is common in sandy, shallow places from Cape Cod to
Florida, but the fishery is most productive about the eastern end of
Long Island and in Narragansett Bay. Farther north is a very much
larger species (_P. islandicus_) especially abundant on the Grand
Banks, off Newfoundland, where it forms an important food of the cod
and other fishes. It is well known to cooks, who use it in baking
their fish confections _en coquille_. A large number of other species
are distributed throughout the world, one (_P. jacobæus_), inhabiting
the Mediterranean having the name "pilgrim shell" in allusion to the
fact that in the days of medieval religious pilgrimages, those who had
visited the shrine of Saint James at Santiago de Compostela, Spain, to
pay homage on July 25, were accustomed to wear a scallop shell in their
hats in token of the fact--this mollusk being connected with traditions
of that saint.
Turning to the fresh-water mussels, or naids, as some books call
them, one is staggered to learn that more than 1,500 species have
been named, a large proportion of which belong to the United States,
which is peculiarly hospitable to them because of our many rivers and
lakes, together with the prevalence of limestone rocks, whose constant
dissolution in water supplies the store of calcareous matter that these
thick-shelled mollusks require. All belong to the family Unionidæ, in
which two divisions are noted--one (Anodon) in which the mussel has a
comparatively elongated thin shell with no "teeth" in the hinges; and
the other (Unio) in which the shell is thick, various in shape from
an oval to a triangle, and has prominent umbones, beneath which the
valves (which are always alike) are hinged together by interlocking
teeth embedded in a somewhat elastic gristle. The interior of all these
unios is richly nacreous, and consequently pearls are produced in the
same way as in the marine pearl-bearing shells; and some of the finest
known gems have been derived from them, in this country and abroad, as
well as innumerable specimens of moderate value. These mollusks like
clear streams or lakes with a sandy bottom, and are not to be looked
for in stagnant weedy waters. They keep an erect position, the nibs of
the shell half buried in the sand, and move slowly about, plowing a
path and dragging themselves along by means of the powerful foot, but
keeping the short siphons at the other (or longer) end of the shell
well above the mud.
We come next to our market clams. These are of two distinct
kinds--"hard" and "soft," or quahog and long clam, as they are
distinctively called. The quahog is a thick-shelled, roundish mollusk
with a distinctly heart-shaped outline when looked at endwise. It
dwells in fairly deep water, standing on its nibs half buried in the
sand, like a wedge, and moving slowly about. Young ones become the
"little necks" of our summer tables.
The soft clam belongs to a different race. Its elongated shell is thin
and chalky, is loosely hinged, and gapes widely at both ends, and
although it is used much as food, especially in chowders, it is by no
means as good as the hard clam. Its principal value, indeed, is as
bait in the cod fisheries, and for this purpose enormous quantities
are gathered. It lives in, rather than on, muddy beaches, sometimes
in crowds of thousands, its shell deeply buried, and its long siphons
reaching up to suck in water and food when the tide covers the flat.
When the tide is out, a tiny hole in the sand and a spurt of water show
the clammer where to dig, and his spade quickly unearths the clam.
The second class, Amphineura, contains the chitons and their relatives.
These chitons are flattened mollusks protected by an armature of eight
crosswise plates, overlapping like shingles, which creep about the
rocks close to shore, and when lifted curl up like sowbugs. The most
interesting thing about the chitons is the fact that they are provided
with excellent visual organs, "the whole dorsal surface of some forms
being studded with eyes, of which not less than 8,000 occasionally
exist on a single specimen." Many of them are complete, with cornea,
lens, and a pigment layer within the iris.
SNAILS AS TYPES OF GASTROPODS
The gastropods (Gastropoda), including the snails and slugs, limpets,
whelks, periwinkles, sea hares and the like, are Mollusca having the
mantle completely enveloping the body, and the shell, when present, in
a single piece, and usually in spiral form. There is a well-developed
ventral foot, on which the animal creeps, and in front of it a distinct
head bearing eyes and tentacles. These organs retain their normal
bilaterality, but the body is, as a rule, inequilateral. The cause
of this is the fact that on the animal's back is developed from the
first a shell, which, with its contents, amounts to a relatively
large weight, and it naturally falls over to one side. The mouth is
armed with a flat, distensible, ribbon-like organ, studded with rows
of chitinous teeth, that serves as a rasp and a boring instrument,
and which is called an odontophore, or, in snails, a radula. Most
gastropods are carnivorous.
The lowest in rank are the shell-less, or "naked" gastropods known as
"sea slugs," "sea hares," and so forth. One Mediterranean species of
Aplysia secretes a purple liquid utilized by the ancients as a dye, and
this is still sought for in Portugal, where storms sometimes cast vast
quantities of the mollusk on the beaches.
We come now to the great group of mollusks inhabiting fresh waters
and dry land--the snails, whose group name is "pulmonates," that is,
possessors of lungs, and breathing air. On the generally accepted
theory that all these are descended from marine ancestors, and have
gradually acquired the faculty of living on land, it would be natural
to look for a series of mollusks that were amphibious, and, as it were,
half-way fitted for a terrestrial existence, and such intermediates
exist in all parts of the world. The little black Melampus, which
covers the mud of tide flats on both the Atlantic and Pacific coasts
in tens of thousands, and seems just as happy when the tide is out as
when it is in, or when it is simply refreshed by the spray, is a good
example. A near relative, Carychium, is still more emancipated from the
sea.
First among these pulmonates are those common in ponds and still
streams the world over, of the family Limneidæ, called limneids or pond
snails. They are in various forms. Some are limpet-shaped (Ancylus),
some are flatly coiled (Planorbis), but most of them have shells drawn
out into a graceful spiral; in all cases the shell is not composed of
lime, but of the thin, fragile, horny substance "chitin." The best
known one is _Limnea stagnalis_, which sometimes reaches a length of
two inches, and inhabits almost every quiet piece of water in North
America, and in Europe and all Asia except India and China.
These water snails of our ponds and ditches are exclusively vegetable
feeders, and must come to the surface at frequent intervals to breathe,
letting out a bubble of vitiated air, and taking in a fresh supply.
Should the pond dry up in summer the limneids burrow down into the mud,
and remain in that heat trance called æstivation until the autumnal
rains refill the basin and let them come forth. The small kinds
called "physas," exceedingly common everywhere in this country and
Europe, differ from Limnea in having the shell partly enveloped in the
turned-up fringed edges of the mantle, and by being coiled from right
to left instead of clockwise. This reversal occasionally occurs in
individuals of all gastropods, which are then said to be "sinistral,"
as opposed to the normal "dextral" coiling; but in the physas it is the
rule.
Next come the wholly terrestrial pulmonates--snails and slugs,
distinguished from the pond snails, which have only one pair of
tentacles at the bases of which the eyes are embedded in the skin,
by having two pairs of "horns," one of which carries the eyes on
their tips--good eyes, which may be quickly withdrawn out of harm's
way by inversion of the tubular stalks. The thick, extensible foot is
surmounted by a body coiled within the shell; and this foot secretes a
viscid fluid that lubricates the creature's path, and often leaves a
silvery trail.
Snails are mainly vegetarians. The mouth lies just under the front
tentacles, and its upper lip is armed with a horny, crescentic "jaw."
Within the mouth is the lingual ribbon, which may be brought up
against the cutting edge of the jaw. This tongue is studded with rows
of infinitesimal, flinty teeth, the radula of our big white-lipped
snail, a quarter of an inch long, furnishing room for 11,000 of these
denticles; and as all of them point backward the tongue easily seizes
and draws into the mouth whatever the jaw nips off. Substantially
the same sort of "tongue" is possessed by all the gastropods, but
the arrangement and shape of the microscopic denticles is different
in every species, and this is one of the "characters" used in
classification. With it the carnivorous rasp away their food; and by
bending it double and using it as a gimlet bandits like Nassa, the
oyster pest, drill through other shells and devour the occupant. You
may pick up on any seabeach scores of examples of the work of these
borers. In Europe some kinds of slugs and snails do great damage in
gardens, but we have little to complain of in this respect.
Largely dependent on moisture, the young snails that are hatched in
midsummer at once seek retreats, and may be looked for under leaves,
logs, and loose stones in the woods and pastures. Most American
snails are solitary, and will be found lurking in the moss beside
mountain brooklets--a favorite spot for the glassy vitrinas--hiding
in the crevices of rocky banks and old walls, crawling at the edge
of swampy pools, creeping in and out of the crannies of bark on aged
trees, or clinging to the underside of succulent leaves. Some forms,
very beautiful in their ornamentation when magnified, are so minute
that they might be encircled by the letter o in this type, yet you
will soon come to perceive them amid the grains of mud adhering to the
undersurface of a soaked chip or rotten log.
For fresh-water species various resorts are to be searched. Go to
the torrents with rocky bottoms for the paludinas and periwinkles
(Melania); to quiet brooks for physas and coil shells (Planorbis); for
limneas to the reeking swamps and weedy ponds. By pulling up the weeds
gently, you may get small species that otherwise easily escape your
dipper or net. In the Southern States and in the tropics certain forms
are to be picked off bushes and mangrove trees like fruit, especially
the round "apple snails" (Ampullaria) as big as your fist.
SEA SHELLS IN NATURE AND ART
Other familiar forms of gastropods are the limpets, keyhole and
half-deck; the abalones, so much used in the making of ornaments; and
the many small sorts of "periwinkles" studding the rocks and hiding
among the seaweeds of every coast. Then there are the pyramidal top
shells (Trochus), the bulging, wide-mouthed turbans (Turbo), and the
open-whorled wentletraps (Scalaria) which years ago were so rare that
collectors paid $100 or more for a good specimen. The two former
kinds are on sale in all seaside shops, with the natural rough brown
exterior ground away until they gleam outside in the prismatic glory
of the nacre layers that lie underneath. A group of heavy shells of
carnivorous tropical mollusks furnishes ornaments for the mantelshelf
also. These include the knobby volutes, often richly colored in
marbled patterns or in spiral rows of round spots; the olives, whose
ovate shells are sometimes dark purple, sometimes beautifully marked,
and always glossy, because enfolded during life inside flaps of the
mantle that completely protect them; the miters, that take their name
from their resemblance in shape to the headdress of a bishop, and
show splendid decorations in tints of red and orange; and the strong,
spiny murexes, a small Mediterranean species, which is the principal
source from which the ancients derived their Tyrian purple dye--a
coloring matter yielded by treatment of the blood of many species,
including one of the commonest little mollusks (Purpura) on our own
coast, which old-fashioned New Englanders yet utilize sometimes for
making an indelible ink for marking clothing. To this family belong the
"drills" that destroy thousands of dollars worth of oysters annually
in Long Island Sound by boring through them. Near relatives are the
whelks (Buccinum), extensively eaten in England; and two of the largest
and commonest shells on our eastern sand beaches, known to northern
fishermen as "winkles" and along the southern coast as "conchs."
These (Fulgur and Sycotyphus) are big, pear-shaped creatures with
chalky white shells that crawl about near shore, seizing and devouring
anything they can overcome, and working havoc on planted oyster beds;
they deposit their eggs in parchmentlike capsules shaped like gun wads
and connected into a long chain that are often thrown up on the beach,
where they are called sea necklaces.
Of great beauty in their rich variety of color and pattern are the
tropical cone shells, of which a large number of species are known,
some so rare as to bring great prices in the conchological market.
Their bite is poisonous. Equally numerous in species are the charmingly
decorated auger shells, some (Pleurotoma) spindle-shaped, others
(Terebra) that would serve as models for a church spire. Near them is
classified that white mollusk (Natica) whose globular shell is perhaps
the commonest relic of the sea seen on our northern beaches, and
sometimes is as large as a man's fist; to it belong the curious "sand
saucers" to be found in August, which contain its eggs. These naticas
are predatory, and burrowing their way through the loose sand come upon
and devour other shellfish, boring a circular, nicely countersunk hole
through their armor and feeding on its inmate; their depredations on
the northern oyster beds are a serious matter.
Well known and always admired are the cowries, smooth, brightly colored
shells, shaped like an olive with a gash down the length of one side.
This long and narrow aperture is usually toothed, and it is only in the
young that any indication of a typical spiral growth is discernible.
The money cowrie of Africa is small and cream-white.
Lastly a word must be said about the largest of known gastropods, the
big "conchs" or wing shells (Strombus), the helmet shells (Cassis),
and the tuns (Dolium). They are West Indian. The species most commonly
seen in the United States, forming a border for flower beds in seaside
villages, is _Strombus gigas_, with a delicate orange-red or pink
interior, from which are cut most of the shell cameos offered to art
lovers. This shell, like the great spiral triton of the South Seas, is
also converted into a horn much used in foggy weather by the spongers
and small coasters of Floridian and West Indian waters. The helmet
shell, a heavy, rounder and smoother mollusk than the Strombus, is also
extensively used in cameo cutting, especially the African black helmet,
in which a white outer layer covers an almost black underlayer on the
broad lip. Dolium has a large, globose but thin shell, ornamented with
revolving ribs.
The class Scaphopoda is composed of a single family (Dentalidæ) known
as tusk shells, because the little shells, one to two inches long, are
shaped like an elephant's tusk, open at both ends. The structure of the
occupant is so singular, the animal lacking head, heart, gills, and
some other ordinary features, that naturalists believe it is a hopeless
degenerate. One of the species of the Pacific coast is famous as the
shell strung as ornaments and serving practically as money among the
northwestern Indians until very recent times, under the name "hiqua."
NAUTILUS, DEVILFISH, AND SQUID
We have now arrived at the last and highest division of the
Mollusca--the Cephalopoda, the class of the nautilus, ammonite, and
other fossil forms, and of the squid, cuttles, and octopuses of our
modern seas. The cephalopods are very different in shape, activity, and
in their higher organization and intelligence, from other mollusks,
but their general anatomy is the same. The special characteristic,
as indicated by the name, is the fact that the head is surrounded by
tentaclelike extensions of the "foot," which is here fused in part
with the head, and divided into the long "foot arms," which are the
instruments by which these predatory creatures obtain their prey. The
underpart of the foot forms a tube called the funnel (or siphon).
Through the funnel the animal expels water from the mantle cavity, and
thus propels itself through the water. When the siphon is in its normal
position the animal swims backward; but it can be turned back over
the edge of the mantle, giving a forward movement. In cephalopods the
sexes are separate, the male being often much smaller than the female.
The eggs are usually laid in gelatinous capsules, commonly known in
New England as "sea grapes," and the development is direct, that is,
without any free-swimming larval stage.
The class is divided into two subclasses: 1. _Tetrabranchiata_,
cephalopods with four plumelike gills inside the mantle; and 2.
_Dibranchiata_, with only two such gills. In the first subclass belong
all those very ancient cephalopods called in a general way ammonites,
goniatites, orthoceratites, etc., that are found in such great numbers
and astonishing variety in the Paleozoic rocks, from the Ordovician
age onward, although but few groups survived beyond the Carboniferous
period, and only two families can be traced as high as the Tertiary
deposits, one of which--that of the nautilus--survives to the present
day as the final remnant of one of the conspicuous and interesting
populations of the primitive ocean.
The pearly or chambered nautilus is one of several species inhabiting
the East Indies and the coral region of the South Pacific seas,
creeping along the bottom in deep water, most numerously at the
depth of about 1,000 feet. Hence the animal is not often taken
alive, although the smoothly coiled and handsome shells are cast
on the beaches in great numbers; and little is known of its habits
or embryology. It is a soft lumpish sort of creature, with a great
number of short arms and tentacles around the mouth, none armed with
suckers. It begins life as a mere globule covered by a minute hood of
shell; but presently, growing too large for this hood, it enlarges it
by additions to the rim, and then forms behind its body a partition
(septum) across the shell, cutting off the part in which it was born.
As growth advances, this enlarging and partitioning continues until the
nautilus has attained its full size. Then, as before, it occupies only
the outermost chamber, behind which the whole interior of the shell is
divided by the septa into chambers, abandoned and empty, but filled
with a gas that buoys it up in the water. Oriental artists are fond of
grinding away the dull exterior of the shell and exposing the gleaming
nacre underneath; and of carving in this mother-of-pearl picturesque
designs, examples of which are often to be seen in curiosity shops.
This is not only the last remnant of the great group of ancient
nautiloids, but one of the smallest, for some of the Paleozoic coiled
forms were as big as a washtub, and the straight ones were often six
feet long.
The Dibranchiata, on the other hand, are comparatively modern, as their
ancestry dates back only to the Trias, and our seas still harbor a long
list of living representatives. This subclass has two divisions: 1.
Octopoda--octopods, the eightarmed argonaut and other octopuses; and
2. Decapoda--decapods, the ten-armed cuttlefishes, or calamaries, and
the squids.
[Illustration: THE CHAMBERED NAUTILUS--SECTION
_H_, Head. _T_, Tentacles. _E_, Eye. _M_, Muscles. _S_, Shell. _A_,
Air Chambers]
The octopods have a saclike body with eight arms of about equal size,
in some kinds thick and short, in others long and snaky. Every arm has
along its underside a double row of round, muscular suckers without
horny rims; and whatever is seized by one or more of these arms is
drawn into the mouth at their base, where it is bitten by a beaklike
jaw of sharp horn, and further devoured by means of a toothed tongue
similar to the radula of gastropods. Nearly all are tropical, but some
species exist in deep water considerably to the northward. Certain
species are used as food in many parts of the world, and are considered
a delicacy in Italy and other Mediterranean countries. The fishermen
of Japan and the Philippines capture them by the simple process of
lowering big earthen urns and leaving them on the bottom overnight;
when they are hauled up in the morning many will contain entrapped
devilfish, as sailors call them, which at once go to market.
A very singular octopod is the little argonaut, or "paper sailor." Its
body is not larger than a walnut--that is the body of the female, for
the male is only a tenth of that bigness. Its home is mainly in the
tropics and in deep water, but in the summer spawning season it rises
to the surface, and is occasionally met with far northward on the Gulf
Stream, drifting, apparently, in a snug little boat. The two dorsal
arms are expanded into broad, roundish membranes at their ends, and old
stories said that they were used as sails--a supposition of much use to
poets; but the "boat," shaped somewhat like the shell of the nautilus,
is not a shell proper, but a membranous pouch secreted by the mantle in
spawning time, and not vitally attached to the body, but held in place
beneath it by the two broadened arms, and serving as a receptacle for
eggs and a cradle for the embryos hatching from them.
Turning now to the Decapoda, we treat of things much nearer home and
familiar on both sides of the continent, for these are the cuttlefish
and squids, none of which have an external shell, but possess an
interior brace to their muscles either of lime or of chitin. The
cuttlefish proper, or calamaries, are those of the family Sepiadæ,
which have an oval, flattened body bordered by a fin; and two of the
ten arms are, in the female, in the form of long, slender tentacles.
In addition to being edible and easy to get, as they stay near shore,
their calcareous back brace is the "cuttlebone" fed to cage birds; and
they furnish the substance from which the drawing ink called "sepia"
is made--principally in Rome. This is a brownish black liquid that the
animal jets out through its siphon when it thinks itself in danger in
order to make an inky cloud in the water behind which, as a sort of
smoke screen, it may run and hide. Other cephalopods use this means of
escape.
The squids, however, are all elongated in shape, and have finlike
expansions of the mantle only on the tail. Two of their arms are long
and slender, and are broadened at the tips, and studded with suckers.
These suckers in some squids are strengthened by a horny rim, or by
recurved hooks, or by both. The eyes are large, perfectly formed,
and as serviceable as those of the fishes on which they prey. These,
and some other animals, including small ones of their own kind, they
capture by darting backward, swinging quickly to one side and seizing
the victim in their sucker-bearing arms. They themselves are devoured
by whales, seals, and many kinds of fishes; and enormous quantities
of squids of various species are annually collected by fishermen for
use as bait in the Newfoundland fisheries. In place of the calcareous
cuttlebone of the sepia the squids have their bodies stiffened by an
internal strip of chitinous substance called the "pen."
Squids are of all sizes from an inch to twelve feet in length; then
there is a surprising jump to the giants (Architeuthis) of the North
Atlantic, which, when the tentacles are stretched out in front, may
measure seventy-five feet from tip to tail. These are little different
in structure or habits from their smaller brethren that exist in so
many species near all coasts and throughout the midseas right around
the globe; but their huge size makes them fit antagonists of the sperm
whale, which hunts them, and whose hide often bears a record, left by
their powerful suckers, to show how hardly some big squid struggled
for life. These monsters are the greatest invertebrates known in
present or past time; and it is probable that the long wriggling arms
of one and another, glimpsed at the surface, may account for some of
the sea serpent stories brought home by apparently perfectly honest
sailors, especially those which in many cases recount that the supposed
"serpent" was in conflict with a whale. Carcasses of these gigantic
squids are occasionally cast on the shores of Labrador and Greenland.
CHAPTER X
ANIMALS WITH JOINTED FRAMES
The phylum Arthropoda embraces an immense assemblage of small animals,
inhabiting salt and fresh waters, the land, and the air above it.
The typical members of this group have a body divided into segments,
jointed limbs, some of which are modified into jaws, and a more or
less firm external skeleton. The general organization is complex,
with the nervous system and senses well developed, in some divisions
showing powers of perception and brainwork of a very high order. The
chief divisions, or classes, of the Arthropoda are given below in the
order of rank, from those simplest in organization to the most complex.
Members of the first three classes breathe by gills, and are termed
Branchiata, the remainder are air breathers or Tracheata.
_Crustacea_--Crabs, lobsters, shrimps, barnacles, beach fleas.
_Trilobita_--Trilobites; eurypterids (fossil only).
_Xiphosura_--Horseshoe crabs.
_Onychophora_--Peripatus.
_Myriapoda_--Centipedes; millipedes.
_Arachnoidea_--Spiders, mites, ticks, scorpions.
_Insecta_--Insects.
As several of these classes contain many subdivisions, and thousands
or even tens of thousands of species, all that is possible is to give
the reader such an account of each important group, as will enable him
to assign to their proper place such arthropods as he may encounter in
his rambles, or in his reading, and to learn something of the manner of
life in the various groups.
CRABS AND THEIR SMALL RELATIVES
"Everyone," says Dr. Calman, "has some acquaintance with the animals
that are grouped by naturalists under the name Crustacea. The edible
crabs, lobsters, prawns, and shrimps are at least superficially
familiar, either as brought to the table, or as displayed in the
fishmonger's.... Many, however, will be surprised that the barnacles
coating the rocks on the seashore, the sand hoppers of the beach, and
the wood lice of our gardens, are members of the same class. Still
less is it suspected that the living species of the group number many
thousands, presenting strange diversities of structure and habit, and
playing an important part in the general economy of nature."
The great majority of crustaceans are aquatic animals, breathing by
gills or by the general surface of the body, having two pairs of
"feelers," or antennæ, on the front part of the head, and at least
three pairs of jaws. Most crustaceans are hatched from eggs, usually
in a form very different from their parents; and they reach the adult
state only after passing through a series of transformations quite
as remarkable as those that a caterpillar undergoes in becoming a
butterfly. All crustaceans, except a few much modified land forms,
breathe by means of feathery or platelike gills which are always
an appendage of the legs, where they appear as one or more lobes.
Colorless blood propelled by the heart wanders into spaces in these
lobes, and there lies separated from the water by a mere film
of tissue, through which oxygen is absorbed from the water. Most
crustaceans are covered, at least in part, by some sort of shelly coat
composed of a combination of the horny substance "chitin" with lime,
which reaches its highest state in the big lobsters and crabs. This not
only protects and gives support to the internal organs, but also to the
muscles by which the animal moves. In other words it plays the part of
a skeleton. As it does not increase in size after it is once formed,
and cannot stretch much, the crab must cast its shell at intervals
as it grows. The new covering, which had been formed underneath the
old, before molting, is at first quite soft, and the animal rapidly
increases in size owing to the absorption of water. The shell then
gradually hardens by the deposition of lime salt.
The reader who may not hitherto have understood the difference between
"hard" and "soft-shelled" crabs is now instructed; and it is observable
that the figurative expression "a hard-shell," when applied to a man,
signifies that he must undergo a complete change before his ideas will
be enlarged.
The simplest of the crustaceans are those small creatures of the
subclass Branchiopoda (gill-footed) that swarm in our waters, both
salt and fresh. Lakes, ponds and ditches abound in a variety of minute
or even microscopic species that, in gathering food from equally
small bits of dead organic matter, as well as from living plants and
animalcules, perform an important service as scavengers--a service,
in fact, performed by all crustaceans in a greater degree than by any
other single group of animals. They also furnish the basis of food
for the whole body of aquatic life, since it is upon these minute
crustaceans that fish fry, tadpoles, insect larvæ, caddis flies, and
so on, must mainly depend. One of them is Daphnia, familiar to keepers
of aquariums. Another is Cyclops, a favorite with microscopists and
abundant in stagnant ponds, which is a member of the group called
copepods that form an important part of the oceanic plankton, where
they are the chief consumers of the minute algæ; but they also occur
at all depths. In arctic waters the copepods are so abundant that
they form the principal part of the food of certain fishes and of the
whalebone whales. These, and their minute relatives, the ostracods,
produce a large part of the phosphorescence of the sea, and some of
them exhibit bright colors.
All these are free swimmers, but nearly related to them are the
barnacles (Cirripedia) whose larvæ float about for a time near shore,
and then settle down and attach themselves by their hinder parts to a
rock or some other support, and begin to secrete an armature of limy
overlapping plates that forms a strong cup in which they sit, often in
a crowd that whitens a big rock. When the tide is low these sessile
"acorn shells" are tightly closed, but when the water returns, bringing
its load of invisible food, the animal stands up, as it were, and
thrusting out its feathery legs sweeps the water to capture a meal--a
beautiful sight to watch. The relation of the plates in the barnacle's
cup to those in the coat of the higher Crustacea is more easily seen in
the more pelagic "goose barnacle," whose hinder part is extended into a
tough, flexible stalk, while the fore part is covered by plates. This
kind is fond of attaching itself to floating timber, to ships' bottoms,
or even to the surface of whales, and thus floats or is carried
all over the watery globe. To it belongs the ridiculous myth of the
barnacle geese.
Great numbers of crustaceans of more advanced types live in the open
sea, and at all depths; and many of them assume extraordinary shapes.
The space between tide marks, and the mud of salt marshes and tidal
creeks abound in a wide variety of species, some of which are familiar
to everyone who lives at or visits the seashore. Thus the sand and rows
of drifted seaweed on all our eastern beaches are likely to harbor
flocks of amphipods, well called "sea fleas" or "sand hoppers," which
sometimes jump away before you in hundreds as you walk along.
Here, too, are to be found the pretty, burrowing "mole crabs," or
"ivory crabs," so called from their shining white jackets; and a host
of other species with strange forms and habits haunt the margins of
tropical and Oriental seas. All these are bandits, preying on whatever
they can catch, and between times guarding themselves from capture
by fishes, bigger crabs, and other enemies, by lying in mud burrows,
to the bottom of which they are quick to retreat. The big arm of the
fiddler crab, held across its face, closes its burrow like a door. One
sort, the hermit crab, has all its hinder parts naked, and so backs
into an empty snail shell, curling its taillike soft abdomen around the
central column of the shell and so dragging it about with it, with its
armored head and thorax sticking out of the mouth of the shell. As it
grows it becomes too large for its first shell, and from time to time
must leave it and find a larger tenement in which to ensconce itself--a
perilous transfer. Let me quote some notes I made on a New England
shore to give a picture of crustacean life there in summer.
"The lady crabs were plentiful, always alert, and inclined to be
pugnacious at our intrusion. The first one I met instantly rose upright
at the surface of the water, and when I made an advance it sprang
half way out of the water and cracked its pincer claws together as if
supposing it would reach, or at any rate frighten me. Perhaps it was my
shadow it clutched at so viciously. If so, the crab probably concluded
its huge antagonist to be an intangible ghost upon which the most
powerful claws could have no effect, for an instant later it _backed
down_--literally and swiftly--to the bottom, and in a twinkling had
wriggled tailwise into the mud and out of sight. When with my shovel
I routed madame out of that retreat, she indignantly scuttled off
too briskly to be followed, and will have great tales to tell of her
adventure.
"The stone and fiddler crabs were as common and comical as usual;
and I made the acquaintance of a new one called Gebia, which was a
small, semi-transparent, bluish white, washed-out, bloodless specimen,
shaped somewhat like a crawfish and carrying bunches of roe beneath
its abdomen. It looked like a miniature lobster made of glass and
filled with milk. Then in the eelgrass there was a funny isopod, called
Caprella. It was half an inch or so long, and clung by its hinder feet
to the grass, waving its body up and down in search of minute prey.
Other isopods and amphipods were exposed by turning over stones or
digging in the sand at the edge of the water--small, pale, shapeless
crustacea, which are flattened laterally so that they must lie on
their sides, and when uncovered will kick about with feet and tail in
laughable anxiety to get under something. Under the stones we found
the tubes made by a certain species; and when we captured the active
little architect and put him in a bucket of clean water, he instantly
began to gather grains of sand and stone and to join them together Into
a shield under which he might hide. We found that these grains were
joined together by spiderlike threads, which the amphipod spins from
two pairs of small legs under the middle of his body, secreting a fluid
that hardens in the water. Another (Hippa) about the size and shape of
a robin's egg, but with a thin shell of mother-of-pearl (so to speak),
gave us great amusement by its extraordinary celerity in burrowing, so
that we could hardly seize it before it had squirmed down out of reach
into the wet sand."
The edible crabs (Cancer) live in the shallow region just below ebb
tide, for they cannot endure exposure to air as well as other species,
and live by scavenging. The lobsters are inhabitants of still deeper
water, especially where it is somewhat rocky, and devour more carrion
than living fish. That miniature of the lobster, the fresh-water
crawfish, which is also edible, dwells in deep burrows in wet
lands--burrows that are really wells half filled with water. Various
species of these and other edible forms of Crustacea are found all over
the world.
MILLIPEDES AND CENTIPEDES
The myriapods (class Myriapoda) are those unpleasant creatures more
commonly known as centipedes, millipedes, or thousand-legged worms.
They have a wormlike form, with the body divided into segments, a
distinct head with antennæ, jaws and several single eyes, and a
varying number of air tubes, or tracheæ; two sexes exist, and eggs are
laid in the ground within cases formed by the mother of pellets of mud.
They vary in size from an almost invisible minuteness to a length in
some tropical species of six or more inches. The centipedes (Chilopoda)
are those flattened forms so often seen in and about rotting wood and
vegetation or in moist ground, their bodies looking like a chain of
plates joined together by flexible skin, each section having a single
pair of legs, usually very short, but in one sort (Cermatia) each leg
is longer than the body, and the hinder pair twice as long, matched by
two very long feelers. Most of them are predacious, feeding on anything
they can catch, and their strong jaws exude poison. The larger ones may
inflict a very painful bite if incautiously handled.
[Illustration: GIANT CENTIPEDE
(_Scolopendra gigas_)]
Another group, the Diploda, are known as galley worms, or millipedes,
and have two pairs of bristlelike legs on each segment. Here the
body is as round as that of an earthworm, and is incased in a hard,
chitinous shell, usually red-brown in color; and when disturbed they
coil up and emit an acrid, unpleasant odor as a defense.
WEAVERS OF SILKEN TRAPS
The class Arachnida, which contains the scorpions, spiders, mites
and their allies, connects the Crustacea with the Insects; and
some naturalists include within it the eurypterids and king crabs,
classified in this book with the Crustacea. All live on land and
breathe air except a small group of allies (Pycnogonida) which are
marine, and may be found on the rocks, and clinging to wharf piles,
etc., on our coasts as well as elsewhere; they appear to be all legs,
and are known to New England fishermen as "no-body crabs." The class
includes seven orders, the lowest in rank of which is that of the
scorpions (Scorpionida).
Scorpions are inhabitants of warm countries, and some tropical American
species are six inches in length, but those of our Southern States
are smaller. They have slender bodies consisting of a cephalothorax
and a long abdomen ending in a sharp sting through which two poison
glands inject poison into the wound made by it, the effect of which
may be very severe on a man, and is fatal to the insects and other
small creatures on which scorpions prey; this "tail" with the sting
is usually carried curled up over the back. The body is protected by
chitinous plates above and below. The legs are four. From the head
spring two great, crablike, pincer claws. When these seize an insect
they hand it back to two small but powerful appendages at their base
which act as jaws. Between them is a small mouth. Scorpions are
nocturnal in habit, hiding by day in crevices, and wandering about at
night; thus they are likely to seek such dark retreats toward morning
as a person's boots; and in hot, dry regions travelers must be cautious
about examining their clothing and baggage to avoid being stung. The
scorpions retain their eggs until hatched. The young when born differ
little except in size from their parents, and are cared for with much
solicitude by the mother, who carries them around with her for some
time, hanging by their pincers to her body. The race is ancient, fossil
remains occurring as early, at least, as the Carboniferous age.
The second order, Pseudoscorpionida, includes the "book scorpions,"
a series of minute, stingless, scorpion-shaped creatures found in
moss, under the bark of trees, or more often on flies. A third order,
Pedipalpida, is that of the scorpion spiders, or "whip scorpions" of
the tropics; the fourth, Solpugida, contains certain ugly creatures
intermediate between scorpions and spiders; and the fifth order,
Phalangida, is that of the small-bodied, vastly long-legged things
called "harvestmen" in England and daddy longlegs by us, which run
about in the summer heat, and feed on minute insects. They abound in
all the warmer parts of the world, and in great variety, South America
showing some very bizarre forms. This brings us to the sixth order,
Araneida--the spiders.
THE SPIDERS AND THEIR WEBS
Spiders are usually thought and spoken of as "insects" by the layman.
Many persons call almost every creature an insect that is small and
supposed to be useless, or suspected of harmfulness. But spiders
are different from insects properly so called in many important
particulars of structure and habits. Spiders have four pairs of legs,
while insects have six legs. The spherical abdomen, which is cut off
from the head by a deep constriction, shows no segmentation, and on its
floor are large glands (the arachnidium) producing the silk which is
exuded from three pairs of tubes with sievelike openings, at the end
of the abdomen, called the spinnerets. Their nervous system is highly
developed, and they show much intelligence. Spiders are of two sexes,
but the male is usually much smaller than his mate.
When egg-laying time comes the female forms a little silken bed
attached to grass, or underneath a stone, or stuck to some object,
or placed in a burrow, or hung like a hammock by long guy lines, and
deposits in it eggs like drops of jelly. One sort places this under
water, forming a nest like an inverted cup and filling it with bubbles
of air, and spending much of its time in this real diving bell. A
common garden spider (Lycosa) forms globular cocoons, and drags them
around attached to the spinnerets, regardless of jars and bumps. In
a large section of the tribe this is all the use that is made of the
silk, which differs from that of insects (caterpillars) in being made
up of a great number of finer threads laid together while soft enough
to unite into one.
It is a common habit with spiders to draw out a thread behind them as
they walk, and in this way they make the great quantities of threads
that sometimes cover a field of grass. This is the gossamer often so
annoying to us in late summer, but a thing of beauty when glistening
with dew.
The gossamer of autumn, however, is made by the very small spiders of
the genus Erigones, which hide in the herbage, but in the fine weather
that comes after the first frosts climb to the tops of posts, fences
and tall weeds, in company with the young of larger kinds, and "turning
their spinnerets upward allow threads to be drawn out by ascending
currents of air, until sometimes the spiders are lifted off their feet
and carried long distances." These are the "ballooning spiders" of
which one hears. In this way the whole country is overspread with lines
and tangles of trailing silken threads that whiten our clothes and
stick to our faces.
Three or four hundred species of spiders might be obtained in almost
any locality in this country by diligent search, and thousands of
foreign species are known; hence only a few conspicuous examples may be
mentioned here. The tribe may be divided according to habits into two
groups of families: 1. The hunting spiders, which run on the ground or
on plants, catching insects by chase or by strategy; and 2. The cobweb
spiders, which make webs to catch insects, and live all the time in the
web or in a nest near it.
In the former group are the Drassidæ, a family of small, varicolored
spiders that run about on the ground or in bushes, one large genus of
which (Clubiana) includes pale, or purely white species; their cocoons
are baglike or tubular. The most conspicuous genus is Misumena, in
which the species are white or brightly colored, and which spend their
days among flowers, waiting in rigid attitude for an insect to alight
near them on which they may pounce. Spiders can see well for four or
five inches, but not much beyond that. The Attidæ are small, hairy, or
scaly jumping spiders, often brightly colored, that are found in open
places and on the tops of low plants, whence they leap on their prey,
or make long jumps to escape danger. To the next family, Lycosidæ,
belong the large spiders most often seen in fields and pastures. They
are fond of dry, sandy places, where the females live in silk-lined
holes. These lycosids are long-legged, rapid runners, and capture
their game by running it down. To this family belongs the famous
tarantula of southern Europe, fabled to produce a madness (tarantism)
in a person bitten that could be cured only by dancing to music of a
certain lively measure called "tarantella." (The so-called "tarantula"
of our southwestern desert region, is, however, another species.) A
common northern spider (_Lycosa carolinensis_) is its equal in size,
(the longest legs covering a spread of three inches), and in color,
black with gray legs. Still larger is another North American lycosid
(_Dolomedes tenebrosus_), gray with spiny legs ringed with dark and
light gray, which spreads four inches.
These big ugly creatures, and the bites of spiders generally, are
regarded with unnecessary dread by most persons. The jaws (mandibles)
are close together at the front of the head. They are two-jointed, the
basal joint stout, and the end joint or claw slender and sharp-pointed.
The claw has near its point a small hole, which is the outlet of
the poison gland. "The poison kills or disables the insects which
are captured by the spider. Its effect on the human skin varies in
different persons. Sometimes it has no effect at all; oftener it causes
some soreness and itching ... and cases have been known in which it
caused serious inflammation which lasted a long time. Spiders seldom
bite and only in self-defense, the bites so commonly charged to them
being often the work of other animals."
In the family Agalenidæ we meet with the first of the web makers. These
are spiders of moderate size, characterized by a big head marked off
from the thorax by converging grooves. Their natural home is in the
grass, where their flat, closely woven sheets of silk, almost invisible
by reason of their transparency, but brought into plain view when
coated with dew or dust, are spread everywhere. They also are fond of
getting into cellars and old buildings, and constructing webs across
corners, bracketwise. Somewhere the web sinks like a narrow funnel into
a short tube in which the owner hides, watching hungrily until a fly
alights on his silken platform.
"The Therididæ," says Emerton, "are the builders of the loose and
apparently irregular webs in the upper corners of rooms, in fences
and among rocks, and between the leaves and branches of low trees and
bushes. They are generally small, soft and light-colored spiders, with
the abdomen large and round and the legs slender and usually without
spines.... Most of the Therididæ live always in their webs, hanging by
their feet, back downward. The webs have in some part a more closely
woven space under which the spider stands." These spiders are quick
to avail themselves of any chance to spin their shapeless meshes of
almost invisible silk, which few regard as real "webs," in closets,
cellars, and all over the house or barn. Many of them are adorned with
gay colors or striking patterns, and some are much feared, especially
_Latrodectus mactans_, about half an inch long, which is black with
scarlet spots. It is common from Canada to Chile, and everywhere is
considered fatally poisonous--why, it is difficult to say.
Last of our list, and highest in rank, are the Epeiridæ, the "orb
weavers," as they are often called, who make those regular spiral nets
which are in our mind's eye when we think of cobwebs. Most of the
moderately large and handsome house and garden spiders are of this
family, and everyone can easily examine their work, although it is less
easy to watch them at it, as the webs are built and repaired at night.
Among the obscurer and foreign species the abdomen often shows humps,
points and long forward-reaching horns that make them exceedingly
grotesque, and doubtless difficult to handle by birds and other
creatures that seize them as food.
One of the round webs of the Epeiridæ consists of several radiating
lines, varying in different species from a dozen to seventy, crossed
by two spirals--an inner spiral that begins in the center and winds
outward, and an outer spiral that begins at the edge of the web and
winds inward. The inner spiral is made of smooth thread, like that of
the rays, to which dust will not cling; the outer spiral is made of
more elastic thread which, when fresh, is covered with fine drops of
sticky liquid.
"In beginning a web, after the radiating threads are finished, the
spider fastens them more firmly at the center and corrects the
distances between them by [inserting] several short, irregular
threads, and then begins the inner spiral, with the turns at first
close together and then widening ... until they are as far apart as
the spider can reach with the spinnerets [resting] on one and the
front feet on the next, and so goes on nearly to the outside of the
web, where it stops abruptly. The spider usually rests a moment, and
then begins, sometimes at another part of the web, the outer sticky
spiral.... As soon as the inner spiral is found in the way a part of it
is cut out, and by the time the outer spiral is finished the inner is
reduced to the small and close portion near the center.... The whole
making of the web seems to be done entirely by feeling, and is done as
well in the dark as in daylight. When the spider is active and the food
supply good, a fresh web is made every day, the old one being torn down
and thrown away."
[Illustration: AMERICAN GARDEN SPIDER
(_Epeira vulgaris_)]
As a rule these orb weavers do not stay in the web in the daytime, but
hide away in their nests made in some near-by but concealed place; and
their egg cocoons are hidden in all sorts of places.
All of the spiders that have been considered so far belong to the
division of the class that has but a single pair of lungs. A second
division has been made for those having two pairs of lungs, composed
of a single family, the Mygalidæ, consisting of the so-called
"bird-catching" spiders and the trapdoor spiders. The great mygale
of Guiana has a body sometimes two inches long, and its legs will
span eight or nine inches of space. It is hairy all over, intensely
black, and a terror to all small creatures, even catching small birds,
according to tradition; but proof of this is wanting.
The trapdoor spiders are those of the genera Cteniza and Atypus which
dig and inhabit vertical holes in the soil, lined with silk and closed
at the top by a hinged stopper or "trapdoor." Several species occur in
southern Europe, one of which has a second door hanging by a silken
hinge half way down the shaft; and in case of trouble the spider goes
below it and pushes it above its head, so that the intruder is deceived
into thinking it has opened an empty nest. _Cteniza californica_ is the
common species of our Southwest. The cover of the hole is made of dirt
fastened together with threads, and is lined, like the tube, with silk,
and fastened by a thick hinge of silk. The spider holds the door shut
from inside. These underground homes are safe retreats for the spiders
during the day, and nesting places in which their eggs are deposited
and young reared; at night the spiders go forth in search of prey.
MITES AND TICKS
Mites and ticks are classified with the spiders as degenerate relatives
of arachnoid stock. Ticks are large enough to be seen without a
magnifying glass, and some become half an inch long. Ticks are wholly
parasitic. The female lays several thousand eggs at one time on the
ground or just beneath the surface. "The young 'seed ticks' that hatch
from these in a few days soon crawl up on some near-by blade of grass,
or on a bush or shrub, and wait quietly until some animal comes along.
If the animal comes close enough they leave the grass or other support
and cling to their new-found host." These parasites are the agents of
the spread of several infectious diseases of cattle, the worst of which
is the destructive Texas fever, and of mankind, as spotted fever and
other ills resulting from the presence of blood parasites.
CHAPTER XI
FROM BUTTERFLIES TO BEETLES AND BEES
The generally accepted classification of the insects divides them into
more than twenty orders, and these into hundreds of families whose
species, already catalogued, are three times as numerous as all other
known animals together. "There are, for example," as Lutz remarks,
"15,000 species of insects to be found within fifty miles of New York
City; more than 2,000 of these are either moths or butterflies."
Insects as a class are characterized primarily by the division of the
body, when adult, into three clearly defined regions--the head, the
thorax or fore body, and the abdomen or hind body. All insects have
three pairs of legs, distinguishing them from the eight-legged spiders,
and from the many-footed myriapods and other arthropods, and most of
them have one or two pairs of wings, borne like the legs on the thorax,
the abdomen never bearing either. The head consists of four segments,
but in most cases the first three are consolidated into the hindmost,
and are represented only by the appendages they bear. The foremost of
these are the mouth organs, of which there are three pairs: the most
anterior are the mandibles, next the maxillæ, and then the labium,
the two latter bearing articulated prolongations known respectively
as maxillar and labial palpi. The mouth has an upper lip (labrum) and
contains a tongue. These mouth parts are variously modified, and by
these modifications insects may be classified in two groups: "First,
those in which the jaws and maxillæ are free, adapted for biting, as
in the locust or grasshopper; and second, those in which the jaws and
maxillæ are more or less modified to suck up or lap up liquid food, as
in the butterfly, bee, and bug." It is in this latter group that we
find those having those interesting relations with plants that result
in cross-fertilization of flowers.
[Illustration: A FLORIDA KATYDID
(_Cyrtophyllus floridensis_)]
From the forehead spring a pair of antennæ, which are not only
"feelers," but the bearers of other senses. They are jointed, and
exceedingly various in form and service. Some are mere stubs, others
long and slender as a whiplash, or they may be thickened at the end,
as commonly in butterflies, or bear rows of hairs on each side, giving
them in some cases a beautiful plumelike appearance. With their
antennæ insects inspect by touch whatever they come in contact with,
and test the shape of what they may be constructing, such as cells
for their eggs. They recognize one another, and apparently exchange
communications, or become aware of a stranger, and the ants induce
their captive aphids to let down the honeydew by stroking them with
their antennæ; but in many of these cases, if not all, additional
information is derived through the antennæ by reason of the senses of
hearing and of smell which many of them certainly possess. Ears, or
organs sensitive to vibrations, and delicate hairs and other processes
connected with nerves responding to touch are found in various other
parts of insects' bodies, but the feelers are preeminently the seat of
the sense of smell.
The eyes of insects are of two kinds, simple and compound. The simple
eyes are small and practically useless single ones (ocelli) situated
in a triangle of three on the top of the head. The compound eyes are
on the side of the head, and are covered by a transparent layer of
the chitinous skin (cornea), divided by delicate lines into square
areas (facets). Beneath each facet of the cornea is an "ommatidium,"
optically separated from its neighbors by black pigment, and consisting
of an outer segment or "vitreous body" and an inner segment or
"retinula" formed of sensory cells. In some such eyes the ommatidia
are few, but in others extremely numerous, so that the eyes cover a
large space; some hawk moths are said to have 27,000 facets. The
nature of the picture conveyed to the mind by such an eye has aroused
much discussion. Photographs taken through the eye of a dragon fly show
that, though the eye is compounded of many lenses and sensitive areas
(retinulæ) corresponding to them, yet the whole eye throws one image
on the retina. However complex such an eye may be, it is devoid of any
focusing arrangement and can only receive a clear image when the retina
and the object are separated by the focal length of the lenses. Hence
the need for active movement on the part of creatures having them.
The head is connected with the thorax by a neck often protected by the
overlapping front of the "tergum," or chitinous plate that covers the
thorax. The thorax consists of three segments, named from the front
backward "prothorax," "mesothorax," and "metathorax." These and a few
other technical terms are in such constant use in describing insects
that it is important to know them. The under (ventral) surface of the
thorax is protected by another plate named "sternum." The armor is not
continuous all around the body as in the crustaceans, but that on the
upper surface is connected with the sternum by a seam of soft skin
along the sides of the body.
Each segment of the thorax bears a pair of legs, each of which consists
of a stout, flattened "coxa," nearest the body; a small second part,
the "trochanter"; a third, the "femur"; a fourth, the "tibia"; and
finally the "tarsus," or foot, terminating in a pair of claws, bristly
on their under surface to give adhesive power. It is by means of these
stiff hairs, and not by any suction or stickiness, that flies are able
to walk on the ceiling and on vertical surfaces.
The wings of such insects as fly arise from the tergum of the thorax,
and are in two pairs except in the flies, where there is but one,
the hinder pair being represented by two little protuberances called
"halteres." Usually the wings are strengthened by rods called "veins,"
and the patterns of venation vary in different groups, and form one of
the means of classification.
The abdomen consists normally of ten segments, and contains most of the
digestive and all of the reproductive organs, above which runs the main
blood vessel, and below it the highly organized nervous system, the
chief ganglion of which, in the head, is termed "brain."
The breathing of insects, although rhythmical in its inhalation and
alternate exhalation, is not to the same purpose as ours. Respiration
goes on by means of a system of branching tubes (tracheæ) that ramify
throughout the body, and to which air is admitted through nine or more
openings in the side of the body guarded by valves called "spiracles."
The buzzing of flies, "singing" of mosquitoes, and the like, are sounds
made in these spiracles, not by their rapid wings. At intervals the
tracheæ are enormously enlarged to form air sacs. These no doubt,
lighten the body, but they probably serve also to provide a reservoir
of air from which the fine branches are filled by diffusion, and into
which the carbon dioxide is discharged. The circulation of oxygen in
adult insects, however, is never by means of the blood, but simply
by absorption by the tissues into which the excessively attenuated
tracheal tubes penetrate.
Insects are bisexual, and male and female are always separate
individuals. Except in a few abnormal cases among the most lowly, eggs
are produced and deposited in some favorable place for hatching.
SOME PRIMITIVE GROUPS
Insects go back in geologic history to the middle of the Paleozoic
age, and their remains are numerous and much differentiated in
Carboniferous rocks, when the orders Aptera, Orthoptera, Neuroptera,
and Hemiptera (the last represented in the Silurian by ancestral forms
of the bedbug and the cockroach--the oldest fossils yet discovered)
were flourishing. The beetles and ants first appear in the Trias, the
true flies, in the Jurassic, and the butterflies and moths, wasps, and
bees not until the Tertiary. This indicates an evolutionary progress
in structure with advancing time, as elsewhere in biology. The most
primitive type (Aptera) is still with us in the skipping silver fish
and snow fleas, or spring-tails, that annoy us in various situations.
They are wingless, very simple in organization, and without any
larval metamorphosis. Not much better are the Mayflies, or dayflies
(Ephemeridæ), that sometimes in early summer arise in enormous numbers
from lake shores and rivers, and then quickly disappear. Most of them
live, in truth, only a single day (or night), a single one of the many
American species surviving three weeks. During their brief life the
female drops into water several hundred eggs where they presently hatch
into swimming or crawling larvæ that next year, or perhaps not until
the third spring, creep out on land, molt, and fly abroad in ephemeral
crowds.
It is not a long step from these Mayflies to the dragon flies and
damsel flies (Odonata), which also belong to the water spaces of the
country, and are among the most interesting of all the insect tribes,
and the most beautiful, as they dart and curvet over the surface of
some glassy pool that reflects the steel-blue or peacock-green sheen
of their long slender bodies, and the black bars that alone make their
narrow and almost transparent wings visible. They are known by many
ridiculous names, as "darning needles," "snake doctors," etc., but
there is no harm in them; on the contrary they are to be encouraged,
for they consume, especially in their larval stages in the water, a
vast number of mosquitoes, gnats, and other troublesome "bugs." The
adults capture their food on the wing, and are hawklike in the agility
with which they turn and dodge in pursuit of their active prey. The
actual catching is done with the feet, which curve far forward, and
are studded with spines that give a sure grip on anything caught
between them; they assist, too, in clinging to plants, but the legs
are ill-adapted to walking. The wings are very powerful; are of a
glassy texture, and never folded; they are crossed by a great many
veins, breaking the surface into innumerable small squarish areas, and
bear markings that distinguish each of the two or three hundred North
American species.
Dragon flies, and their cousins, the smaller and more graceful,
low-flying damsel flies, pair as a rule in flight. In some of the
families the female descends below the surface of the water, and is
able by special apparatus to insert her eggs beneath the skin of a
plant; others place them in plant stems above the water, or simply drop
them at the surface, whence they sink to the bottom. The "nymphs," as
aquatic larvæ like this, with incomplete metamorphosis, are termed, go
about preying on anything they can seize and eat, and possess some very
peculiar temporary adaptations to their underwater career. After a time
the nymph (which is the "dragon" in dragon fly) changes from a rather
slender to a broad and flattened creature and crawls out of the water.
Soon its skin splits, and an adult dragon fly emerges.
Closely allied to the dragon flies are the stone flies, or alder flies
(Percoptera), whose ugly and predacious nymphs are so well known
to anglers as "dobsons," "crawlers," and by many other local and
opprobrious names, because they make excellent bait for still-water
fishing. The adult is that great, thin-winged creature called
"hellgrammite" (_Corydalis cornuta_), with a wing spread of four
inches, and possessed in the female of powerful biting jaws, which,
as in all insects, work horizontally and not up and down as among
vertebrates. In the male the jaws are extended into long, curved,
piercing organs which cross when at rest, and which are fully an inch
in length, but fortunately they are not used as jaws, but for holding.
Related to these is a group of well-known insects belonging to the
old order Neuroptera, but now placed in separate orders, all with
lacelike wings and an incomplete metamorphosis. They include the ant
lions, the useful aphis lions, the scorpion flies (Panorpa), the lovely
lace-winged flies, and the caddis flies, which make larval cases of
bits of stick, or of shells or fragments of stone, in the bottoms of
rapid streams. From somewhere in this group, probably, the ancestral
Lepidoptera branched off to develop into the butterflies and moths of
the present day. Next to them are the earwigs (Dermaptera), beetlelike
insects very conspicuous in Europe, but little noticed in this country.
A MUSICAL TRIBE
Out of this confusing array of rather primitive groups we come to an
extensive and well-defined order, the types of which are familiar to
the most careless of observers in all parts of the world. This is the
order Orthoptera, "straight wings," which includes the cockroaches,
mantids, walkingsticks, grasshoppers, locusts, katydids, crickets, and
their humbler kinfolk.
Cockroaches are native to all the warmer parts of the world, and we
have a common large brown one, and some others of our own; but the pest
of our kitchens is the small Oriental species whose origin was Asiatic,
and which probably accompanied the earliest westward wanderings of
mid-Asian men, and established themselves as boarders by the camp fires
of the cave men. At any rate, the "black beetles," as the British call
them--wrongly in both particulars--are now settled wherever ships have
gone or civilized goods have been carried. As they first began to be
really troublesome in New York City about the time when the Croton
water was introduced (1842) they got the local name "Croton bug," but
they are the world-wide _Blatta orientalis_, scampering around where
they are not wanted, carrying a queer packet of eggs under the tail.
[Illustration: TWO WALKINGSTICKS
(_Diapheromera femorata_)]
The mantids--of which a common species in the Southern States is known
as "mule killers" because of the superstition that its saliva poisons
stock--and the gaunt "walkingstick" insects that mimic twigs so well
that they are not seen as often as they might be, introduce us to the
great tribe of grasshoppers or locusts--two words that it has worried
bookmakers to keep straight. The grasshoppers fall into two families,
distinguished among other points by the length of the antennæ. The
short-horned ones (Acrididæ) are properly called locusts, and the
long-horned family (Tetigonidæ) are better known as grasshoppers,
despite the fact that until recently the books called this family
Locustidæ. To the Acrididæ belong the locusts that in years past have
worked such havoc now and then in the West, when vast swarms came from
the Rocky Mountains to the new farms along the eastern border of
the plains, and ate up the young grass and crops, leaving the ground
looking as if swept by fire. It is a story older than written history
in all plains districts of southern Asia, Asia Minor, Egypt, and
northern and south-central Africa, where no earthquake, or tornado,
or other reaction of nature against man's interference with natural
conditions, is so dreaded as a visitation of migratory locusts. In
this country any such "plagues" as half ruined Kansas forty years or
so ago need no longer be anticipated, because the plowing on ranches
and other disturbance of the ground in which the locusts lay their eggs
is now so extensive, and the methods of checking small flocks are so
well understood, that the vast surplus generations that constituted a
migration in search of food in the old days are no longer born.
All the Orthoptera are musical, or at any rate noisy, and make their
rattling or piercing notes as instrumentalists, not as vocalists.
"Some species," writes Frank E. Lutz, "make a rasping sound by rubbing
their hind legs against their front wings (tegmina). Others rattle,
while flying, their hind wings against the tegmina. These sounds
are primarily amorous serenades, and Nature's serenades without
attentive ears would be even more curious than the ears for which the
grasshoppers perform. In this family there is an auditory organ on each
side of the first abdominal segment, just above and back of the place
where the large hind femora start. Notice the clear round spot on the
next grasshopper you catch.... Few have not heard the masculine debates
as to whether Katy did or didn't, but many do not know by sight the
small, green, long-horned, stockily built disputants, both of whom
usually stay high in trees. The musical apparatus of the male--the
musician--is at the tegmina, and the leaflike wing covers, broadly
curving entirely around the body, act as sounding boards. The female's
wing covers do not have the thick rasp veins at their bases."
A third family, the Gryllidæ, contains the crickets--burrowing mole
crickets, ordinary black crickets dwelling in the herbage, and several
kinds of tree crickets that look like ghosts of their kind. All add to
the noise of a summer evening by rubbing the roughened surface of their
wing covers together--chirping to ears that are situated in the shins
of the listening cricketesses.
THE TRUE BUGS
Skipping the white ants or termites, which are few and comparatively
harmless in this country, but in the tropics make vast trouble for
house-holders; the various sorts of lice and the little black thrips
that destroys onions and some fruits, we come to the great assemblage
that entomologists call "bugs," limiting the word to the order
Hemiptera, which now must be considered.
The two features, basally common to all the immensely diverse members
of the order, are the character of: 1. The feeding organs; and 2. The
wings--in each case very distinct from that of all other insects. The
bugs have highly developed piercing and sucking jaws. The mandibles and
first maxillæ are transformed into stylets, often barbed toward the
tip; these work to and fro within the groove of a stout-jointed beak
(rostrum) which is formed by the union of the second maxillæ. The head
is usually triangular in shape, as viewed from above.
As to the second characteristic, the bugs are distinguished by the
modification of the fore wings into partly horny covers for the
entirely membranous hinder wings. This feature divides the order into
two suborders, Homoptera and Heteroptera. In the first this hardening
is little evident; but in the Heteroptera--where not wingless, as in
certain families--the fore wings are stiff and lie flat on the back
when closed, whereas in the Homoptera they are somewhat humped over the
back, and droop down on each side a little. The triangular space marked
on the back by the closed wings is a ready mark by which to recognize a
hemipteran, or true bug.
The Hemiptera display a greater diversity of form than any other order
of insects, and vary in size from almost microscopic scales to fat
cicadas and "giant" water bugs. "Some pass their lives in the upper
parts of trees, others chiefly on the lower limbs; still others prefer
the protection of roots, stones or rubbish on the ground; a large
number of species select a home beneath the surface of the earth,
often in the holes of ants or other insects; a conspicuous assemblage
of dull-colored forms occurs only in the crevices or under the bark
of trees and shrubs; while a host of others skim over the surface of
placid waters, and a few are found remote from land upon the rarely
disturbed waves of the tropical and subtropical oceans.... While the
greater number derive their food either from the sap of vegetables,
or the blood of fishes, animals and man, there are others which are
satisfied with the strong fluid that accumulates beneath damp, decaying
bark of trees, or still others which enjoy the juices of fungi or
ferns.... Those which creep about in search of living prey are often
furnished with curved or hooked forelegs, suitable for seizing and
holding creatures when in motion, such as caterpillars and other larvæ."
The Homoptera include the immense and destructive family Coccidæ,
the bark lice, scale insects, and mealy bugs, among which, however,
are the useful producers of lacs and such dyes as cochineal. Related
to them are the Aleyrodidæ, the destructive "white flies," and the
Aphidæ, almost infinite in number and in harmfulness to fruit trees and
cultivated plants; also the queerly shaped leaf hoppers and similar
minute, plant-sucking forms.
It is one of the curiosities of zoölogy that associated with these
minutiæ we find a family of bugs of large size--the cicadas, whose loud
"singing" by the male in autumn gives them the name "locust," and often
becomes annoying when one wants to sleep where trees are near by. The
noise is made by vibrating membranes stretched over a pair of sound
chambers, situated, one on each side, near the base of the abdomen.
The cicada lays its eggs in slits cut in the bark. The newly hatched
young drops to the ground and, burrowing into it, feeds by sucking the
juices of roots. The time spent in the ground varies according to the
species in various parts of the world. In the case of our "periodical"
cicada it lasts about seventeen years, whence we call that species
"seventeen-year locust," and know it, when a great swarm comes out of
the ground and ascends the trees, by the humming of the crowd which
sounds like the vibration of telegraph wires in the poles.
[Illustration: SEVENTEEN-YEAR LOCUSTS
1-4, pupæ, increasing in age; 5-15, the locust imago struggling out
of the pupa; 16, 17, 18, the imago stretching its wings; 19, empty
pupacase; 20, 21, perfect locust. (Smithsonian Institution.)]
The Heteroptera, or proper "bugs," are a much larger assemblage, a few
kinds of which have attracted popular notice. The long catalogue
begins with the small "water boatmen" that live an active predatory
life on the bottom of streams and ponds. Other common aquatic families
are the Notonectidæ, that swim on their backs, the Nepidæ, or "water
scorpions," one of whose genera is that of the slender, long-legged
"skaters" that glide so swiftly across the glassy surface of still
waters. Then there are the great water bugs (Belostoma), which all
over the world are the tigers of quiet rivers and ponds, pouncing from
their concealed lairs on even minnows, small frogs, and anything else
they can catch and kill. These great brown bandits are sometimes two
inches long. Some of the tropical species are strange in form and have
extraordinary habits in caring for eggs and young.
Leaving the aquatic group, we come to certain troublesome plant-sucking
bugs, and to the bedbug, which claims the longest lineage of any known
insect, for the remains of perfectly recognizable ancestors are found
in Ordovician rocks dating from early in the Paleozoic time. Skipping
the lace bugs, red bugs, or "cotton stainers," and others, we come to
a series of families that are among the worst pests of the farmer and
gardener, the chinch bug, squash bug, cabbage bug and many others, the
aggregate effect of whose ravages causes a loss of millions of dollars'
worth of crops every year, not only in this country, but everywhere
that grain, vegetables, and fruit are cultivated; and in most cases it
is not the native but introduced species that does the most damage.
GILDED BUTTERFLIES AND DUSTY MOTHS
The butterflies and moths, whose beauty attracts more collectors than
any other group of insects, constitute the order Lepidoptera, the
meaning of which term is "scaly winged," in reference to the fact that
the hairs that clothe and ornament the wings are scalelike. Butterflies
have club-shaped antennæ, and belong to the division Rhopalocera.
Moths are Heterocera. Some of the moths, especially the males, have
feathered antennæ, some threadlike, while a few tropical ones have
"club" antennæ, so that this distinction is not perfect. The pupæ
of butterflies are not protected by cocoons, as are those of most
moths, and are usually called "chrysalides" (singular, "chrysalis").
Butterflies in general only fly during the daylight, when few moths are
stirring, and usually hold their wings erect when at rest, while moths
hold them flat or folded against the body.
The Lepidoptera undergo a complete larval metamorphosis, and the
process is more familiar to general readers than in the case of
other insects. From the eggs, which are often objects of great
beauty when examined through a lens, are hatched wormlike creatures
that grow rapidly by repeated moltings of the skin into full-sized
"caterpillars"; those of certain moths develop in community nests,
but ordinarily they live singly. All have three pairs of thoracic
legs, and a variable number of temporary "prolegs" near the rear
of the body. Caterpillars may be smooth, round, and colorless, or
coated with a heavy fur, or bristling with knobs, tufts of hairs, and
other appendages, and brightly ornamented with color; and many of
these peculiarities appear to be wholly defensive in purpose. Some
caterpillars give off, when alarmed, disgusting and acrid fluids, and
the hairs of others irritate venomously the skin of anyone handling
them, and probably account for the fact that few birds will touch
certain species. All caterpillars feed voraciously--in fact, this is
the only time in the life of many species when food is taken, the adult
moths and butterflies as a rule being neither willing nor able to eat.
At a certain time, having completed its final molt, the caterpillar
arranges itself according to the custom of its race, and subsides into
a pupa.
[Illustration: CATERPILLAR OF THE MILKWEED BUTTERFLY
(_Anosia plexippus._) (Smithsonian Institution.)]
A century ago men interested in butterflies spoke of themselves
as aurelians, explaining that "aurelia" was a proper name for the
butterfly pupa because of the golden ornaments it usually bore. Really,
however, this characteristic, so marked in the gilt "buttons" of our
common milkweed butterfly, pertained to only a single family--the
Nymphalidæ. When the nymphalid caterpillar reaches the turning point,
it withdraws the abdomen a little from the cracking skin, exudes a
little sticky silk which it fastens to its support, then hooks the tip
of its abdomen firmly into this silk; this done, hanging thus by its
tail, the caterpillar finally shakes off its coat and, as a chrysalis
(a Greek word of the same general sense as the Latin _aurelia_) the
pupa hangs, head down and inert, until the following spring.
The butterflies of greatest size and most splendid coloring belong to
the family Nymphalidæ, whose hundreds of species are scattered all over
the warmer parts of the world. Here belong those gorgeous tropical
ones, whose wings, sometimes with a spread of five inches, emulate the
prismatic hues of the "eyes" in a peacock's tail, and which are so
often seen mounted as lovely ornaments in curiosity shops; and here
also is classified that strange "leaf butterfly" of Malaysia, whose
wings when closed so perfectly imitate a leaf of the tree on which it
alights that the sharpest eyes can hardly find it. Here, too, belong
our brown-streaked "fritillaries," such as the vanessas, and darker
ones like our mourning cloak, and many others well known to amateurs.
[Illustration: FINAL MOLTING OF A NYMPHALID CATERPILLAR
(a) Before shedding skin. (b) In act of shedding skin. (c) Trying to
catch hold of silk button. (Smithsonian Institution.)]
All of this family have their chrysalides hung by the tail; but in
the remainder of the butterfly families they are held in an upright
position by a loop of silk that passes around them like a girdle. Such
are the "coppers," the "blues," the "hair streaks" and many other
small, gayly colored species (Lycænidæ) common in summer, to which
season they add so beautiful an interest. In another large family, the
Papilionidæ, are found the great yellow and black "swallowtails," which
are almost exclusively American, and several dark blue or purple-marked
species, with "tails" to their wings, that attract the attention of the
most careless as they lazily flit among the flowers. In this family,
too, are the sulphur-yellow butterflies that dance over the roads and
fields in little flocks; and, alas, the white ones whose caterpillars
are so injurious to cabbages and similar vegetables. The last family
(Hesperidæ) contains small, rather obscurely marked, butterflies that
connect the Rhopalocera with the Heterocera, or moths.
[Illustration: CATERPILLAR BEGINNING ITS COCOON
Photo, A. N. Mirzaoff]
In fact the distinction between the two divisions of Lepidoptera is
one of convenience rather than of science, for it marks difference of
habits rather than of structure. Instead of a naked pupa, that of the
moth is inclosed in some sort of envelope called a "cocoon." This may
be an earthen cell underground, or a woolly tuft fastened to some such
support as the bark of a tree, or a leaf rolled and tied by silken
threads into a tube, or a burrow in dead wood, or a paperlike case
fastened to a twig; but in every case some special provision is made
for the easy emergence of the imago when the time comes for its birth
as a moth. The moths themselves do no harm. Their few weeks of life
are devoted entirely to mating and putting their eggs in just those
places where the larvæ they will never see can have the food proper for
them and the best chance for life--a matter of marvelous instincts and
adaptations. Few of them, except the hawk moths, eat at all. That is
done in the caterpillar stage, when many sorts become destructive of
the labor and hopes of the farmer and gardener and orchardist, or make
havoc in stores of grain and meal, and in garments of wool and fur,
carpets, and cabinets of natural history specimens.
[Illustration: MOTH AND EGGS
Photo, A. N. Mirzaoff]
Most of the moths are small, inconspicuous, grayish or brownish
creatures whose markings, very lovely when closely examined, so closely
resemble in their mottlings the places where the moths rest during the
day, that they are comparatively safe from the birds, monkeys and other
enemies that seek to catch and eat them. Some, however, are of large
size and brighter hue. Thus the silkworm moths of the Orient (and of
our own land) may measure four or five inches across the outstretched
wings, as does the cecropia and others that flit about evening lights;
and a near relative among us is the exquisite, long-tailed, luna moth,
which is pale green with chestnut edgings; many others in this group
are almost as "richy bedight" as butterflies. It is these that make the
large papery cocoons so easily seen in the fall in trees and bushes.
A remarkable family (Bombycidæ) is that of the hawk moths, which much
resemble in shape and action humming birds. They are day flyers,
but most active in the morning and evening twilights, and hover on
whirring wings before a flower, while with their long, tubular tongues
they suck its nectar, for these moths feed as well as do their fat,
uprearing, bulldoglike caterpillars, to which they owe another common
name for the family--that of sphinx moths. Their pupæ are lodged under,
on, or near the ground in a loose cocoon, and are to be recognized
by an appendage, curled around like a jug handle, in which lies the
chrysalis' long tongue.
FLIES AND THEIR HYGIENIC IMPORTANCE
Flies, scientifically speaking, are only those insects of the order
of Diptera, distinguished by having only one pair of fully developed
wings. They pass through a complete metamorphosis, and the larva is in
all cases a "grub" or "maggot" destitute of legs. It is rarely enclosed
in a cocoon but lies buried in the ground, floats in the water, or is
protected by the last larval skin which, separating from the pupa skin,
remains around it as a hard case. Flies and their larvæ live in the
most diverse manner. Some flies attack backboned animals and suck their
blood, some prey on smaller insects, some suck honey, and some find
their food in decaying animal and vegetable matter. A large number of
dipterous larvæ eat refuse, many feed inside growing vegetable tissues,
and some prey, or are parasitic, on other insects. More than 10,000
species of true flies have already been named in the United States
alone. The order contains all the different species and varieties of
fleas, mosquitoes, sand flies, gnats, midges and gall flies. Then come
the blood-sucking gadflies, and half a dozen families allied to them;
the scavenging syrphus flies, the bots that trouble cattle, the house
flies and stable flies of deservedly bad repute; and, lastly, the
horseflies, bee parasites, and botflies. The popular interest in these
insects is confined to the flies of our houses and stables, and to the
mosquitoes. In fact it is in the relation that the flies mentioned,
and some others, bear to public health and comfort, that this group of
insects is important at all to any but the special student.
BEETLES AND THEIR GRUBS
The beetles (order Coleoptera) make up a very distinct and natural
group of insects, characterized by the horny or leathery texture of
their forewings, or "elytra," which serve as cases for the folding
membranous hind wings alone used in flight. These elytra, when closed,
usually cover the whole hind body. They are strengthened with ridges
around their edges, and marked with a series of longitudinal furrows
and often also with impressed dots. The hind wings are sometimes very
small or wanting; in such cases the elytra are often fused together
along their middle edges (suture). The head is usually extended from
behind forward, having therefore a large crown and a small face; the
feelers are very inconstant in form; the mandibles are always developed
as strong biting jaws; the prothorax is free and movable; its tergite
(pronotum) is a very prominent feature in all beetles, reaching back to
the origin of the elytra.
The beetles undergo a complete metamorphosis, and the larvæ, called
"grubs," have various shapes, while the pupa is "free," that is,
closely similar in development and appearance to the adult. Beetles
are world-wide in distribution and more than 100,000 species have been
catalogued. They are divided into a great number of families, among
which those mentioned below contain the most noteworthy forms.
The tiger beetles are large-headed, predacious forms, most numerous
in the tropics, which live in holes in the soil and rush out to seize
passing prey. The ground beetles (Carabidæ) are a very extensive
family, represented in all parts of the world, and are insect hunters,
destroying hosts of injurious insects. Most of them are black or brown.
The Dyticidæ and Hydrophilidæ are aquatic families, including some
of the largest and fiercest of carnivorous beetles, the terrors of
ponds and marshes, where they prey not only on other insects and their
young, but on tadpoles, small fishes, etc.; and their grubs are quite
as savage. The rove beetles (Staphylinidæ) are a very large family of
narrow, elongated species, which are very active; they feed mostly on
small insects, worms and snails. The carrion beetles belong to the
family Silphidæ, the smaller among which live in moss and under tree
bark, and the larger genera contain the noted "burying beetles." Some
groups of very minute, ground-keeping species lead to the familiar
"ladybirds" (Coccinellidæ), a large and world-wide family of small,
rounded beetles, usually brightly spotted, which frequent plants of all
sorts, and feed chiefly on aphids. Some quaint superstitions pertain
to these pretty insects, that should be attracted rather than repelled
when they visit window gardens and greenhouses, which they will
endeavor to clear of the "greenfly" and similar injurious plant lice.
Passing over several inconspicuous families we come to the dermestids,
very small, dark-colored beetles of elliptical outline, some of whose
genera are among the worst of household pests, and have been spread by
commerce throughout the civilized world.
Some of the dermestids are troublesome as museum pests; others attack
food in the pantry, store, or warehouse. "Drugs do not escape their
attack, species devouring even cantharides and tobacco; woolen and silk
goods, feathers and furs, are ruined if left long exposed to their
depredations; and one species is accused of biting young doves....
_Anthrenus scrophulariæ_, probably introduced into America from Europe,
has received the names carpet beetle and buffalo bug, on account
of its habit, both as larvæ and imago, of destroying carpets. This
beetle measures about four-fifths of an inch in length, and is black,
brick-red and white, the last crossing the back in two zigzag lines.
The point of attack is the nailed-down edge or the lines of the seams."
Who has not been amused at the labors of the big black beetles that
one meets in summer on dusty paths rolling balls of fibrous material.
These "dung beetles" are the American cousin of the scarab of the
ancient Egyptians, which typified to them many mystical ideas connected
with life, present and eternal. With its shovellike head and broad
forelegs the beetle gathers and compacts the material it wants, and
begins to roll it, sometimes with the help, more often against the
struggles, of another beetle toward a prepared nest-hole. Arrived there
an egg may be inserted into it, and then the rounded mass is left as
food for the grub to be hatched from the egg; if no egg is inserted,
the ball becomes simply a mass of stored food to be eaten by its
maker. Processes vary among the 7,000 or more known species of this
cosmopolitan family.
Not all of this great family are dung beetles, however, or scarablike.
Here belong the May bugs and June beetles that come blundering around
lighted country residences in the evenings; and it is their fat white
grubs that, hatched from eggs buried in the ground, devour the roots
of the grass and other plants, spoiling the lawns and strawberry beds.
The robin is their most effective enemy. Among the lesser genera are
those of the rose bugs, hated pests of the horticulturist and fruit
grower. In that section of the family known as the cetonians are found
the giants of the race, the West African "goliaths," four inches long;
the tropical American Hercules beetle, exceeding six inches long, half
of which belongs to the forward-reaching horn of its helmet, the South
American elephant beetle which is even more bulky, and several other
giants, the males of which have the head ornamented with fearsome
protuberances.
Other families of beetles are the Buprestidæ, whose larvæ are injurious
to trees by boring into their wood; the Elateridæ, or snap beetles,
which arch their bodies and leap when they happen to fall on their
backs, and among which are found the many varieties of brilliant
"fireflies" for which the American tropics are famous. The larvæ of
the elaters mostly live in decaying wood, and are the justly hated
"wireworms" of our gardens. Then there are the Meloidæ, that include
the blister beetles, or oil beetles, one of which is the cantharides of
the pharmacopoeia; and there are a great many more.
HONEY MAKERS AND PLANT STINGERS
A long shelf is required in the naturalist's library for the books
relating to the Hymenoptera of America alone--our wasps, bees, ants,
and their smaller relatives, which engage everybody's attention by
their social habits and amazing display of instincts. Besides these
three principal and familiar groups the Hymenoptera include a host of
other insects of great but inconspicuous importance. In large part
these are parasitic on other insects or their larvæ, or even on their
eggs, and some are the most minute insects known, virtually invisible
to the unaided eye. Scarcely larger are the makers (Cynipidæ) of the
galls so commonly seen on trees and plants in which they breed. Another
group (Chalcidoidea) cause the swellings that disfigure plants by
placing their young within their tissues, such as the "joint worms"
that ruin grain; and here, again, many species are parasitic on grubs.
Then there are the sawflies (Tenthredinidæ), resembling bees, whose
ovipositors are like a pair of saws with which these insects are able
to bore holes into wood, within which the egg is placed and the young
larva burrows; of these are many and various kinds, all injurious to
trees, garden shrubs and plants, each kind restricted to a particular
sort of plant.
Perhaps even more numerous are the ichneumon flies, whose service in
the world seems to be to keep the insect hosts down to the number
possible to exist and at the same time to allow men and other animals
to live. Their method of life is to deposit their eggs on or in the
bodies of other insects, usually in the larval stage, where they
hatch and thrive by the slow death of the host. The ichneumon flies
are the dread of all other insects, most of whose adaptations for
self-preservation are directed against this insidious and universal
enemy to insect life.
None of the foregoing Hymenoptera live in colonies or by social
methods. That plan belongs to the four most advanced divisions--wasps,
bees, termites, and ants. Even among the wasps and bees, however, the
larger number of species live alone or in single families, each female
constructing a solitary receptacle for her purpose underground, in
soft wood or otherwise. Most species store with the egg placed there
half-dead insects, or pollen, etc., as food for the grub, which
receives no further attention; but a few, such as the big digger wasp
(Bembex) take food to the grubs daily. Another class of both wasps and
bees form nests of several cells containing eggs, and thus in spring
families are originated by fertilized females that have survived the
winter. As the larvæ develop in succession they are fed by the mother,
and presently mature sufficiently to aid her in caring for the younger
grubs. Out of such family nests, or "combs" of paper cells, often
attached to the ceilings of sheds and porches of rural houses, have
apparently developed the mutually helpful societies of bees and ants,
which are often of surprising extent and permanency.
The prosperity of these social insect communities, whose instincts,
habits, and products amaze us, is due to an organized division of
labor in the community between three classes of "citizens"--(1) the
comparatively few males, whose whole duty is to fertilize the queen
mother and supply the community with progeny; (2) the selected and
specially nourished "queen"; (3) a vast number of nonreproductive
females, the "workers," that build and guard the nest, gather and
preserve stores of food (honey), and nurse and rear the young. In
some groups the duties of the workers are subdivided among classes
that differ in size and equipment. It is these female workers, or
their correlatives among the solitary bees and wasps, that sting,
their useless ovipositors having been transformed by the addition of
poison into deadly weapons by which they procure their prey, or defend
themselves, or both. It is this division of labor, and attendant
habits, that especially characterize the higher Hymenoptera, and give
to the order the supreme rank it occupies among insects.
CHAPTER XII
AT THE DOORWAY OF THE "UPPER CLASSES"
PRIME SIGNIFICANCE OF THE NOTOCHORD
We have been considering up to this point one of the two primary and
natural divisions of the animal kingdom--that into Invertebrates and
Vertebrates. Although these are terms made familiar by long usage, and
refer to the absence or presence of a backbone composed of jointed
sections (vertebræ), a truer conception of the distinction is had by
regarding the first as animals whose skeleton, or frame, that gives
support to the muscles and other soft parts of the body, is exterior;
and the second as animals whose skeleton is interior. The one is, in
scientific language, an "exoskeleton," or more or less hardened outer
shell from the inner surface of which the organs grow and maintain
their attachment and leverage for work or protection; and the other an
"endoskeleton," around which the organs and integument are accumulated
by growth, and by means of which the animal's strength is maintained,
the interior bones--of which the chief is the spinal column, or
backbone--giving a firm fulcrum for the operation of the muscles and a
support and protection for the vital organs.
* * * * *
All the vertebrates are included in a single phylum--Chordata. This
term has supplanted in zoölogy the old term Vertebrata (now reserved
as a class name only), because it is more comprehensive and precise.
Professor Harmer says:
"The axis of the backbone of all vertebrates is formed by an elastic
rod known as the 'notochord,' which lasts throughout life in some of
the lowest forms, but in the higher forms appears only in the embryo.
The universal occurrence of this structure has been regarded as the
most important characteristic of the Vertebrata and their allies, which
are accordingly grouped together in the phylum Chordata. The members
of this phylum are further distinguished from other animals by several
important features. Of these one of the most important appears to be
the existence of lateral outgrowths of the pharynx, which unite with
the skin of the neck and form a series of perforations leading to the
exterior. These structures are the gill slits, and in the fishes their
walls give rise to vascular folds or gills. With the assumption of a
terrestrial life the higher vertebrates lost their gills as functional
organs, respiration being then performed by entirely different organs,
the lungs. But even in these cases, the gill slits appear in the
embryo.... Another fundamental characteristic of the Chordata is given
by the central nervous system, which lies entirely above the alimentary
canal, just dorsal to the notochord. Not only does this position of the
nerve centers distinguish the Chordata from the Invertebrates, but a
further point of difference is found in the development."
This definition requires the inclusion of various creatures very unlike
"vertebrates," and the phylum therefore embraces three subdivisions:
1. Adelochorda--marine wormlike creatures having a notochord in
the anterior of the body, and gill slits, both persistent; 2.
Urochorda--the ascidians or tunicates, small marine creatures, some
fixed along shores, others free-swimming and in some cases united into
swimming colonies (e. g., the salpæ), the tadpolelike larvæ of which
show a notochord in the tail; and 3. Vertebrata.
This last great subphylum is divisible into seven grand natural groups
with the rank of classes, namely:
1. _Acrania_--Lancelets (Amphioxus).
2. _Cyclostomata_--Lampreys; hags.
3. _Pisces_--Fishes.
4. _Amphibia_--Amphibians.
5. _Reptilia_--Reptiles.
6. _Aves_--Birds.
7. _Mammalia_--Mammals.
The first of these seven classes, the Acrania, has usually, heretofore,
been set apart as a subdivision equal in rank to the subphyla
Adelochorda and Urochorda, and the remaining six classes were grouped
into a coordinate subphylum Craniata, denoting that they alone have
a distinct head (cranium); the reason was that its members, the
lancelets, have no spine, but only a notochord, which, however, extends
from end to end of the body above the digestive organs, and persists
in the adult and throughout life. The lancelets (amphioxus) are small,
fish-shaped creatures that burrow in the sand of the seashore, usually
leaving only the head exposed, and sucking in a continuous current
of water which brings with it minute food. They breathe through gill
slits. The reproduction is bisexual, and by eggs.
The significance of the Acrania in this phylum is that they represent
a very early ancestral stage of the stock from which the higher
vertebrates (Craniata) have developed, and from which they themselves,
of course, have also diverged to a certain degree; and it is because
they retain many primitive characteristics that the study of their life
histories has engaged the attention of so many eminent zoölogists and
has thrown so much light on the evolutionary history of the "higher
animals," or vertebrates.
THE ROUNDMOUTHS--LAMPREYS AND HAGS
Popularly included among fishes, the lampreys and hags of the class
Cyclostomata (roundmouths) differ from true fishes by the possession of
a suctorial mouth devoid of functional jaws, by the single olfactory
organ, and by the absence of lateral appendages, or paired fins.
They have an eellike form and method of travel, and some species are
a yard in length. They are bisexual, discharging both eggs and milt
into the water to become fertilized by accidental contact. Lampreys
ascend the rivers to spawn, however, and there make little heaps of
pebbles, carried and piled with the mouth, in which the eggs find some
protection from the many egg-eaters in all streams. Most, if not all,
of the migratory parents die after spawning. From the eggs hatch larvæ
that undergo a metamorphosis. Lampreys live on small crustaceans,
worms, and so forth, eat carrion, and also attack living fishes. The
tongue, like the interior of the mouth, is armed with teeth. They
are in the habit of attaching themselves to stones in order to hold
themselves against a river current, breathing meanwhile by taking water
directly into the pouchlike gill chambers and expelling it, instead of
sucking it through the mouth and passing it out of the gill slits. In
ancient Rome the big sea lampreys of the Mediterranean were eaten as a
delicacy, and even cultivated in landlocked ponds, and they are still
highly prized in some parts of Europe.
The hags are an even more primitive group of cyclostomes that live in
the mud of shallow seas and are too abundant on both our coasts, where
they are a pest of the fisheries. Their general habits are similar
to those of lampreys, but wherever possible they attach themselves
to fish on which they feed. The hag is particularly destructive to
fishes caught on "set lines" of hooks, or in nets, and the loss thus
resulting on the coasts of California, in Japan, and in some European
fisheries is very serious. As these cyclostomes have no scales or
other h
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