Encyclopaedia Britannica, 11th Edition, "Gassendi, Pierre" to "Geocentric"
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Title: Encyclopaedia Britannica, 11th Edition, "Gassendi, Pierre" to "Geocentric"
Author: Various
Release date: August 31, 2011 [eBook #37282]
Most recently updated: January 8, 2021
Language: English
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Transcriber's notes:
(1) Numbers following letters (without space) like C2 were originally
printed in subscript. Letter subscripts are preceded by an
underscore, like C_n.
(2) Characters following a carat (^) were printed in superscript.
(3) Side-notes were relocated to function as titles of their respective
paragraphs.
(4) Macrons and breves above letters and dots below letters were not
inserted.
(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek
letters.
(6) The following typographical errors have been corrected:
ARTICLE GAUDEN, JOHN: "... and on the fact that it was admitted by
Clarendon, who should have had means of being acquainted with the
truth." 'should' amended from 'sould'.
ARTICLE GAWAIN: "In the later Historia of Geoffrey of Monmouth, and
its French translation by Wace, Gawain plays an important and
'pseudo-historic' rôle." 'Geoffrey' amended from 'Goeffrey'.
ARTICLE GAYA: "... and at which a religious fair is held each
September, attended by 10,000 to 20,000 pilgrims." '20,000' amended
from '20,0000'.
ARTICLE GECKO: "The arrangement of the lamellae and pads differs
much in the various genera and is used for classificatory
purposes." 'classificatory' amended from 'classificactory'.
ARTICLE GEDDES, ALEXANDER: "Although under ecclesiastical censures,
he had never swerved from a consistent profession of faith as a
Catholic; and on his death-bed he duly received the last rites of
his communion." 'Although' amended from 'Athough'.
ARTICLE GELSEMIUM: "It was first described in 1640 by John
Parkinson, who grew it in his garden from seed sent by Tradescant
from Virginia; at the present time it is but rarely seen, even in
botanical gardens, in Great Britain." 'Britain' amended from
'Britian'.
ARTICLE GEM: "From the Byzantine period downward one peculiarity of
gem-engraving becomes noticeable." 'peculiarity' amended from
'peculiarty'.
ARTICLE GENEALOGY: "... or that Bilhan points to an old clan
associated with Reuben (Gen. xxxv. 22) or Edom (Bilhan, Gen. xxxvi.
27), ..." 'Bilhan' amended from 'Bilhah'.
ARTICLE GENTIANACEAE: "... bright blue corolla, is visited by
bumble bees; and G. verna, with a still longer narrower tube, is
visited by Lepidoptera." 'bumble' amended from 'humble'.
ARTICLE GENTZ, FRIEDRICH VON: "... but partly supplemented in
Österreichs Teilnahme an den Befreiungskriegen (Vienna, 1887) ..."
'Österreichs' amended from 'Öesterreichs'.
ENCYCLOPAEDIA BRITANNICA
A DICTIONARY OF ARTS, SCIENCES, LITERATURE
AND GENERAL INFORMATION
ELEVENTH EDITION
VOLUME XI, SLICE V
Gassendi, Pierre to Geocentric
ARTICLES IN THIS SLICE:
GASSENDI, PIERRE GEFLE
GASTEIN GEGENBAUR, CARL
GASTRIC ULCER GEGENSCHEIN
GASTRITIS GEIBEL, EMANUEL
GASTROPODA GEIGE
GASTROTRICHA GEIGER, ABRAHAM
GATAKER, THOMAS GEIJER, ERIK GUSTAF
GATCHINA GEIKIE, SIR ARCHIBALD
GATE GEIKIE, JAMES
GATEHOUSE GEIKIE, WALTER
GATES, HORATIO GEILER VON KAISERSBERG, JOHANN
GATESHEAD GEINITZ, HANS BRUNO
GATH GEISHA
GATLING, RICHARD JORDAN GEISLINGEN
GATTY, MARGARET GEISSLER, HEINRICH
GAU, JOHN GELA
GAUDEN, JOHN GELADA
GAUDICHAUD-BEAUPRÉ, CHARLES GELASIUS
GAUDRY, JEAN ALBERT GELATI
GAUDY GELATIN
GAUERMANN, FRIEDRICH GELDERLAND (duchy)
GAUGE GELDERLAND (province of Holland)
GAUHATI GELDERN
GAUL, GILBERT WILLIAM GELL, SIR WILLIAM
GAUL GELLERT, CHRISTIAN FÜRCHTEGOTT
GAULT GELLERT
GAUNTLET GELLIUS, AULUS
GAUR (ruined city of India) GELLIVARA
GAUR (wild ox) GELNHAUSEN
GAUSS, KARL FRIEDRICH GELO
GAUSSEN, FRANÇOIS SAMUEL LOUIS GELSEMIUM
GAUTIER, ÉMILE THÉODORE LÉON GELSENKIRCHEN
GAUTIER, THÉOPHILE GEM
GAUTIER D'ARRAS GEM, ARTIFICIAL
GAUZE GEMBLOUX
GAVARNI GEMINI
GAVAZZI, ALESSANDRO GEMINIANI, FRANCESCO
GAVELKIND GEMISTUS PLETHO, GEORGIUS
GAVESTON, PIERS GEMMI PASS
GAVOTTE GENDARMERIE
GAWAIN GENEALOGY
GAWLER GENELLI, GIOVANNI BUONAVENTURA
GAY, JOHN GENERAL
GAY, MARIE FRANÇOISE SOPHIE GENERATION
GAY, WALTER GENESIS
GAYA GENET
GAYAL GENEVA (New York, U.S.A.)
GAYANGOS Y ARCE, PASCUAL DE GENEVA (Switzerland)
GAYARRÉ, CHARLES ÉTIENNE ARTHUR GENEVA CONVENTION
GAY-LUSSAC, JOSEPH LOUIS GENEVA, LAKE OF
GAZA, THEODORUS GENEVIÈVE, ST
GAZA GENEVIÈVE, OF BRABANT
GAZALAND GENGA, GIROLAMO
GAZEBO GENISTA
GAZETTE GENIUS
GEAR GENUS, STÉPHANIE DE SAINT-AUBIN
GEBER GENNA
GEBHARD TRUCHSESS VON WALDBURG GENNADIUS II.
GEBWEILER GENOA
GECKO GENOVESI, ANTONIO
GED, WILLIAM GENSONNÉ, ARMAND
GEDDES, ALEXANDER GENTIAN
GEDDES, ANDREW GENTIANACEAE
GEDDES, JAMES LORRAINE GENTILE
GEDDES, SIR WILLIAM DUGUID GENTILE DA FABRIANO
GEDYMIN GENTILESCHI, ARTEMISIA and ORAZIO DE'
GEE, THOMAS GENTILI, ALBERICO
GEEL, JACOB GENTLE
GEELONG GENTLEMAN
GEESTEMÜNDE GENTZ, FRIEDRICH VON
GEFFCKEN, FRIEDRICH HEINRICH GEOCENTRIC
GEFFROY, MATHIEU AUGUSTE
GASSENDI[1] [GASSEND], PIERRE (1592-1655), French philosopher, scientist
and mathematician, was born of poor parents at Champtercier, near Digne,
in Provence, on the 22nd of January 1592. At a very early age he gave
indications of remarkable mental powers and was sent to the college at
Digne. He showed particular aptitude for languages and mathematics, and
it is said that at the age of sixteen he was invited to lecture on
rhetoric at the college. Soon afterwards he entered the university of
Aix, to study philosophy under P. Fesaye. In 1612 he was called to the
college of Digne to lecture on theology. Four years later he received
the degree of doctor of theology at Avignon, and in 1617 he took holy
orders. In the same year he was called to the chair of philosophy at
Aix, and seems gradually to have withdrawn from theology. He lectured
principally on the Aristotelian philosophy, conforming as far as
possible to the orthodox methods. At the same time, however, he followed
with interest the discoveries of Galileo and Kepler, and became more and
more dissatisfied with the Peripatetic system. It was the period of
revolt against the Aristotelianism of the schools, and Gassendi shared
to the full the empirical tendencies of the age. He, too, began to draw
up objections to the Aristotelian philosophy, but did not at first
venture to publish them. In 1624, however, after he had left Aix for a
canonry at Grenoble, he printed the first part of his _Exercitationes
paradoxicae adversus Aristoteleos_. A fragment of the second book was
published later at La Haye (1659), but the remaining five were never
composed, Gassendi apparently thinking that after the _Discussiones
Peripateticae_ of Francesco Patrizzi little field was left for his
labours.
After 1628 Gassendi travelled in Flanders and Holland. During this time
he wrote, at the instance of Mersenne, his examination of the mystical
philosophy of Robert Fludd (_Epistolica dissertatio in qua praecipua
principia philosophiae Ro. Fluddi deteguntur_, 1631), an essay on
parhelia (_Epistola de parheliis_), and some valuable observations on
the transit of Mercury which had been foretold by Kepler. He returned to
France in 1631, and two years later became provost of the cathedral
church at Digne. Some years were then spent in travelling through
Provence with the duke of Angoulême, governor of the department. The
only literary work of this period is the _Life of Peiresc_, which has
been frequently reprinted, and was translated into English. In 1642 he
was engaged by Mersenne in controversy with Descartes. His objections to
the fundamental propositions of Descartes were published in 1642; they
appear as the fifth in the series contained in the works of Descartes.
In these objections Gassendi's tendency towards the empirical school of
speculation appears more pronounced than in any of his other writings.
In 1645 he accepted the chair of mathematics in the Collège Royal at
Paris, and lectured for many years with great success. In addition to
controversial writings on physical questions, there appeared during this
period the first of the works by which he is known in the history of
philosophy. In 1647 he published the treatise _De vita, moribus, et
doctrina Epicuri libri octo_. The work was well received, and two years
later appeared his commentary on the tenth book of Diogenes Laërtius,
_De vita, moribus, et placitis Epicuri, seu Animadversiones in X. librum
Diog. Laër_. (Lyons, 1649; last edition, 1675). In the same year the
more important _Syntagma philosophiae Epicuri_ (Lyons, 1649; Amsterdam,
1684) was published.
In 1648 ill-health compelled him to give up his lectures at the Collège
Royal. He travelled in the south of France, spending nearly two years at
Toulon, the climate of which suited him. In 1653 he returned to Paris
and resumed his literary work, publishing in that year lives of
Copernicus and Tycho Brahe. The disease from which he suffered, lung
complaint, had, however, established a firm hold on him. His strength
gradually failed, and he died at Paris on the 24th of October 1655. A
bronze statue of him was erected by subscription at Digne in 1852.
His collected works, of which the most important is the _Syntagma
philosophicum_ (_Opera_, i. and ii.), were published in 1658 by Montmort
(6 vols., Lyons). Another edition, also in 6 folio volumes, was
published by N. Averanius in 1727. The first two are occupied entirely
with his _Syntagma philosophicum_; the third contains his critical
writings on Epicurus, Aristotle, Descartes, Fludd and Lord Herbert, with
some occasional pieces on certain problems of physics; the fourth, his
_Institutio astronomica_, and his _Commentarii de rebus celestibus_; the
fifth, his commentary on the tenth book of Diogenes Laërtius, the
biographies of Epicurus, N.C.F. de Peiresc, Tycho Brahe, Copernicus,
Georg von Peuerbach, and Regiomontanus, with some tracts on the value of
ancient money, on the Roman calendar, and on the theory of music, to all
which is appended a large and prolix piece entitled _Notitia ecclesiae
Diniensis_; the sixth volume contains his correspondence. The _Lives_,
especially those of Copernicus, Tycho and Peiresc, have been justly
admired. That of Peiresc has been repeatedly printed; it has also been
translated into English. Gassendi was one of the first after the revival
of letters who treated the _literature_ of philosophy in a lively way.
His writings of this kind, though too laudatory and somewhat diffuse,
have great merit; they abound in those anecdotal details, natural yet
not obvious reflections, and vivacious turns of thought, which made
Gibbon style him, with some extravagance certainly, though it was true
enough up to Gassendi's time--"le meilleur philosophe des littérateurs,
et le meilleur littérateur des philosophes."
Gassendi holds an honourable place in the history of physical science.
He certainly added little to the stock of human knowledge, but the
clearness of his exposition and the manner in which he, like Bacon,
urged the importance of experimental research, were of inestimable
service to the cause of science. To what extent any place can be
assigned him in the history of philosophy is more doubtful. The
_Exercitationes_ on the whole seem to have excited more attention than
they deserved. They contain little or nothing beyond what had been
already advanced against Aristotle. The first book expounds clearly,
and with much vigour, the evil effects of the blind acceptance of the
Aristotelian dicta on physical and philosophical study; but, as is the
case with so many of the anti-Aristotelian works of this period, the
objections show the usual ignorance of Aristotle's own writings. The
second book, which contains the review of Aristotle's dialectic or
logic, is throughout Ramist in tone and method. The objections to
Descartes--one of which at least, through Descartes's statement of it
in the appendix of objections in the _Meditationes_ has become
famous--have no speculative value, and in general are the outcome of
the crudest empiricism. His labours on Epicurus have a certain
historical value, but the want of consistency inherent in the
philosophical system raised on Epicureanism is such as to deprive it
of genuine worth. Along with strong expressions of empiricism we find
him holding doctrines absolutely irreconcilable with empiricism in any
form. For while he maintains constantly his favourite maxim "that
there is nothing in the intellect which has not been in the senses"
(_nihil in intellectu quod non prius fuerit in sensu_), while he
contends that the imaginative faculty (_phantasia_) is the counterpart
of sense--that, as it has to do with material images, it is itself,
like sense, material, and essentially the same both in men and brutes;
he at the same time admits that the intellect, which he affirms to be
immaterial and immortal--the most characteristic distinction of
humanity--attains notions and truths of which no effort of sensation
or imagination can give us the slightest apprehension (Op. ii. 383).
He instances the capacity of forming "general notions"; the very
conception of universality itself (_ib._ 384), to which he says
brutes, who partake as truly as men in the faculty called _phantasia_,
never attain; the notion of God, whom he says we may imagine to be
corporeal, but understand to be incorporeal; and lastly, the reflex
action by which the mind makes its own phenomena and operations the
objects of attention.
The _Syntagma philosophicum_, in fact, is one of those eclectic
systems which unite, or rather place in juxtaposition, irreconcilable
dogmas from various schools of thought. It is divided, according to
the usual fashion of the Epicureans, into logic (which, with Gassendi
as with Epicurus, is truly _canonic_), physics and ethics. The logic,
which contains at least one praiseworthy portion, a sketch of the
history of the science, is divided into theory of right apprehension
(_bene imaginari_), theory of right judgment (_bene proponere_),
theory of right inference (_bene colligere_), theory of right method
(_bene ordinare_). The first part contains the specially empirical
positions which Gassendi afterwards neglects or leaves out of account.
The senses, the sole source of knowledge, are supposed to yield us
immediately cognition of individual things; phantasy (which Gassendi
takes to be material in nature) reproduces these ideas; understanding
compares these ideas, which are particular, and frames general ideas.
Nevertheless, he at the same time admits that the senses yield
knowledge--not of things--but of qualities only, and holds that we
arrive at the idea of thing or substance by induction. He holds that
the true method of research is the analytic, rising from lower to
higher notions; yet he sees clearly, and admits, that inductive
reasoning, as conceived by Bacon, rests on a general proposition not
itself proved by induction. He ought to hold, and in disputing with
Descartes he did apparently hold, that the evidence of the senses is
the only convincing evidence; yet he maintains, and from his special
mathematical training it was natural he should maintain, that the
evidence of reason is absolutely satisfactory. The whole doctrine of
judgment, syllogism and method is a mixture of Aristotelian and Ramist
notions.
In the second part of the _Syntagma_, the physics, there is more that
deserves attention; but here, too, appears in the most glaring manner
the inner contradiction between Gassendi's fundamental principles.
While approving of the Epicurean physics, he rejects altogether the
Epicurean negation of God and particular providence. He states the
various proofs for the existence of an immaterial, infinite, supreme
Being, asserts that this Being is the author of the visible universe,
and strongly defends the doctrine of the foreknowledge and particular
providence of God. At the same time he holds, in opposition to
Epicureanism, the doctrine of an immaterial rational soul, endowed
with immortality and capable of free determination. It is altogether
impossible to assent to the supposition of Lange (_Gesch. des
Materialismus_, 3rd ed., i. 233), that all this portion of Gassendi's
system contains nothing of his own opinions, but is introduced solely
from motives of self-defence. The positive exposition of atomism has
much that is attractive, but the hypothesis of the _calor vitalis_
(vital heat), a species of _anima mundi_ (world-soul) which is
introduced as physical explanation of physical phenomena, does not
seem to throw much light on the special problems which it is invoked
to solve. Nor is his theory of the weight essential to atoms as being
due to an inner force impelling them to motion in any way reconcilable
with his general doctrine of mechanical causes.
In the third part, the ethics, over and above the discussion on
freedom, which on the whole is indefinite, there is little beyond a
milder statement of the Epicurean moral code. The final end of life is
happiness, and happiness is harmony of soul and body (_tranquillitas
animi et indolentia corporis_). Probably, Gassendi thinks, perfect
happiness is not attainable in this life, but it may be in the life to
come.
The _Syntagma_ is thus an essentially unsystematic work, and clearly
exhibits the main characteristics of Gassendi's genius. He was
critical rather than constructive, widely read and trained thoroughly
both in languages and in science, but deficient in speculative power
and original force. Even in the department of natural science he shows
the same inability steadfastly to retain principles and to work from
them; he wavers between the systems of Brahe and Copernicus. That his
revival of Epicureanism had an important influence on the general
thinking of the 17th century may be admitted; that it has any real
importance in the history of philosophy cannot be granted.
AUTHORITIES.--Gassendi's life is given by Sorbière in the first
collected edition of the works, by Bugerel, _Vie de Gassendi_ (1737;
2nd ed., 1770), and by Damiron, _Mémoire sur Gassendi_ (1839). An
abridgment of his philosophy was given by his friend, the celebrated
traveller, Bernier (_Abrégé de la philosophie de Gassendi_, 8 vols.,
1678; 2nd ed., 7 vols., 1684). The most complete surveys of his work
are those of G.S. Brett (_Philosophy of Gassendi_, London, 1908),
Buhle (_Geschichte der neuern Philosophie_, iii. 1, 87-222), Damiron
(_Mémoires pour servir à l'histoire de philosophie au XVII^e siècle_),
and P.F. Thomas (_La Philosophie de Gassendi_, Paris, 1889). See also
Ritter, _Geschichte der Philosophie_, x. 543-571; Feuerbach, _Gesch.
d. neu. Phil. von Bacon bis Spinoza_, 127-150; F.X. Kiefl, _P.
Gassendis Erkenntnistheorie und seine Stellung zum Materialismus_
(1893) and "Gassendi's Skepticismus" in _Philos. Jahrb._ vi. (1893);
C. Güttler, "Gassend oder Gassendi?" in _Archiv f. Gesch. d. Philos._
x. (1897), pp. 238-242. (R. Ad.; X.)
FOOTNOTE:
[1] It was formerly thought that _Gassendi_ was really the genitive
of the Latin form _Gassendus_. C. Güttler, however, holds that it is
a modernized form of the O. Fr. _Gassendy_ (see paper quoted in
bibliography).
GASTEIN, in the duchy of Salzburg, Austria, a side valley of the Pongau
or Upper Salzach, about 25 m. long and 1¼ m. broad, renowned for its
mineral springs. It has an elevation of between 3000 and 3500 ft. Behind
it, to the S., tower the mountains Mallnitz or Nassfeld-Tauern (7907
ft.) and Ankogel (10,673 ft.), and from the right and left of these
mountains two smaller ranges run northwards forming its two side walls.
The river Ache traverses the valley, and near Wildbad-Gastein forms two
magnificent waterfalls, the upper, the Kesselfall (196 ft.), and the
lower, the Bärenfall (296 ft.). Near these falls is the Schleierfall
(250 ft.), formed by the stream which drains the Bockhart-see. The
valley is also traversed by the so-called Tauern railway (opened up to
Wildbad-Gastein in September 1905), which goes to Mallnitz, piercing the
Tauern range by a tunnel 9260 yds. in length. The principal villages of
the valley are Hof-Gastein, Wildbad-Gastein and Böckstein.
HOF-GASTEIN, pop. (1900) 840, the capital of the valley, is also a
watering-place, the thermal waters being conveyed here from
Wildbad-Gastein by a conduit 5 m. long, constructed in 1828 by the
emperor Francis I. of Austria. Hof-Gastein was, after Salzburg, the
richest place in the duchy, owing to its gold and silver mines, which
were already worked during the Roman period. During the 16th century
these mines were yielding annually 1180 lb. of gold and 9500 lb. of
silver, but since the 17th century they have been much neglected and
many of them are now covered by glaciers.
WILDBAD-GASTEIN, commonly called _Bad-Gastein_, one of the most
celebrated watering-places in Europe, is picturesquely situated in the
narrow valley of the Gasteiner Ache, at an altitude of 3480 ft. The
thermal springs, which issue from the granite mountains, have a
temperature of 77°-120° F., and yield about 880,000 gallons of water
daily. The water contains only 0.35 to 1000 of mineral ingredients and
is used for bathing purposes. The springs are resorted to in cases of
nervous affections, senile and general debility, skin diseases, gout and
rheumatism. Wildbad-Gastein is annually visited by over 8500 guests. The
springs were known as early as the 7th century, but first came into fame
by a successful visit paid to them by Duke Frederick of Austria in 1436.
Gastein was a favourite resort of William I. of Prussia and of the
Austrian imperial family, and it was here that, on the 14th of August
1865, was signed the agreement known as the Gastein Convention, which by
dividing the administration of the conquered provinces of Schleswig and
Holstein between Austria and Prussia postponed for a while the outbreak
of war between the two powers. It was also here (August-September 1879)
that Prince Bismarck negotiated with Count Julius Andrássy the
Austro-German treaty, which resulted in the formation of the Triple
Alliance.
See Pröll, _Gastein, Its Springs and Climate_ (Vienna, 5th ed., 1893).
GASTRIC ULCER (ulcer of the stomach), a disease of much gravity,
commonest in females, and especially in anaemic domestic servants. It is
connected in many instances with impairment of the circulation in the
stomach and the formation of a clot in a small blood-vessel
(thrombosis). It may be due to an impoverished state of the blood
(anaemia), but it may also arise from disease of the blood-vessels, the
result of long-continued indigestion and gastric catarrh.
When clotting takes place in a blood-vessel the nutrition of that
limited area of the stomach is cut off, and the patch undergoes
digestion by the unresisted action of the gastric juices, an ulcer being
formed. The ulcer is usually of the size of a silver threepence or
sixpence, round or oval, and, eating deeply, is apt to make a hole right
through the coats of the stomach. Its usual site is upon the posterior
wall of the upper curvature, near to the pyloric orifice. It may undergo
a healing process at any stage, in which case it may leave but little
trace of its existence; while, on the other hand, it may in the course
of cicatrizing produce such an amount of contraction as to lead to
stricture of the pylorus, or to a peculiar hour-glass deformity of the
stomach. Perforation is in most cases quickly fatal, unless previously
the stomach has become adherent to some neighbouring organ, by which the
dangerous effects of this occurrence may be averted, or unless the
condition has been promptly recognized and an operation has been quickly
done. Usually there is but one ulcer, but sometimes there are several
ulcers.
The symptoms of ulcer of the stomach are often indefinite and obscure,
and in some cases the diagnosis has been first made on the occurrence of
a fatal perforation. First among the symptoms is pain, which is present
at all times, but is markedly increased after food. The pain is situated
either at the lower end of the breast-bone or about the middle of the
back. Sometimes it is felt in the sides. It is often extremely severe,
and is usually accompanied with localized tenderness and also with a
sense of oppression, and by an inability to wear tight clothing. The
pain is due to the movements of the stomach set up by the presence of
the food, as well as to the irritation of the inflamed nerve filaments
in the floor of the ulcer. Vomiting is a usual symptom. It occurs either
soon after the food is swallowed or at a later period, and generally
relieves the pain and discomfort. Vomiting of blood (haematemesis) is a
frequent and important symptom. The blood may show itself in the form of
a brown or coffee-like mixture, or as pure blood of dark colour and
containing clots. It comes from some vessel or vessels which the
ulcerative process has ruptured. Blood is also found mixed with the
discharges from the bowels, rendering them dark or tarry-looking. The
general condition of the patient with gastric ulcer is, as a rule, that
of extreme ill-health, with pallor, emaciation and debility. The tongue
is red, and there is usually constipation. In most of the cases the
disease is chronic, lasting for months or years; and in those cases
where the ulcers are large or multiple, incomplete healing may take
place, relapses occurring from time to time. But the ulcers may give
rise to no marked symptoms, and there have been instances where fatal
perforation suddenly took place, and where post-mortem examination
revealed the existence of long-standing ulcers which had given rise to
no suggestive symptoms. While gastric ulcer is to be regarded as
dangerous, its termination, in the great majority of cases, is in
recovery. It frequently, however, leaves the stomach in a delicate
condition, necessitating the utmost care as regards diet. Occasionally
the disease proves fatal by sudden haemorrhage, but a fatal result is
more frequently due to perforation and the escape of the contents of the
stomach into the peritoneal cavity, in which case death usually occurs
in from twelve to forty-eight hours, either from shock or from
peritonitis. Should the stomach become adherent to another organ, and
fatal perforation be thus prevented, chronic "indigestion" may persist,
owing to interference with the natural movements of the stomach.
Stricture of the pylorus and consequent dilatation of the stomach may be
caused by the cicatrization of an ulcer.
The patient should at once be sent to bed and kept there, and allowed
for a while nothing stronger than milk and water or milk and lime water.
But if bleeding has recently taken place no food whatever should be
allowed by the stomach, and the feeding should be by nutrient enemata.
As the symptoms quiet down, eggs may be given beaten up with milk, and
later, bread and milk and home-made broths and soups. Thus the diet
advances to chicken and vegetables rubbed through a sieve, to custard
pudding and bread and butter. As regards medicines, iron is the most
useful, but no pills of any sort should be given. Under the influence of
rest and diet most gastric ulcers get well. The presence of
healthy-looking scars upon the surface of the stomach, which are
constantly found in operating upon the interior of the abdomen, or as
revealed in post-mortem examinations, are evidence of the truth of this
statement. It is unlikely that under the treatment just described
perforation of the stomach will take place, and if the surgeon is called
in to assist he will probably advise that operation is inadvisable.
Moreover, he knows that if he should open the abdomen to search for an
ulcer of the stomach he might fail to find it; more than that, his
search might also be in vain if he opened the stomach itself and
examined the interior. Serious haemorrhages, however, may make it
necessary that a prompt and thorough search should be made in order that
the surgeon may endeavour to locate the ulcer, and, having found it,
secure the damaged vessel and save the patient from death by bleeding.
Perforation of a gastric ulcer having taken place, the septic germs,
which were harmless whilst in the stomach, escape with the rest of the
contents of the stomach into the general peritoneal cavity. The
immediate effects of this leakage are sudden and severe pain in the
upper part of the abdomen and a great shock to the system (collapse).
The muscles of the abdominal wall become hard and resisting, and as
peritonitis appears and the intestines are distended with gas, the
abdomen is distended and becomes greatly increased in size and ceases to
move, the respiratory movements being short and quick. At first, most
likely, the temperature drops below normal, and the pulse quickens.
Later, the temperature rises. If nothing is done, death from the septic
poisoning of peritonitis is almost certain.
The treatment of ruptured gastric ulcer demands immediate operation. An
incision should be made in the upper part of the middle line of the
abdomen, and the perforation should be looked for. There is not, as a
rule, much difficulty in finding it, as there are generally deposits of
lymph near the spot, and other signs of local inflammation; moreover,
the contents of the stomach may be seen escaping from the opening. The
ulcer is to be closed by running a "purse-string" suture in the healthy
tissue around it, and the place is then buried in the stomach by picking
up small folds of the stomach-wall above and below it and fixing them
together by suturing. This being done, the surface of the stomach, and
the neighbouring viscera which have been soiled by the leakage, are
wiped clean and the abdominal wound is closed, provision being made for
efficient drainage. A large proportion of cases of perforated gastric
ulcer thus treated recover. (E. O.*)
GASTRITIS (Gr. [Greek: gastêr], stomach), an inflammatory affection of
the stomach, of which the condition of catarrh, or irritation of its
mucous membrane, is the most frequent and most readily recognized. This
may exist in an acute or a chronic form, and depends upon some
condition, either local or general, which produces a congested state of
the circulation in the walls of the stomach (see DIGESTIVE ORGANS:
_Pathology_).
_Acute Gastritis_ may arise from various causes. The most intense forms
of inflammation of the stomach are the toxic conditions which follow the
swallowing of corrosive poisons, such as strong mineral acids of alkalis
which may extensively destroy the mucous membrane. Other non-corrosive
poisons cause acute degeneration of the stomach wall (see POISONS).
Acute inflammatory conditions may be secondary to zymotic diseases such
as diphtheria, pyaemia, typhus fever and others. Gastritis is also
caused by the ingestion of food which has begun to decompose, or may
result from eating unsuitable articles which themselves remain
undigested and so excite acute catarrhal conditions. These give rise to
the symptoms well known as characterizing an acute "bilious attack,"
consisting in loss of appetite, sickness or nausea, and headache,
frontal or occipital, often accompanied with giddiness. The tongue is
furred, the breath foetid, and there is pain or discomfort in the region
of the stomach, with sour eructations, and frequently vomiting, first of
food and then of bilious matter. An attack of this kind tends to subside
in a few days, especially if the exciting cause be removed. Sometimes,
however, the symptoms recur with such frequency as to lead to the more
serious chronic form of the disease.
The treatment bears reference, in the first place, to any known source
of irritation, which, if it exist, may be expelled by an emetic or
purgative (except in cases due to poisoning). This, however, is seldom
necessary, since vomiting is usually present. For the relief of sickness
and pain the sucking of ice and counter-irritation over the region of
the stomach are of service. Further, remedies which exercise a soothing
effect upon an irritable mucous membrane, such as bismuth or weak
alkaline fluids, and along with these the use of a light milk diet, are
usually sufficient to remove the symptoms.
_Chronic Gastric Catarrh_ may result from the acute or may arise
independently. It is not infrequently connected with antecedent disease
in other organs, such as the lungs, heart, liver or kidneys, and it is
especially common in persons addicted to alcoholic excess. In this form
the texture of the stomach is more altered than in the acute form,
except in the toxic and febrile forms above referred to. It is
permanently in a state of congestion, and its mucous membrane and
muscular coat undergo thickening and other changes, which markedly
affect the function of digestion. The symptoms are those of dyspepsia in
an aggravated form (see DYSPEPSIA), of which discomfort and pain after
food, with distension and frequently vomiting, are the chief; and the
treatment must be conducted in reference to the causes giving rise to
it. The careful regulation of the diet, alike as to the amount, the
quality, and the intervals between meals, demands special attention.
Feeding on artificially soured milk may in many cases be useful. Lavage
or washing out of the stomach with weak alkaline solutions has been used
with marked success in the treatment of chronic gastritis. Of medicinal
agents, bismuth, arsenic, nux vomica, and the mineral acids are all of
acknowledged efficacy, as are also preparations of pepsin.
GASTROPODA, the second of the five classes of animals constituting the
phylum Mollusca. For a discussion of the relationship of the Gastropoda
to the remaining classes of the phylum, see MOLLUSCA.
The Gastropoda are mainly characterized by a loss of symmetry,
produced by torsion of the visceral sac. This torsion may be resolved
into two successive movements. The first is a ventral flexure in the
antero-posterior or sagittal plane; the result of this is to
approximate the two ends of the alimentary canal. In development, the
openings of the mantle-cavity and the anus are always originally
posterior; later they are brought forward ventrally. During this first
movement flexure is also produced by the coiling of the visceral sac
and shell; primitively the latter was bowl-shaped; but the ventral
flexure, which brings together the two extremities of the digestive
tube, gives the visceral sac the outline of a more or less acute cone.
The shell necessarily takes this form also, and then becomes coiled in
a dorsal or anterior plane--that is to say, it becomes exogastric.
This condition may be seen in embryonic _Patellidae, Fissurellidae_
and _Trochidae_ (fig. 1, A), and agrees with the method of coiling of
a mollusc without lateral torsion, such as _Nautilus_. But ultimately
the coil becomes ventral or endogastric, in consequence of the second
torsion movement then apparent.
[Illustration: From Lankester's Treatise on Zoology.
FIG. 1.--Three stages in the development of Trochus, during the
process of torsion. (After Robert.)
A, Nearly symmetrical larva (veliger).
B, A stage 1½ hours later than A.
C, A stage 3½ hours later than B.
f, Foot.
op, Operculum.
pac, Pallial cavity.
ve, Velum.]
The shell is represented as fixed, while the head and foot rotate from
left to right. In reality the head and foot are fixed and the shell
rotates from right to left.
The second movement is a lateral torsion of the visceral mass, the
foot remaining a fixed point; this torsion occurs in a plane
approximately at right angles to that of the first movement, and
carries the pallial aperture and the anus from behind forwards. If, at
this moment, the animal were placed with mouth and ventral surface
turned towards the observer, this torsion carries the circumanal
complex in a clockwise direction (along the right side in dextral
forms) through 180° as compared with its primitive condition. The
(primitively) right-hand organs of the complex thus become left-hand,
and vice versa. The visceral commissure, while still surrounding the
digestive tract, becomes looped; its right half, with its proper
ganglion, passes to the left side over the dorsal face of the
alimentary canal (whence the name supra-intestinal), while the left
half passes below towards the right side, thus originating the name
infra-intestinal given to this half and to its ganglion. Next, the
shell, the coil of which was at first exogastric, being also included
in this rotation through 180°, exhibits an endogastric coiling (fig.
1, B, C). This, however, is not generally retained in one plane, and
the spire projects, little by little, on the side which was originally
left, but finally becomes right (in dextral forms, with a clockwise
direction, if viewed from the side of the spire; but counter-clockwise
in sinistral forms). Finally, the original symmetry of the circumanal
complex vanishes; the anus leaves the centre of the pallial cavity and
passes towards the right side (left side in sinistral forms); the
organs of this side become atrophied and disappear. The essential
feature of the asymmetry of Gastropoda is the atrophy or disappearance
of the primitively left half of the circumanal complex (the right half
in sinistral forms), including the gill, the auricle, the osphradium,
the hypobranchial gland and the kidney.
In dextral Gastropods the only structure found on the topographically
right side of the rectum is the genital duct. But this is not part of
the primitive complex. It is absent in the most primitive and
symmetrical forms, such as _Haliotis_ and _Pleurotomaria_. Originally
the gonads opened into the kidneys. In the most primitive existing
Gastropods the gonad opens into the right kidney (_Patellidae,
Trochidae, Fissurellidae_). The gonaduct, therefore, is derived from
the topographically right kidney. The transformation has been
actually shown to take place in the development of Paludina. In a
dextral Gastropod the shell is coiled in a right-handed spiral from
apex to mouth, and the spiral also projects to the right of the median
plane of the animal.
[Illustration: From Lankester's _Treatise on Zoology_.
FIG. 2.--Four stages in the development of a Gastropod showing the
process of body torsion. (After Robert.)
A, Embryo without flexure.
B, Embryo with ventral flexure of the intestine.
C, Embryo with ventral flexure and exogastric shell.
D, Embryo with lateral torsion and an endogastric shell.
a, Anus.
f, Foot.
m, Mouth.
pa, Mantle.
pac, Pallial cavity.
ve, Velum.]
When the shell is sinistral the asymmetry of the organs is usually
reversed, and there is a complete situs _inversus viscerum_, the
direction of the spiral of the shell corresponding to the position of
the organs of the body. _Triforis, Physa, Clausilia_ are examples
of sinistral Gastropods, but reversal also occurs as an individual
variation among forms normally dextral. But there are forms in which
the involution is "hyperstrophic," that is to say, the turns of the
spire projecting but slightly, the spire, after flattening out
gradually, finally becomes re-entrant and transformed into a false
umbilicus; at the same time that part which corresponds to the
umbilicus of forms with a normal coil projects and constitutes a false
spire; the coil thus appears to be sinistral, although the asymmetry
remains dextral, and the coil of the operculum (always the opposite to
that of the shell) sinistral (e.g. _Lanistes_ among Streptoneura,
_Limacinidae_ among Opisthobranchia). The same, _mutatis mutandis_,
may occur in sinistral shells.
[Illustration: FIG 3.--Sketch of a model designed so as to show the
effect of torsion or rotation of the visceral hump in Streptoneurous
Gastropoda.
A, Unrotated ancestral condition.
B, Quarter-rotation.
C, Complete semi-rotation (the limit).
an, Anus.
ln, rn, Primarily left nephridium and primarily right nephridium.
lvg, Primarily left (subsequently the sub-intestinal) visceral
ganglion.
rvg, Primarily right (subsequently the sub-intestinal) visceral
ganglion.
cerg, Cerebral ganglion.
plg, Pleural ganglion.
pedg, Pedal ganglion.
abg, Abdominal ganglion.
bucc, Buccal mass.
W, Wooden arc representing the base-line of the wall of the visceral
hump.
x, 'x, Pins fastening the elastic cord (representing the visceral
nerve loop) to W.]
The problem of the causes of the torsion of the Gastropod body has
been much discussed. E.R. Lankester in the ninth edition of this work
attributed it to the pressure of the shell and visceral hump towards
the right side. He referred also to the nautiloid shell of the larva
falling to one side. But these are two distinct processes. In the
larva a nautiloid shell is developed which is coiled exogastrically,
that is, dorsally, and the pallial cavity is posterior or ventral
(fig. 2, C): the larva therefore resembles _Nautilus_ in the relations
of body and shell. The shell then rotates towards the left side
through 180°, so that it becomes ventral or endogastric (fig. 2, D).
The pallial cavity, with its organs, is by this torsion moved up the
_right_ side of the larva to the dorsal surface, and thus the left
organs become right and vice versa. In the subsequent growth of the
shell the spire comes to project on the right side, which was
originally the left. Neither the rotation of the shell as a whole nor
its helicoid spiral coiling is the immediate cause of the torsion of
the body in the individual, for the direction of the torsion is
indicated in the segmentation of the ovum, in which there is a
complete reversal of the cleavage planes in sinistral as compared
with dextral forms. The facts, however, strongly suggest that the
original cause of the torsion was the weight of the exogastric shell
and visceral hump, which in an animal creeping on its ventral surface
necessarily fell over to one side. It is not certain that the
projection of the spire to the originally left side of the shell has
anything to do with the falling over of the shell to that side. The
facts do not support such a suggestion. In the larva there is no
projection at the time the torsion takes place. In some forms the
coiling disappears in the adult, leaving the shell simply conical as
in _Patellidae, Fissurellidae_, &c., and in some cases the shell is
coiled in one plane, e.g. _Planorbis_. In all these cases the torsion
and asymmetry of the body are unaffected.
The characteristic torsion attains its maximum effect among the
majority of the Streptoneura. It is followed in some specialized
Heteropoda and in the Euthyneura by a torsion in the opposite
direction, or detorsion, which brings the anus farther back and
untwists the visceral commissure (see Euthyneura, below). This
conclusion has shown that the Euthyneura do not represent an archaic
form of Gastropoda, but are themselves derived from streptoneurous
forms. The difference between the two sub-classes has been shown to be
slight; certain of the more archaic Tectibranchia (_Actaeon_) and
Pulmonata (_Chilina_) still have the visceral commissure long and not
untwisted. The fact that all the Euthyneura are hermaphrodite is not a
fundamental difference; several Streptoneura are so, likewise
_Valvata, Oncidiopsis, Marsenina, Odostomia, Bathysciadium,
Entoconcha_.
_Classification._--The class Gastropoda is subdivided as follows:
Sub-class I. Streptoneura.
Order 1. Aspidobranchia.
Sub-order 1. Docoglossa.
" 2. Rhipidoglossa.
Order 2. Pectinibranchia.
Sub-order 1. Taenioglossa.
Tribe 1. Platypoda.
" 2. Heteropoda.
Sub-order 2. Stenoglossa.
Tribe 1. Rachiglossa.
" 2. Toxiglossa.
Sub-class II. Euthyneura.
Order 1. Opisthobranchia.
Sub-order 1. Tectibranchia.
Tribe 1. Bullomorpha.
" 2. Aplysiomorpha.
" 3. Pleurobranchomorpha.
Sub-order 2. Nudibranchia.
Tribe 1. Tritoniomorpha.
" 2. Doridomorpha.
" 3. Eolidomorpha.
" 4. Elysiomorpha.
Order 2. Pulmonata.
Sub-order 1. Basommatophora.
" 2. Stylommatophora.
Tribe 1. Holognatha.
" 2. Agnatha.
" 3. Elasmognatha.
" 4. Ditremata.
Sub-Class I.--STREPTONEURA
In this division the torsion of the visceral mass and visceral
commissure is at its maximum, the latter being twisted into a figure of
eight. The right half of the commissure with its ganglion is
supra-intestinal, the left half with its ganglion infra-intestinal. In
some cases each pleural ganglion is connected with the opposite branch
of the visceral commissure by anastomosis with the pallial nerve, a
condition which is called dialyneury; or there may be a direct
connective from the pleural ganglion to the visceral ganglion of the
opposite side, which is called zygoneury. The head bears only one pair
of tentacles. The radular teeth are of several different kinds in each
transverse row. The heart is usually posterior to the branchia
(proso-branchiate). The sexes are usually separate.
The old division into Zygobranchia and Azygobranchia must be abandoned,
for the Azygobranchiate Rhipidoglossa have much greater affinity to the
Zygobranchiate _Haliotidae_ and _Fissurellidae_ than to the
Azygobranchia in general. This is shown by the labial commissure and
pedal cords of the nervous system, by the opening of the gonad into the
right kidney, and by other points. Further, the _Pleurotomariidae_ have
been discovered to possess two branchiae. The sub-class is now divided
into two orders: the Aspidobranchia in which the branchia or ctenidium
is bipectinate and attached only at its base, and the Pectinibranchia in
which the ctenidium is monopectinate and attached to the mantle
throughout its length.
[Illustration: FIG. 4.--The Common Limpet (_Patella vulgata_) in its
shell, seen from the pedal surface. (Lankester.)
x, y, The median antero-posterior axis.
a, Cephalic tentacle.
b, Plantar surface of the foot.
c, Free edge of the shell.
d, The branchial efferent vessel carrying aerated blood to the
auricle, and here interrupting the circlet of gill lamellae.
e, Margin of the mantle-skirt.
f, Gill lamellae (_not_ ctenidia, but special pallial growths,
comparable with those of Pleurophyllidia).
g, The branchial efferent vessel.
h, Factor of the branchial advehent vessel.
i, Interspaces between the muscular bundles of the root of the foot,
causing the separate areae seen in fig. 5, c.]
Order I. ASPIDOBRANCHIA.--These are the most primitive Gastropods,
retaining to a great degree the original symmetry of the organs of the
pallial complex, having two kidneys, in some cases two branchiae, and
two auricles. The gonad has no accessory organs and except in
_Neritidae_ no duct, but discharges into the right kidney.
[Illustration: FIG. 5.--Dorsal surface of the Limpet removed from its
shell and deprived of its black pigmented epithelium; the internal
organs are seen through the transparent body-wall. (Lankester.)
c, Muscular bundles forming the root of the foot, and adherent to
the shell.
e, Free mantle-skirt.
em, Tentaculiferous margin of the same.
i, Smaller (left) nephridium.
k, Larger (right) nephridium.
l, Pericardium.
lx, Fibrous septum, behind the pericardium.
n, Liver.
int, Intestine.
ecr, Anterior area of the mantle-skirt over-hanging the head
(cephalic hood).]
Forms adapted to terrestrial life and to aerial respiration occur in
various divisions of Gastropods, and do not constitute a single
homogeneous group. Thus the _Helicinidae_, which are terrestrial, are
now placed among the Aspidobranchia. In these there are neither
branchia nor osphradium, and the pallial chamber which retains its
large opening serves as a lung. Degeneration of the shell occurs in
some members of the order. It is largely covered by the mantle in some
_Fissurellidae_, is entirely internal in _Pupilia_ and absent in
_Titiscaniidae_.
The common limpet is a specially interesting and abundant example of
the more primitive Aspidobranchia. The foot of the limpet is a nearly
circular disk of muscular tissue; in front, projecting from and raised
above it, are the head and neck (figs. 4, 13). The visceral hump forms
a low conical dome above the sub-circular foot, and standing out all
round the base of this dome so as completely to overlap the head and
foot, is the circular mantle-skirt. The depth of free mantle-skirt is
greatest in front, where the head and neck are covered in by it. Upon
the surface of the visceral dome, and extending to the edge of the
free mantle-skirt, is the conical shell. When the shell is taken away
(best effected by immersion in hot water) the surface of the visceral
dome is found to be covered by a black-coloured epithelium, which may
be removed, enabling the observer to note the position of some organs
lying below the transparent integument (fig. 5). The muscular columns
(c) attaching the foot to the shell form a ring incomplete in front,
external to which is the free mantle-skirt. The limits of the large
area formed by the flap over the head and neck (ecr) can be traced,
and we note the anal papilla showing through and opening on the right
shoulder, so to speak, of the animal into the large anterior region of
the sub-pallial space. Close to this the small renal organ (i, mediad)
and the larger renal organ (k, to the right and posteriorly) are seen,
also the pericardium (l) and a coil of the intestine (int) embedded
in the compact liver.
[Illustration: Fig. 6.--Anterior portion of the same Limpet, with the
overhanging cephalic hood removed. (Lankester.)
a, Cephalic tentacle.
b, Foot.
c, Muscular substance forming the root of the foot.
d, The capito-pedal organs of Lankester (= rudimentary ctenidia).
e, Mantle-skirt.
f, Papilla of the larger nephridium.
g, Anus.
h, Papilla of the smaller nephridium.
i, Smaller nephridium.
k, Larger nephridium.
l, Pericardium.
m, Cut edge of the mantle-skirt.
n, Liver.
p, Snout.]
On cutting away the anterior part of the mantle-skirt so as to expose
the sub-pallial chamber in the region of the neck, we find the right
and left renal papillae (discovered by Lankester in 1867) on either
side of the anal papilla (fig. 6), but no gills. If a similar
examination be made of the allied genus _Fissurella_ (fig. 17, d), we
find right and left of the two renal apertures a right and left
gill-plume or ctenidium, which here as in _Haliotis_ and
_Pleurotomaria_ retain their original paired condition. In _Patella_
no such plumes exist, but right and left of the neck are seen a pair
of minute oblong yellow bodies (fig. 6, d), which were originally
described by Lankester as orifices possibly connected with the
evacuation of the generative products. On account of their position
they were termed by him the "capito-pedal orifices," being placed near
the junction of head and foot. J.W. Spengel has, however, in a most
ingenious way shown that these bodies are the representatives of the
typical pair of ctenidia, here reduced to a mere rudiment. Near to
each rudimentary ctenidium Spengel has discovered an olfactory patch
or osphradium (consisting of modified epithelium) and an olfactory
nerve-ganglion (fig. 8). It will be remembered that, according to
Spengel, the osphradium of mollusca is definitely and intimately
related to the gill-plume or ctenidium, being always placed near the
base of that organ; further, Spengel has shown that the nerve-supply
of this olfactory organ is always derived from the visceral loop.
Accordingly, the nerve-supply affords a means of testing the
conclusion that we have in Lankester's capito-pedal bodies the
rudimentary ctenidia. The accompanying diagrams (figs. 9, 10) of the
nervous systems of _Patella_ and of _Haliotis_, as determined by
Spengel, show the identity in the origin of the nerves passing from
the visceral loop to Spengel's olfactory ganglion of the Limpet, and
that of the nerves which pass from the visceral loop of _Haliotis_ to
the olfactory patch or osphradium, which lies in immediate relation on
the right and on the left side to the right and left gill-plumes
(ctenidia) respectively. The same diagrams serve to demonstrate the
streptoneurous condition of the visceral loop in Aspidobranchia.
[Illustration: FIG. 7.--The same specimen viewed from the left front,
so as to show the sub-anal tract (ff) of the larger nephridium, by
which it communicates with the pericardium. o, Mouth; other letters as
in fig. 6.]
Thus, then, we find that the limpet possesses a symmetrically disposed
pair of ctenidia in a rudimentary condition, and justifies its
position among Aspidobranchia. At the same time it possesses a
totally distinct series of _functional_ gills, which are not derived
from the modification of the typical molluscan ctenidium. These gills
are in the form of delicate lamellae (fig. 4, f), which form a series
extending completely round the inner face of the depending
mantle-skirt. This circlet of gill-lamellae led Cuvier to class the
limpets as Cyclobranchiata, and, by erroneous identification of them
with the series of metamerically repeated ctenidia of _Chiton_, to
associate the latter mollusc with the former. The gill-lamellae of
_Patella_ are processes of the mantle comparable with the plait-like
folds often observed on the roof of the branchial chamber in other
Gastropoda (e.g. _Buccinum_ and _Haliotis_). They are termed pallial
gills. The only other molluscs in which they are exactly represented
are the curious Opisthobranchs _Phyllidia_ and _Pleurophyllidia_ (fig.
55). In these, as in _Patella_, the typical ctenidia are aborted, and
the branchial function is assumed by close-set lamelliform processes
arranged in a series beneath the mantle-skirt on either side of the
foot. In fig. 4, d, the large branchial vein of _Patella_ bringing
blood from the gill-series to the heart is seen; where it crosses the
series of lamellae there is a short interval devoid of lamellae.
[Illustration: Fig. 8.--A, Section in a plane vertical to the surface
of the neck of _Patella_ through a, the rudimentary ctenidium
(Lankester's organ), and b, the olfactory epithelium (osphradium); c,
the olfactory (osphradial) ganglion. (After Spengel.)
B, Surface view of a rudimentary ctenidium of _Patella_ excised and
viewed as a transparent object. (Lankester.)]
[Illustration: Fig. 9.--Nervous system of _Patella_; the visceral loop
is lightly shaded; the buccal ganglia are omitted. (After Spengel.)
ce, Cerebral ganglia.
c'e, Cerebral commissure.
pl, Pleural ganglion.
pe, Pedal ganglion. p'e, Pedal nerve.
s, s', Nerves (right and left) to the mantle.
o, Olfactory ganglion, connected by nerve to the streptoneurous
visceral loop.]
The heart in _Patella_ consists of a single auricle (not two as in
_Haliotis_ and _Fissurella_) and a ventricle; the former receives the
blood from the branchial vein, the latter distributes it through a
large aorta which soon leads into irregular blood-lacunae.
The existence of two renal organs in _Patella_, and their relation to
the pericardium (a portion of the coelom), is important. Each renal
organ is a sac lined with glandular epithelium (ciliated cell, with
concretions) communicating with the exterior by its papilla, and by a
narrow passage with the pericardium. The connexion with the
pericardium of the smaller of the two renal organs was demonstrated by
Lankester in 1867, at a time when the fact that the renal organ of the
Mollusca, as a rule, opens into the pericardium, and is therefore a
typical nephridium, was not known. Subsequent investigations carried
on under the direction of the same naturalist have shown that the
larger as well as the smaller renal sac is in communication with the
pericardium. The walls of the renal sacs are deeply plaited and thrown
into ridges. Below the surface these walls are excavated with
blood-vessels, so that the sac is practically a series of
blood-vessels covered with renal epithelium, and forming a meshwork
within a space communicating with the exterior. The larger renal sac
(remarkably enough, that which is aborted in other Anisopleura)
extends between the liver and the integument of the visceral dome very
widely. It also bends round the liver as shown in fig. 12, and forms
a large sac on half of the upper surface of the muscular mass of the
foot. Here it lies close upon the genital body (ovary or testis), and
in such intimate relationship with it that, when ripe, the gonad
bursts into the renal sac, and its products are carried to the
exterior by the papilla on the right side of the anus (Robin, Dall).
This fact led Cuvier erroneously to the belief that a duct existed
leading from the gonad to this papilla. The position of the gonad,
best seen in the diagrammatic section (fig. 13), is, as in other
Aspidobranchia, devoid of a special duct communicating with the
exterior. This condition, probably an archaic one, distinguishes the
Aspidobranchia from other Gastropoda.
[Illustration: FIG. 10.--Nervous system of _Haliotis_; the visceral
loop is lightly shaded; the buccal ganglia are omitted. (After
Spengel.)
ce, Cerebral ganglion.
pl.pe, The fused pleural and pedal ganglia.
pe, The right pedal nerve.
ce.pl, The cerebro-pleural connective.
ce.pe, The cerebro-pedal connective.
s, s', Right and left mantle nerves.
ab, Abdominal ganglion or site of same.
o, o, Right and left olfactory ganglia and osphardia receiving nerve
from visceral loop.]
[Illustration: FIG. 11.--Nervous system of _Fissurella_. (From
Gegenbaur, after Jhering.)
pl, Pallial nerve.
p, Pedal nerve.
A, Abdominal ganglia in the streptoneurous visceral commissure, with
supra- and sub-intestine ganglion on each side.
B, Buccal ganglia.
C, C, Cerebral ganglia.
es, Cerebral commissure.
o, Otocysts attached to the cerebro-pedal connectives.]
[Illustration: FIG. 12.--Diagram of the two renal organs (nephridia),
to show their relation to the rectum and to the pericardium.
(Lankester.)
f, Papilla of the larger nephridium.
g, Anal papilla with rectum leading from it.
h, Papilla of the smaller nephridium, which is only represented by
dotted outlines.
l, Pericardium indicated by a dotted outline--at its right side are
seen the two reno-pericardial pores.
ff, The sub-anal tract of the large nephridium given off near its
papilla and seen through the unshaded smaller nephridium.
ks.a, Anterior superior lobe of the large nephridium.
ks.l, Left lobe of same.
ks.p, Posterior lobe of same.
ks.i, Inferior sub-visceral lobe of same.]
[Illustration: FIG. 13.--Diagram of a vertical antero-postero median
section of a Limpet. Letters as in figs. 6, 7, with following
additions. (Lankester.)
q, Intestine in transverse section.
r, Lingual sac (radular sac).
rd, Radula.
s, Lamellated stomach.
t, Salivary gland.
u, Duct of same.
v, Buccal cavity
w, Gonad.
br.a, Branchial advehent vessel (artery).
br.v, Branchial efferent vessel (vein).
bv, Blood-vessel.
odm, Muscles and cartilage of the odontophore.
cor, Heart within the pericardium.]
[Illustration: FIG. 14.--Vertical section in a plane running right and
left through the anterior part of the visceral hump of _Patella_ to
show the two renal organs and their openings into the pericardium.
(J.T. Cunningham.)
a, Large or external or right renal organ.
ab, Narrow process of the same running _below_ the intestine and
leading by k into the pericardium.
b, Small or median renal organ.
c, Pericardium.
d, Rectum.
e, Liver.
f, Manyplies.
g, Epithelium of the dorsal surface.
h, Renal epithelium lining the renal sacs.
i, Aperture connecting the small sac with the pericardium.
k, Aperture connecting the large sac with the pericardium.]
The digestive tract of _Patella_ offers some interesting features. The
odontophore is powerfully developed; the radular sac is
extraordinarily long, lying coiled in a space between the mass of the
liver and the muscular foot. The radula has 160 rows of teeth with
twelve teeth in each row. Two pairs of salivary ducts, each leading
from a salivary gland, open into the buccal chamber. The oesophagus
leads into a remarkable stomach, plaited like the manyplies of a
sheep, and after this the intestine takes a very large number of turns
embedded in the yellow liver, until at last it passes between the two
renal sacs to the anal papilla. A curious ridge (spiral? valve) which
secretes a slimy cord is found upon the inner wall of the intestine.
The general structure of the Molluscan intestine has not been
sufficiently investigated to render any comparison of this structure
of _Patella_ with that of other Mollusca possible. The eyes of the
limpet deserve mention as examples of the most primitive kind of eye
in the Molluscan series. They are found one on each cephalic tentacle,
and are simply minute open pits or depressions of the epidermis, the
epidermic cells lining them being pigmented and connected with nerves
(compare fig. 14, art. CEPHALOPODA). The limpet breeds upon the
southern English coast in the early part of April, but its development
has not been followed. It has simply been traced as far as the
formation of a diblastula which acquires a ciliated band, and becomes
a nearly spherical trochosphere. It is probable that the limpet takes
several years to attain full growth, and during that period it
frequents the same spot, which becomes gradually sunk below the
surrounding surface, especially if the rock be carbonate of lime. At
low tide the limpet (being a strictly intertidal organism) is exposed
to the air, and (according to trustworthy observers) quits its
attachment and walks away in search of food (minute encrusting algae),
and then once more returns to the identical spot, not an inch in
diameter, which belongs, as it were, to it. Several million
limpets--twelve million in Berwickshire alone--are annually used on
the east coast of Britain as bait.
Sub-order 1. _Docoglossa._--Nervous system without dialyneury. Eyes
are open invaginations without crystalline lens. Two osphradia present
but no hypobranchial glands nor operculum. Teeth of radula beam-like,
and at most three marginal teeth on each side. Heart has only a single
auricle, neither heart nor pericardium traversed by rectum. Shell
conical without spire.
Fam. 1.--_Acmaeidae._ A single bipectinate ctenidium on left side.
Acmaea, without pallial branchiae, British. Scurria, with pallial
branchiae in a circle beneath the mantle.
Fam. 2.--_Tryblidiidae._ Muscle scar divided into numerous
impressions. _Tryblidium_, Silurian.
Fam. 3.--_Patellidae_. No ctenidia but pallial branchiae in a circle
between mantle and foot. _Patella_, pallial branchiae forming a
complete circle, no epipodial tentacles, British. _Ancistromesus_,
radula with median central tooth. _Nacella_, epipodial tentacles
present. _Helcion_, circlet of branchiae interrupted anteriorly,
British.
Fam. 4.--_Lepetidae._ Neither ctenidia nor pallial branchiae.
_Lepeta_, without eyes. _Pilidium. Propilidium._
Fam. 5.--_Bathysciadidae._ Hermaphrodite; head with appendage on
right side; radula without central tooth. _Bathysciadium_, abyssal.
Sub-order 2. RHIPIDOGLOSSA.--Aspidobranchia with a palliovisceral
anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens;
ctenidia, osphradia and hypobranchial glands paired or single. Radula
with very numerous marginal teeth arranged like the rays of a fan.
Heart with two auricles; ventricle traversed by the rectum, except in
the _Helicinidae_. An epipodial ridge on each side of the foot and
cephalic expansions between the tentacles often present.
Fam. 1.--_Pleurotomariidae_. Shell spiral; mantle and shell with an
anterior fissure; two ctenidia; a horny operculum. _Pleurotomaria_,
epipodium without tentacles. Genus includes several hundred extinct
species ranging from the Silurian to the Tertiary. Five living
species from the Antilles, Japan and the Moluccas. Moluccan species
is 19 cm. in height.
Fam. 2.--_Bellerophontidae._ 300 species, all fossil, from Cambrian
to Trias.
Fam. 3.--_Euomphalidae._ Also extinct, from Cambrian to Cretaceous.
Fam. 4.--_Haliotidae._ Spire of shell much reduced; two bipectinate
ctenidia, the right being the smaller; no operculum. Haliotis.
Fam. 5.--_Velainiellidae_, an extinct family from the Eocene.
[Illustration: FIG. 15.--_Halio tistuberculata._ d, Foot; i,
tentacular processes of the mantle. (From Owen, after Cuvier.)]
Fam. 6.--_Fissurellidae._ Shell conical; slit or hole in anterior
part of mantle; two symmetrical ctenidia; no operculum.
_Emarginula_, mantle and shell with a slit, British. _Scutum_,
mantle split anteriorly and reflected over shell, which has no slit.
_Puncturella_, mantle and shell with a foramen in front of the apex,
British. _Fissurella_, mantle and shell perforated at apex, British.
Fam. 7.--_Cocculinidae._ Shell conical, symmetrical, without slit or
perforation. _Cocculina_, abyssal.
Fam. 8.--_Trochidae._ Shell spirally coiled; a single ctenidium;
eyes perforated; a horny operculum; lobes between the tentacles.
_Trochus_, shell umbilicated, spire pointed and prominent, British.
_Monodonta_, no jaws, spire not prominent, no umbilicus, columella
toothed. _Gibbula_, with jaws, three pairs of epipodial cirri
without pigment spots at their bases, British. _Margarita_, five to
seven pairs of epipodial cirri with a pigment spot at base of each.
[Illustration: FIG. 16.--_Scutum_, seen from the pedal surface.
(Lankester.)
o, Mouth.
T, Cephalic tentacle.
br, One of the two symmetrical gills placed on the neck.]
[Illustration: FIG. 17.--Dorsal aspect of a specimen of _Fissurella_
from which the shell has been removed, whilst the anterior area of the
mantle-skirt has been longitudinally slit and its sides reflected.
(Lankester.)
a, Cephalic tentacle.
b, Foot.
d, Left (archaic right) gill-plume.
e, Reflected mantle-flap.
fi, The fissure or hole in the mantle-flap traversed by the
longitudinal incision.
f, Right (archaic left) nephridium's aperture.
g, Anus.
h, Left (archaic right) aperture of nephridium.
p, Snout.]
Fam. 9.--_Stomatellidae._ Spire of shell much reduced; a single
ctenidium. _Stomatella_, foot truncated posteriorly, an operculum
present, no epipodial tentacles. _Gena_, foot elongated posteriorly,
no operculum.
Fam. 10.--_Delphinulidae._ Shell spirally coiled; operculum horny;
intertentacular lobes absent. _Delphinula._
Fam. 11.--_Liotiidae_, shell globular, margin of aperture thickened.
_Liotia_.
Fam. 12.--_Cyclostrematidae._ Shell flattened, umbilicated; foot
anteriorly truncated with angles produced into lobes. _Cyclostrema._
_Teinostoma._
Fam. 13.--_Trochonematidae._ All extinct, Cambrian to Cretaceous.
Fam. 14.--_Turbinidae._ Shell spirally coiled; epipodial tentacles
present; operculum thick and calcareous. _Turbo. Astralium. Molleria.
Cyclonema._
Fam. 15.--_Phasianellidae._ Shell not nacreous, without umbilicus,
with prominent spire and polished surface. _Phasianella._
Fam. 16.--_Umboniidae._ Shell flattened, not umbilicated, generally
smooth; operculum horny. _Umbonium. Isanda._
Fam. 17.--_Neritopsidae._ Shell semi-globular, with short spire;
operculum calcareous, not spiral. _Neritopsis. Naticopsis_,
extinct.
Fam. 18.--_Macluritidae._ Extinct, Cambrian and Silurian.
Fam. 19.--_Neritidae._ Shell with very low spire, without umbilicus,
internal partitions frequently absorbed; a single ctenidium; a
cephalic penis present. _Nerita_, marine. _Neritina_, freshwater,
British. _Septaria_, shell boat-shaped.
Fam. 20.--_Titiscaniidae._ Without shell and operculum, but with
pallial cavity and ctenidium. _Titiscania_, Pacific.
Fam. 21.--_Helicinidae._ No ctenidium, but a pulmonary cavity; heart
with a single auricle, not traversed by the rectum. _Helicina.
Eutrochatella. Stoastoma. Bourceria._
Fam. 22.--_Hydrocenidae._ No ctenidium, but a pulmonary cavity;
operculum with an apophysis. _Hydrocena_, Dalmatia.
Fam. 23.--_Proserpinidae._ No operculum. _Proserpina_, Central
America.
Order 2. PECTINIBRANCHIA.--In this order there is no longer any trace
of bilateral symmetry in the circulatory, respiratory and excretory
organs, the topographically right half of the pallial complex having
completely disappeared, except the right kidney, which is represented
by the genital duct. There is usually a penis in the male. The
ctenidium is monopectinate and attached to the mantle along its whole
length, except in _Adeorbis_ and _Valvata_; in the latter alone it is
bipectinate. There is a single well-developed, often pectinated
osphradium. The eye is always a closed vesicle, and the internal
cornea is extensive. In the radula there is a single central tooth or
none.
[Illustration: FIG. 18.--Animal and shell of _Pyrula laevigata_. (From
Owen.)
a, Siphon.
b, Head-tentacles.
C, Head, the letter placed near the right eye.
d, The foot, expanded as in crawling.
h, The mantle-skirt reflected over the sides of the shell.]
The former classification into Holochlamyda, Pneumochlamyda and
Siphonochlamyda has been abandoned, as it was founded on adaptive
characters not always indicative of true affinities. The order is now
divided into two sub-orders: the Taenioglossa, in which there are
three teeth on each side of the median tooth of the radula, and the
Stenoglossa, in which there is only one tooth on each side of the
median tooth. In the latter a pallial siphon, a well-developed
proboscis and an unpaired oesophageal gland are always present, in the
former they are usually absent. The siphon is an incompletely tubular
outgrowth of the mantle margin on the left side, contained in a
corresponding outgrowth of the edge of the shell-mouth, and serving to
conduct water to the respiratory cavity.
The condition usually spoken of as a "proboscis" appears to be derived
from the condition of a simple rostrum (having the mouth at its
extremity) by the process of _incomplete introversion_ of that simple
rostrum. There is no reason in the actual significance of the word why
the term "proboscis" should be applied to an alternately introversible
and eversible tube connected with an animal's body, and yet such is a
very customary use of the term. The introversible tube may be
completely closed, as in the "proboscis" of Nemertine worms, or it may
have a passage in it leading into a non-eversible oesophagus, as in
the present case, and in the case of the eversible pharynx of the
predatory Chaetopod worms. The diagrams here introduced (fig. 19) are
intended to show certain important distinctions which obtain amongst
the various "introverts," or intro- and e-versible tubes so frequently
met with in animal bodies. Supposing the tube to be completely
introverted and to commence its eversion, we then find that eversion
may take place, either by a forward movement of the side of the tube
near its attached base, as in the proboscis of the Nemertine worms,
the pharynx of Chaetopods and the eye-tentacle of Gastropods, or by a
forward movement of the inverted apex of the tube, as in the proboscis
of the Rhabdocoel Planarians, and in that of Gastropods here under
consideration. The former case we call "pleurecbolic" (fig. 19, A, B,
C, H, I, K), the latter "acrecbolic" tubes or introverts (fig. 19, D,
E, F, G). It is clear that, if we start from the condition of full
eversion of the tube and watch the process of introversion, we shall
find that the pleurecbolic variety is introverted by the apex of the
tube sinking inwards; it may be called acrembolic, whilst conversely
the acrecbolic tubes are pleurembolic. Further, it is obvious enough
that the process either of introversion or of eversion of the tube may
be arrested at any point, by the development of fibres connecting the
wall of the introverted tube with the wall of the body, or with an
axial structure such as the oesophagus; on the other hand, the range
of movement of the tubular introvert may be unlimited or complete. The
acrembolic proboscis or frontal introvert of the Nemertine worms has a
complete range. So has the acrembolic pharynx of Chaetopods, if we
consider the organ as terminating at that point where the jaws are
placed and the oesophagus commences. So too the acrembolic
eye-tentacle of the snail has a complete range of movement, and also
the pleurembolic proboscis of the Rhabdocoel prostoma. The introverted
rostrum of the Pectinibranch Gastropods presents in contrast to these
a limited range of movement. The "introvert" in these Gastropods is
not the pharynx as in the Chaetopod worms, but a prae-oral structure,
its apical limit being formed by the true lips and jaws, whilst the
apical limit of the Chaetopod's introvert is formed by the jaws placed
at the junction of pharynx and oesophagus, so that the Chaetopod's
introvert is part of the stomodaeum or fore-gut, whilst that of the
Gastropod is external to the alimentary canal altogether, being in
front of the mouth, not behind it, as is the Chaetopod's. Further, the
Gastropod's introvert is pleurembolic (and therefore acrecbolic), and
is limited both in eversion and in introversion; it cannot be
completely everted owing to the muscular bands (fig. 19, G), nor can
it be fully introverted owing to the bands (fig. 19, F) which tie the
axial pharynx to the adjacent wall of the apical part of the
introvert. As in all such intro- and e-versible organs, eversion of
the Gastropod proboscis is effected by pressure communicated by the
muscular body-wall to the liquid contents (blood) of the body-space,
accompanied by the relaxation of the muscles which directly pull upon
either the sides or the apex of the tubular organ. The inversion of
the proboscis is effected directly by the contraction of these
muscles. In various members of the Pectinibranchia the mouth-bearing
cylinder is introversible (i.e. is a _proboscis_)--with rare
exceptions these forms have a siphonate mantle-skirt. On the other
hand, many which have a siphonate mantle-skirt are not provided with
an introversible mouth-bearing cylinder, but have a simple
non-introversible rostrum, as it has been termed, which is also the
condition presented by the mouth-bearing region in nearly all other
Gastropoda. One of the best examples of the introversible
mouth-cylinder or proboscis which can be found is that of the common
whelk (_Buccinum undatum_) and its immediate allies. In fig. 23 the
proboscis is seen in an everted state; it is only so carried when
feeding, being withdrawn when the animal is at rest. Probably its use
is to enable the animal to introduce its rasping and licking apparatus
into very narrow apertures for the purposes of feeding, e.g. into a
small hole bored in the shell of another mollusc.
[Illustration: FIG. 19.--Diagrams explanatory of the nature of
so-called proboscides or "introverts." (Lankester.)
A, Simple introvert completely introverted.
B, The same, partially everted by eversion of the sides, as in the
Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic.
C, The same, fully everted.
D, E, A similar simple introvert in course of eversion by the
forward movement, not of its sides, but of its apex, as in the
proboscidean Rhabdocoels = acrecbolic.
F, Acrecbolic (= pleurembolic) introvert, formed by the snout of the
proboscidiferous Gastropod. al, alimentary canal; d, the true mouth.
The introvert is not a simple one with complete range both in
eversion and introversion, but is arrested in introversion by the
fibrous bands at c, and similarly in eversion by the fibrous bands
at b.
G, The acrecbolic snout of a proboscidiferous Gastropod, arrested
short of complete eversion by the fibrous band b.
H, The acrembolic (= pleurecbolic) pharynx of a Chaetopod fully
introverted. al, alimentary canal; at d, the jaws; at a, the mouth;
therefore a to d is stomodaeum, whereas in the Gastropod (F) a to d
is inverted body-surface.
I, Partial eversion of H.
K, Complete eversion of H.]
[Illustration: FIG. 20.--Male of _Littorina littoralis_, Lin., removed
from its shell; the mantle-skirt cut along its right line of
attachment and thrown over to the left side of the animal so as to
expose the organs on its inner face.
a, Anus.
i, Intestine.
r, Nephridium (kidney).
r', Aperture of the nephridium.
c, Heart.
br, Ctenidium (gill-plume).
pbr, Parabranchia (= the osphradium or olfactory patch).
x, Glandular lamellae of the inner face of the mantle-skirt.
y, Adrectal (purpuriparous) gland.
t, Testis.
vd, Vas deferens.
p, Penis.
mc, Columella muscle (muscular process grasping the shell).
v, Stomach.
h, Liver.
N.B.--Note the simple snout or rostrum not introverted as a
"proboscis."]
[Illustration: FIG. 21.--Nervous system of _Paludina_ as a type of the
streptoneurous condition. (From Gegenbaur, after Jhering.)
B, Buccal (suboesophageal) ganglion.
C, Cerebral ganglion.
Co, Pleural ganglion.
P, Pedal ganglion with otocyst attached.
p, Pedal nerve.
A, Abdominal ganglion at the extremity of the twisted visceral
"loop."
sp, Supra-intestinal visceral ganglion on the course of the right
visceral cord.
sb, Sub-intestinal ganglion on the course of the left visceral
cord.]
The very large assemblage of forms coming under this order comprises
the most highly developed predaceous sea-snails, numerous vegetarian
species, a considerable number of freshwater and some terrestrial
forms. The partial dissection of a male specimen of the common
periwinkle, _Littorina littoralis_, drawn in fig. 20, will serve to
exhibit the disposition of viscera which prevails in the group. The
branchial chamber formed by the mantle-skirt overhanging the head has
been exposed by cutting along a line extending backward from the
letters vd to the base of the columella muscle mc, and the whole roof
of the chamber thus detached from the right side of the animal's neck
has been thrown over to the left, showing the organs which lie upon
the roof. No opening into the body-cavity has been made; the organs
which lie in the coiled visceral hump show through its transparent
walls. The head is seen in front resting on the foot and carrying a
median non-retractile snout or rostrum, and a pair of cephalic
tentacles at the base of each of which is an eye. In many Gastropoda
the eyes are not thus sessile but raised upon special eye-tentacles
(figs. 25, 56). To the right of the head is seen the muscular penis p,
close to the termination of the vas deferens (spermatic duct) vd. The
testis t occupies a median position in the coiled visceral mass.
Behind the penis on the same side is the hook-like columella muscle, a
development of the retractor muscle of the foot, which clings to the
spiral column or columella of the shell (see fig. 33). This columella
muscle is the same thing as the muscles adhering to the shell in
_Patella_, and the posterior adductor of Lamellibranchs.
The surface of the neck is covered by integument forming the floor of
the branchial cavity. It has not been cut into. Of the organs lying on
the reflected mantle-skirt, that which in the natural state lay
nearest to the vas deferens on the right side of the median line of
the roof of the branchial chamber is the rectum i', ending in the anus
a. It can be traced back to the intestine i near the surface of the
visceral hump, and it is found that the apex of the coil formed by the
hump is occupied by the liver h and the stomach v. Pharynx and
oesophagus are concealed in the head. The enlarged glandular structure
of the walls of the rectum is frequent in the Pectinibranchia, as is
also though not universal the gland marked y, next to the rectum. It
is the adrectal gland, and in the genera _Murex_ and _Purpura_
secretes a colourless liquid which turns purple upon exposure to the
atmosphere, and was used by the ancients as a dye. Near this and less
advanced into the branchial chamber is the single renal organ or
nephridium r with its opening to the exterior r'. Internally this
glandular sac presents a second slit or aperture which leads into the
pericardium (as is now found to be the case in all Mollusca). The
heart c lying in the pericardium is seen in close proximity to the
renal organ, and consists of a single auricle receiving blood from the
gill, and of a single ventricle which pumps it through the body by an
anterior and posterior aorta. The surface x of the mantle between the
rectum and the gill-plume is thrown into folds which in many
sea-snails (whelks or _Buccinidae_, &c.) are very strongly developed.
The whole of this surface appears to be active in the secretion of a
mucous-like substance. The single gill-plume br lies to the left of
the median line in natural position. It corresponds to the right of
the two primitive ctenidia in the untwisted archaic condition of the
molluscan body, and does not project freely into the branchial cavity,
but its axis is attached (by concrescence) to the mantle-skirt (roof
of the branchial chamber). It is rare for the gill-plume of a
Pectinibranch Gastropod to stand out freely as a plume, but
occasionally this more archaic condition is exhibited as in _Valvata_
(fig. 30). Next beyond (to the left of) the gill-plume we find the
so-called parabranchia, which is here simple, but sometimes lamellated
as in _Purpura_ (fig. 22). This organ has, without reason, been
supposed to represent the second ctenidium of the typical mollusc,
which it cannot do on account of its position. It should be to the
right of the anus were this the case. Spengel showed that the
parabranchia of Gastropods is the typical olfactory organ or
osphradium in a highly developed condition. The minute structure of
the epithelium which clothes it, as well as the origin of the nerve
which is distributed to the parabranchia, proves it to be the same
organ which is found universally in molluscs at the base of each
gill-plume, and tests the indrawn current of water by the sense of
smell. The nerve to this organ is given off from the superior
(original right, see fig. 3) visceral ganglion.
[Illustration: FIG. 22.--Female of _Purpura lapillus_ removed from its
shell; the mantle-skirt cut along its left line of attachment and
thrown over to the right side of the animal so as to expose the organs
on its inner face.
a, Anus.
vg, Vagina.
gp, Adrectal purpuriparous gland.
r', Aperture of the nephridium (kidney).
br, Ctenidium (branchial plume).
br', Parabranchia (= the comb-like osphradium or olfactory organ).]
The figures which are given here of various Pectinibranchia are in
most cases sufficiently explained by the references attached to them.
As an excellent general type of the nervous system, attention may be
directed to that of _Paludina_ drawn in fig. 21. On the whole the
ganglia are strongly individualized in the Pectinibranchia, nerve-cell
tissue being concentrated in the ganglia and absent from the cords. At
the same time, the junction of the visceral loop above the intestine
prevents in all Streptoneura the shortening of the visceral loop, and
it is rare to find a fusion of the visceral ganglia with either
pleural, pedal or cerebral--a fusion which can and does take place
where the visceral loop is not above but below the intestine, e.g. in
the Euthyneura (fig. 48), Cephalopoda and Lamellibranchia. As
contrasted with the Aspidobranchia, we find that in the
Pectinibranchia the pedal nerves are distinctly nerves given off from
the pedal ganglia, rather than cord-like nerve-tracts containing both
nerve-cells or ganglionic elements and nerve-fibres. Yet in some
Pectinibranchia (_Paludina_) a ladder-like arrangement of the two
pedal nerves and their lateral branches has been detected. The
histology of the nervous system of Mollusca has yet to be seriously
inquired into.
The alimentary canal of the Pectinibranchia presents little diversity
of character, except in so far as the buccal region is concerned.
Salivary glands are present, and in some carnivorous forms (_Dolium_)
these secrete free sulphuric acid (as much as 2% is present in the
secretion), which assists the animal in boring holes by means of its
rasping tongue through the shells of other molluscs upon which it
preys. A crop-like dilatation of the gut and a recurved intestine,
embedded in the compact yellowish-brown liver, the ducts of which open
into it, form the rest of the digestive tract and occupy a large bulk
of the visceral hump. The buccal region presents a pair of shelly jaws
placed laterally upon the lips, and a wide range of variation in the
form of the denticles of the lingual ribbon or radula.
Well-developed glandular invaginations occur in different positions on
the foot in Pectinibranchia. The most important of these opens by the
ventral pedal pore, situated in the median line in the anterior half
of the foot. This organ is probably homologous with the byssogenous
gland of Lamellibranchs. The aperture, which was formerly supposed to
be an aquiferous pore, leads into an extensive and often ramified
cavity surrounded by glandular tubules. The gland has been found in
both sub-orders of the Pectinibranchia, in _Cyclostoma_ and _Cypraea_
among the Taenioglossa, in _Hemifusus, Cassis, Nassa, Murex,
Fasciolariidae, Turbinellidae, Olividae, Marginellidae_ and _Conidae_
among the Stenoglossa. It was discovered by J.T. Cunningham that in
_Buccinum_ the egg-capsules are formed by this pedal gland and not by
any accessory organ of the generative system. Such horny egg-capsules
doubtless have the same origin in all other species in which they
occur, e.g. _Fusus, Pyrula, Purpura, Murex, Nassa, Trophon, Voluta_,
&c. The float of the pelagic _Janthina_, to which the egg-capsules are
attached, probably is also formed by the secretion of the pedal gland.
[Illustration: FIG. 23.--A, _Triton variegatum_, to show the proboscis
or buccal introvert (e) in a state of eversion.
a, Siphonal notch of the shell occupied by the siphonal fold of the
mantle-skirt (Siphonochlamyda).
b, Edge of the mantle-skirt resting on the shell.
c, Cephalic eye.
d, Cephalic tentacle.
e, Everted buccal introvert (proboscis).
f, Foot.
g, Operculum.
h, Penis.
i, Under surface of the mantle-skirt forming the roof of the
sub-pallial chamber.
B, Sole of the foot of _Pyrula tuba_, to show a, the pore usually
said to be "aquiferous" but probably the orifice of a gland; b,
median line of foot.]
Other glands opening on or near the foot are: (1) The suprapedal gland
opening in the middle line between the snout and the anterior border
of the foot. It is most commonly found in sessile forms and in
terrestrial genera such as _Cyclostoma_; (2) the anterior pedal gland
opening into the anterior groove of the foot, generally present in
aquatic species; (3) dorsal posterior mucous glands in certain
_Cyclostomatidae_.
The foot of the Pectinibranchia, unlike the simple muscular disk of
the Isopleura and Aspidobranchia, is very often divided into lobes, a
fore, middle and hind lobe (pro-, meso- and meta-podium, see figs. 24
and 25). Very usually, but not universally, the metapodium carries an
operculum. The division of the foot into lobes is a simple case of
that much greater elaboration or breaking up into processes and
regions which it undergoes in the class Cephalopoda. Even among some
Gastropoda (viz. the Opisthobranchia) we find the lobation of the foot
still further carried out by the development of lateral lobes, the
parapodia, whilst there are many Pectinibranchia, on the other hand,
in which the foot has a simple oblong form without any trace of lobes.
The development of the Pectinibranchia has been followed in several
examples, e.g. _Paludina, Purpura, Nassa, Vermetus, Neritina_. As in
other Molluscan groups, we find a wide variation in the early process
of the formation of the first embryonic cells, and their arrangement
as a diblastula, dependent on the greater or less amount of food-yolk
which is present in the egg-cell when it commences its embryonic
changes. In fig. 26 the early stages of _Paludina vivipara_ are
represented. There is but very little food-material in the egg of this
Pectinibranch, and consequently the diblastula forms by invagination;
the blastopore or orifice of invagination coincides with the anus, and
never closes entirely. A well-marked trochosphere is formed by the
development of an equatorial ciliated band; and subsequently, by the
disproportionate growth of the lower hemisphere, the trochosphere
becomes a veliger. The primitive shell-sac or shell-gland is well
marked at this stage, and the pharynx is seen as a new ingrowth (the
stomodaeum), about to fuse with and open into the primitively
invaginated arch-enteron (fig. 26, F).
[Illustration: FIG. 24.--Animal and shell of _Phorus exutus_.
a, Snout (not introversible).
b, Cephalic tentacles.
c, Right eye.
d, Pro- and meso-podium; to the right of this is seen the metapodium
bearing the sculptured operculum.]
[Illustration: FIG. 25.--Animal and shell of _Rostellaria
rectirostris_. (From Owen.)
a, Snout or rostrum.
b, Cephalic tentacle.
c, Eye.
d, Propodium and mesopodium.
e, Metapodium.
f, Operculum.
h', Prolonged siphonal notch of the shell occupied by the siphon, or
trough-like process of the mantle-skirt.]
In other Pectinibranchia (and such variations are representative for
all Mollusca, and not characteristic only of Pectinibranchia) we find
that there is a very unequal division of the egg-cell at the
commencement of embryonic development, as in _Nassa_. Consequently
there is, strictly speaking, no invagination (emboly), but an
overgrowth (epiboly) of the smaller cells to enclose the larger. The
general features of this process and of the relation of the blastopore
to mouth and anus have been explained in treating of the development
of Mollusca generally. In such cases the blastopore may entirely
close, and both mouth and anus develop as new ingrowths (stomodaeum
and proctodaeum), whilst, according to the observations of N.
Bobretzky, the closed blastopore may coincide in position with the
mouth in some instances (_Nassa_, &c.), instead of with the anus. But
in these epibolic forms, just as in the embolic _Paludina_, the embryo
proceeds to develop its ciliated band and shell-gland, passing through
the earlier condition of a trochosphere to that of the veliger. In the
veliger stage many Pectinibranchia (_Purpura, Nassa_, &c.) exhibit, in
the dorsal region behind the head, a contractile area of the
body-wall. This acts as a larval heart, but ceases to pulsate after a
time. Similar rhythmically contractile areas are found on the foot of
the embryo Pulmonate _Limax_ and on the yolk-sac (distended
foot-surface) of the Cephalopod _Loligo_. The preconchylian
invagination or shell-gland is formed in the embryo behind the velum,
on the surface opposite the blastopore. It is surrounded by a ridge of
cells which gradually extends over the visceral sac and secretes the
shell. In forms which are naked in the adult state, the shell falls
off soon after the reduction of the velum, but in _Cenia, Runcina_ and
_Vaginula_ the shell-gland and shell are not developed, and the young
animal when hatched has already the naked form of the adult.
[Illustration: FIG. 26.--Development of the River-Snail, _Paludina
vivipara_. (After Lankester, 17.)
dc, Directive corpuscle (outcast cell).
ae, Arch-enteron or cavity lined by the enteric cell-layer or
endoderm.
bl, Blastopore.
vr, Velum or circlet of ciliated cells.
dv, Velar area or cephalic dome.
sm, Site of the as yet unformed mouth.
f, Foot.
mes, Rudiments of the skeleto-trophic tissues.
pi, The pedicle of invagination, the future rectum.
shgl, The primitive shell-sac or shell-gland.
m, Mouth.
an, Anus.
A, Diblastula phase (optical section).
B, The diblastula has become a trochosphere by the development of
the ciliated ring vr (optical section).
C, Side view of the trochosphere with commencing formation of the
foot.
D, Further advanced trochosphere (optical section).
E, The trochosphere passing to the veliger stage, dorsal view
showing the formation of the primitive shell-sac.
F, Side view of the same, showing foot, shell-sac (shgl), velum
(vr), mouth and anus.
N.B.--In this development the blastopore is not elongated; it
persists as the anus. The mouth and stomodaeum form independently
of the blastopore.]
One further feature of the development of the Pectinibranchia deserves
special mention. Many Gastropoda deposit their eggs, after
fertilization, enclosed in capsules; others, as _Paludina_, are
viviparous; others, again, as the Zygobranchia, agree with the
Lamellibranch Conchifera (the bivalves) in having simple exits for the
ova without glandular walls, and therefore discharge their eggs
unenclosed in capsules freely into the sea-water; such unencapsuled
eggs are merely enclosed each in its own delicate chorion. When
egg-capsules are formed they are often of large size, have tough
walls, and in each capsule are several eggs floating in a viscid
fluid. In some cases all the eggs in a capsule develop; in other cases
one egg only in a capsule (_Neritina_), or a small proportion
(_Purpura, Buccinum_), advance in development; the rest are arrested
either after the first process of cell-division (cleavage) or before
that process. The arrested embryos or eggs are then swallowed and
digested by those in the same capsule which have advanced in
development. This is clearly the same process in essence as that of
the formation of a vitellogenous gland from part of the primitive
ovary, or of the feeding of an ovarian egg by the absorption of
neighbouring potential eggs; but here the period at which the
sacrifice of one egg to another takes place is somewhat late. What it
is that determines the arrest of some eggs and the progressive
development of others in the same capsule is at present unknown.
[Illustration: FIG. 27.--_Oxygyrus Keraudrenii_. (From Owen.)
a, Mouth and odontophore.
b, Cephalic tentacles.
c, Eye.
d, Propodium (B) and mesopodium.
e, Metapodium.
f, Operculum.
h, Mantle-chamber.
i, Ctenidium (gill-plume).
k, Retractor muscle of foot.
l, Optic tentacle.
m, Stomach.
n, Dorsal surface overhung by the mantle-skirt; the letter is close
to the salivary gland.
o, Rectum and anus.
p, Liver.
q, Renal organ (nephridium).
s, Ventricle.
u, The otocyst attached to the cerebral ganglion.
w, Testis.
x, Auricle of the heart.
y, Vesicle on genital duct.
z, Penis.]
In the tribe of Pectinibranchia called Heteropoda the foot takes the
form of a swimming organ. The nervous system and sense organs are
highly developed. The odontophore also is remarkably developed, its
lateral teeth being mobile, and it serves as an efficient organ for
attacking the other pelagic forms on which the Heteropoda prey. The
sexes are distinct, as in all Streptoneura; and genital ducts and
accessory glands and pouches are present, as in all Pectinibranchia.
The Heteropoda exhibit a series of modifications in the form and
proportions of the visceral mass and foot, leading from a condition
readily comparable with that of a typical Pectinibranch such as
_Rostellaria_, with the three regions of the foot strongly marked and
a coiled visceral hump of the usual proportions, up to a condition in
which the whole body is of a tapering cylindrical shape, the foot a
plate-like vertical fin, and the visceral hump almost completely
atrophied. Three steps of this modification may be distinguished as
three families:--_Atlantidae, Carinariidae_ and _Pterotrachaeidae_.
They are true Pectinibranchia which have taken to a pelagic life, and
the peculiarities of structure which they exhibit are strictly
adaptations consequent upon their changed mode of life. Such
adaptations are the transparency and colourlessness of the tissues,
and the modifications of the foot, which still shows in _Atlanta_ the
form common in Pectinibranchia (compare fig. 27 and fig. 24). The
cylindrical body of _Pterotrachaea_ is paralleled by the slug-like
forms of Euthyneura. J.W. Spengel has shown that the visceral loop of
the Heteropoda is streptoneurous. Special to the Heteropoda is the
high elaboration of the lingual ribbon, and, as an agreement with some
of the opisthobranchiate Euthyneura, but as a difference from the
Pectinibranchia, we find the otocysts closely attached to the cerebral
ganglia. This is, however, less of a difference than it was at one
time supposed to be, for it has been shown by H. Lacaze-Duthiers, and
also by F. Leydig, that the otocysts of Pectinibranchia even when
lying close upon the pedal ganglion (as in fig. 21) yet receive their
special nerve (which can sometimes be readily isolated) from the
cerebral ganglion (see fig. 11). Accordingly the difference is one of
position of the otocyst and not of its nerve-supply. The Heteropoda
are further remarkable for the high development of their cephalic
eyes, and for the typical character of their osphradium (Spengel's
olfactory organ). This is a groove, the edges of which are raised and
ciliated, lying near the branchial plume in the genera which possess
that organ, whilst in _Firoloida_, which has no branchial plume, the
osphradium occupies a corresponding position. Beneath the ciliated
groove is placed an elongated ganglion (olfactory ganglion) connected
by a nerve to the supra-intestinal (therefore the primitively dextral)
ganglion of the long visceral nerve-loop, the strands of which cross
one another--this being characteristic of Streptoneura (Spengel).
[Illustration: FIG. 28.--_Carinaria mediterranea_. (From Owen.)
A, The animal. B, The shell removed. C, D, Two views of the shell of
_Cardiopoda_.
a, Mouth and odontophore.
b, Cephalic tentacles.
c, Eye.
d, The fin-like mesopodium.
d', Its sucker.
e, Metapodium.
f, Salivary glands.
h, Border of the mantle-flap.
i, Ctenidium (gill-plume).
m, Stomach.
n, Intestine.
o, Anus.
p, Liver.
t, Aorta, springing from the ventricle.
u, Cerebral ganglion.
v, Pleural and pedal ganglion.
w, Testis.
x, Visceral ganglion.
y, Vesicula seminalis.
z, Penis.]
The Heteropoda belong to the "pelagic fauna" occurring near the
surface in the Mediterranean and great oceans in company with the
Pteropoda, the Siphonophorous Hydrozoa, Salpae, Leptocephali, and
other specially-modified transparent swimming representatives of
various groups of the animal kingdom. In development they pass through
the typical trochosphere and veliger stages provided with boat-like
shell.
Sub-order 1.--TAENIOGLOSSA. Radula with a median tooth and three teeth
on each side of it. Formula 3 : 1 : 3.
Tribe 1.--PLATYPODA. Normal Taenioglossa of creeping habit. The foot
is flattened ventrally, at all events in its anterior part
(_Strombidae_). Otocysts situated close to the pedal nerve-centres.
Accessory organs are rarely found on the genital ducts, but occur in
_Paludina, Cyclostoma, Naticidae, Calyptraeidae_, &c. Mandibles
usually present. This is the largest group of Mollusca, including
nearly sixty families, some of which are insufficiently known from the
anatomical point of view.
Fam. 1.--_Paludinidae_. Pedal centres in the form of ganglionated
cords; kidney provided with a ureter; viviparous; fluviatile.
_Paludina_. _Neothauma_, from Lake Tanganyika. _Tylopoma_, extinct,
Tertiary.
[Illustration: FIG. 29.--_Pterotrachea mutica_ seen from the right
side. (After Keferstein.)
a, Pouch for reception of the snout when retracted.
c, Pericardium.
ph, Pharynx.
oc, Cephalic eye.
g, Cerebral ganglion.
g', Pleuro-pedal ganglion.
pr, Foot (mesopodium).
v, Stomach.
i, Intestine.
n, So-called nucleus.
br, Branchial plume (ctenidium).
w, Osphradium.
mt, Foot (metapodium).
z, Caudal appendage.]
Fam. 2.--_Cyclophoridae_. No ctenidium, pallial cavity transformed
into a lung; aperture of shell circular; terrestrial. _Pomatias_,
shell turriculated. _Diplommatina. Hybocystis. Cyclophorus_, shell
umbilicated, with a short spire and horny operculum. Cyclosurus,
shell uncoiled. _Dermatocera_, foot with a horn-shaped protuberance
at its posterior end. Spiraculum.
Fam. 3.--_Ampullariidae_. To the left of the ctenidium a pulmonary
sac, separated from it by an incomplete septum, amphibious.
_Ampullaria_, shell dextral, coiled. _Lanistes_, shell sinistral,
spire short or obsolete. _Meladomus._
Fam. 4.--_Littorinidae._ Oesophageal pouches present; pedal
nerve-centres concentrated; a pedal penis near the right tentacle.
_Littorina_, shell not umbilicated, littoral habit. _Lacuna_, foot
with two posterior appendages, marine, entirely aquatic.
_Cremnoconchus_, entirely aerial, Indian. _Risella. Tectarius._
Fam. 5.--_Fossaridae._ Head with two lobes in some Rhipidoglossa.
_Fossaria._
Fam. 6.--_Purpurinidae_, extinct.
Fam. 7.--_Planaxidae._ Shell with pointed spire; a short pallial
siphon. Planaxis.
Fam. 8.--_Cyclostomatidae._ Pallial cavity transformed into a lung;
pedal centres concentrated; a deep pedal groove. _Cyclostoma_, shell
turbinated, operculum calcareous, British. _Omphalotropis._
Fam. 9.--_Aciculidae._ Pallial cavity transformed into a lung;
operculum horny; shell narrow and elongated. _Acicula._
Fam. 10.--_Valvatidae._ Ctenidium bipectinate, free; hermaphrodite;
fluviatile. _Valvata_, British.
Fam. 11.--_Rissoidae._ Epipodial filaments present; one or two
pallial tentacles. _Rissoa. Rissoina. Stiva._
Fam. 12.--_Litiopidae._ An epipodium bearing three pairs of
tentacles and an operculigerous lobe with two appendages;
inhabitants of the Sargasso weed. _Litiopa._
Fam. 13.--_Adeorbiidae._ Mantle with two posterior appendages;
ctenidium large and capable of protrusion from pallial cavity.
_Adeorbis_, British.
Fam. 14.--_Jeffreysiidae._ Head with two long labial palps; shell
ovoid; operculum horny, semicircular, carinated. _Jeffreysia._
Fam. 15.--_Homalogyridae._ Shell flattened; no cephalic tentacles.
_Homalogyra_, British. _Ammoniceras._
Fam. 16.--_Skeneidae._ Shell depressed, with rounded aperture;
cephalic tentacles long. _Skenea_, British.
Fam. 17.--_Choristidae._ Shell spiral; four cephalic tentacles; eyes
absent; two pedal appendages. _Choristes._
Fam. 18.--_Assimineidae._ Eyes at free extremities of tentacles.
Assiminea, estuarine, British.
Fam. 19.--_Truncatellidae._ Snout very long, bilobed; foot short.
_Truncatella._
[Illustration: FIG. 30.--_Valvata cristata_, Müll.
o, Mouth.
op, Operculum.
br, Ctenidium (branchial plume).
x, Filiform appendage (? rudimentary ctenidium).
The freely projecting ctenidium of typical form not having its axis
fused to the roof of the branchial chamber is the notable
character of this genus.]
Fam. 20.--_Hydrobiidae._ Shell with prominent spire; penis distant
from right tentacle, generally appendiculated; brackish water or
fluviatile. _Hydrobia_, British. _Baikalia_, from Lake Baikal.
_Pomatiopsis. Bithynella. Lithoglyphus. Spekia_, viviparous, from
Lake Tanganyika. _Tanganyicia. Limnotrochus_, from Lake Tanganyika.
_Chytra. Littorinida. Bithynia_, British, fluviatile. _Stenothyra._
Fam. 21.--_Melaniidae._ Spire of shell somewhat elongated;
mantle-border fringed; viviparous; fluviatile. _Melania. Faunus.
Paludomus. Melanopsis. Nassopsis. Bythoceras_, from Lake
Tanganyika.
Fam. 22.--_Typhobiidae._ Foot wide; shell turriculated, with
carinated whorls, the carinae tuberculated or spiny. _Typhobia.
Bathanalia_, from Lake Tanganyika.
Fam. 23.--_Pleuroceridae._ Like _Melaniidae_, but mantle-border not
fringed and reproduction oviparous. _Pleurocera. Anculotus._
Fam. 24.--_Pseudomelaniidae._ All extinct.
Fam. 25.--_Subulitidae._ All extinct.
Fam. 26.--_Nerineidae._ All extinct.
Fam. 27.--_Cerithiidae._ Shell with numerous tuberculated whorls;
aperture canaliculated anteriorly; short pallial siphon. _Cerithium.
Bittium. Potamides. Triforis. Laeocochlis. Cerithiopsis._
Fam. 28.--_Modulidae._ Shell with short spire; no siphon.
_Modulus._
Fam. 29.--_Vermetidae._ Animal fixed by the shell, the last whorls
of which are not in contact with each other; foot small; two
anterior pedal tentacles. _Vermetus. Siliquaria._
Fam. 30.--_Caecidae._ Shell almost completely uncoiled, in one
plane, with internal septa. _Caecum_, British.
Fam. 31.--_Turritellidae._ Shell very long; head large; foot broad.
_Turritella_, British. _Mesalia. Mathilda._
Fam. 32.--_Struthiolariidae._ Shell conical; aperture slightly
canaliculated; siphon slightly developed. _Struthiolaria._
Fam. 33.--_Chenopodidae._ Shell elongated; aperture expanded; siphon
very short. _Chenopus_, British. _Alaria, Spinigera, Diartema_,
extinct.
Fam. 34.--_Strombidae._ Foot narrow, compressed, without sole.
_Strombus. Pteroceras. Rostellaria. Terebellum._
Fam. 35.--_Xenophoridae._ Foot transversely divided into two parts.
_Xenophorus. Eotrochus_, Silurian.
Fam. 36.--_Capulidae._ Shell conical, not coiled, but slightly
incurved posteriorly; a tongue-shaped projection between snout and
foot. _Capulus. Thyca_, parasitic on asterids. _Platyceras_,
extinct.
[Illustration: FIG. 31.--Shell of _Crucibulum_, seen from below so as
to show the inner whorl b, concealed by the cap-like outer whorl a.]
Fam. 37.--_Hipponycidae._ Shell conical; foot secreting a ventral
calcareous plate; animal fixed. _Hipponyx. Mitrularia._
Fam. 38.--_Calyptraeidae._ Shell with short spire; lateral cervical
lobes present; accessory genital glands. _Calyptraea_, British.
_Crepidula. Crucibulum._
Fam. 39.--_Naricidae._ Foot divided into two, posterior half bearing
the operculum; a wide epipodial velum; shell turbinated. Narica.
Fam. 40.--_Naticidae._ Foot large, with aquiferous system; propodium
reflected over head; eyes degenerate; burrowing habit. _Natica_,
British. _Amaura. Sigaretus._
Fam. 41.--_Lamellariidae._ Shell thin, more or less covered by the
mantle; no operculum. _Lamellaria. Velutina. Marsenina_,
_Oncidiopsis_, hermaphrodite.
Fam. 42.--_Trichotropidae._ Shell with short spire, carinate and
pointed. _Trichotropis._
Fam. 43.--_Seguenziidae._ Shell trochiform, with canaliculated
aperture and twisted columella. _Seguenzia_, abyssal.
Fam. 44.--_Janthinidae._ Shell thin; operculum absent; tentacles
bifid; foot secretes a float; pelagic. _Janthina. Recluzia._
Fam. 45.--_Cypraeidae._ Shell inrolled, solid, polished, aperture
very narrow in adult; short siphon; anus posterior; osphradium with
three lobes; mantle reflected over shell. _Cypraea. Pustularia.
Ovula. Pedicularia_, attached to corals. _Erato_.
[Illustration: FIG. 32.--Animal and shell of _Ovula_.
b, Cephalic tentacles.
d, Foot.
h, Mantle-skirt, which is naturally carried in a reflected condition
so as to cover the sides of the shell.]
Fam. 46.--_Tritonidae._ Shell turriculated and siphonated, thick,
each whorl with varices; foot broad and truncated anteriorly;
pallial siphon well developed; proboscis present. _Triton. Persona._
_Ranella._
Fam. 47.--_Columbellinidae._ All extinct.
Fam. 48.--_Cassididae._ Shell ventricose, with elongated aperture,
and short spire; proboscis and siphon long; operculum with marginal
nucleus. _Cassis. Cassidaria. Oniscia._
Fam. 49--_Oocorythidae._ Shell globular and ventricose; aperture
oval and canaliculated; operculum spiral. _Oocorys_, abyssal.
Fam. 50.--_Doliidae._ Shell ventricose, with short spire, and wide
aperture; no varices and no operculum; foot very broad, with
projecting anterior angles; siphon long. _Dolium. Pyrula._
Fam. 51.--_Solariidae. Solarium. Torinia. Fluxina._
Fam. 52.--_Scalariidae._ Shell turriculated, with elongated spire;
proboscis short; siphon rudimentary. _Scalaria. Eglisia._ Crossea.
Aclis.
The three following families have neither radula nor jaws, and are
therefore called _Aglossa_. They have a well-developed proboscis which
is used as a suctorial organ; some are abyssal, but the majority are
either commensals or parasites of Echinoderms.
Fam. 53.--_Pyramidellidae._ Summit of spire heterostrophic; a
projection, the mentum, between head and foot; operculum present.
_Pyramidella. Turbonilla. Odostomia_, British. _Myxa._
[Illustration: FIG. 33.--Section of the shell of _Triton_, Cuv. (From
Owen.)
a, Apex.
ac, Siphonal notch of the mouth of the shell.
ac to pc, Mouth of the shell.
w, w, Whorls of the shell.
s, s. Sutures.
Occupying the axis, and exposed by the section, is seen the
"columella" or spiral pillar. The upper whorls of the shell are
seen to be divided into separate chambers by the formation of
successively formed "septa."]
Fam. 54.--_Eulimidae._ Visceral mass still coiled spirally; shell
thin and shining. _Eulima_, foot well developed, with an operculum,
animal usually free, but some live in the digestive cavity of
Holothurians. _Mucronalia_, foot reduced, but still operculate, eyes
present, animal fixed by its very long proboscis which is deeply
buried in the tissues of an Echinoderm, no pseudopallium.
_Stylifer_, the operculum is lost, animal fixed by a large proboscis
which forms a pseudopallium covering the whole shell except the
extremity of the spire, parasitic on all groups of Echinoderms.
_Entosiphon_, visceral mass still coiled; shell much reduced,
proboscis very long forming a pseudopallium which covers the whole
body and projects beyond in the form of a siphon, foot and nervous
system present, eyes, branchia and anus absent, parasite in the
Holothurian _Deima blakei_ in the Indian Ocean.
Fam. 55.--_Entoconchidae._ No shell; visceral mass not coiled; no
sensory organs, nervous system, branchia or anus; body reduced to a
more or less tubular sac; hermaphrodite and viviparous; parasitic in
Holothurians; larvae are veligers, with shell and operculum.
_Entocolax_, mouth at free extremity, animal fixed by aboral orifice
of pseudopallium, Pacific. _Entoconcha_, body elongated and tubular,
animal fixed by the oral extremity, protandric hermaphrodite,
parasitic in testes of Holothurians causing their abortion.
_Enteroxenos_, no pseudopallium and no intestine, hermaphrodite,
larvae with operculum.
Tribe 2.--HETEROPODA. Pelagic Taenioglossa with foot large and
laterally compressed to form a fin.
Fam. 1. _Atlantidae._ Visceral sac and shell coiled in one plane;
foot divided transversely into two parts, posterior part bearing an
operculum, anterior part forming a fin provided with a sucker.
_Atlanta. Oxygyrus._
Fam. 2.--_Carinariidae._ Visceral sac and shell small in proportion
to the rest of the body, which cannot be withdrawn into the shell;
foot elongated, fin-shaped, with sucker, but without operculum.
_Carinaria. Cardiopoda._
Fam. 3.--_Pterotrachaeidae._ Visceral sac very much reduced; without
shell or mantle; anus posterior; foot provided with sucker in male
only. _Pterotrachaea. Firoloida. Pterosoma._
Sub-order 2.--STENOGLOSSA. Radula narrow with one lateral tooth on
each side, and one median tooth or none.
Tribe 1.--RACHIGLOSSA. Radula with a median tooth and a single tooth
on each side of it. Formula 1 : 1 : 1. Rudimentary jaws present.
[Illustration: FIG. 34.--Female _Janthina_, with egg-float (a)
attached to the foot; b, egg-capsules; c, ctenidium (gill-plume); d,
cephalic tentacles.]
Fam. 1.--_Turbinellidae._ Shell solid, piriform, with thick folded
columella; lateral teeth of radula bicuspidate. _Turbinella.
Cynodonta. Fulgur. Hemifusus. Tudicla. Strepsidura._
Fam. 2.--_Fasciolariidae._ Shell elongated, with long siphon;
lateral teeth of radula multicuspidate. _Fasciolaria. Fusus.
Clavella. Latirus._
Fam. 3.--_Mitridae._ Shell fusiform and solid, aperture elongated,
columella folded; no operculum; eyes on sides of tentacles. _Mitra.
Turricula. Cylindromitra. Imbricaria._
Fam. 4.--_Buccinidae._ Foot large and broad; eyes at base of
tentacles; operculum horny. _Buccinum. Chrysodomus. Liomesus.
Cominella. Tritonidea. Pisania. Euthria. Phos. Dipsacus._
Fam. 5.--_Nassidae._ Foot broad, with two slender posterior
appendages; operculum unguiculate. _Nassa_, marine, British.
_Canidia_, fluviatile. _Bullia._
Fam. 6.--_Muricidae._ Shell with moderately long spire and canal,
ornamented with ribs, often spiny; foot truncated anteriorly.
_Murex_, British. _Trophon_, British. _Typhis. Urosalpinx.
Lachesis._
Fam. 7.--_Purpuridae._ Shell thick, with short spire, last whorl
large and canal short; aperture wide; operculum horny. _Purpura_,
British. _Rapana. Monoceros. Sistrum. Concholepas._
Fam. 8.--_Haliidae._ Shell ventricose, thin and smooth, with wide
aperture; foot large and thick, without operculum. _Halia._
Fam. 9.--_Cancellariidae._ Shell ovoid, with short spire and folded
columella; foot small, no operculum; siphon short. _Cancellaria._
Fam. 10.--_Columbellidae._ Spire of shell prominent, aperture
narrow, canal very short, columella crenelated; foot large.
_Columbella._
Fam. 11.--_Coralliophilidae._ Shell irregular; radula absent; foot
and siphon short; sedentary animals, living in corals.
_Coralliophila. Rhizochilus. Leptoconchus. Magilus. Rapa._
Fam. 12.--_Volutidae._ Head much flattened and wide, with eyes on
sides; foot broad; siphon with internal appendages. _Valuta.
Guivillea. Cymba._
Fam. 13.--_Olividae._ Foot with anterior transverse groove; a
posterior pallial tentacle; generally burrowing. _Olivia. Olivella.
Ancillaria. Agaronia._
Fam. 14.--_Marginellidae._ Foot very large; mantle reflected over
shell. _Marginella. Pseudomarginella._
Fam. 15.--_Harpidae._ Foot very large; without operculum; shell with
short spire and longitudinal ribs; siphon long. _Harpa._
Tribe 2.--TOXIGLOSSA. No jaws. No median tooth in radula. Formula: 1 :
0 : 1. Poison-gland present whose duct traverses the nerve-collar.
Fam. 1.--_Pleurotomatidae._ Shell fusiform, with elongated spire;
margin of shell and mantle notched. _Pleurotoma. Clavatula.
Mangilia. Bela. Pusionella. Pontiothauma._
Fam. 2.--_Terebridae._ Shell turriculated, with numerous whorls;
aperture and operculum oval; eyes at summits of tentacles; siphon
long. _Terebra._
Fam. 3.--_Conidae._ Shell conical, with very short spire, and narrow
aperture with parallel borders; operculum unguiform _Conus._
Sub-Class II.--EUTHYNEURA
The most important general character of the Euthyneura is the absence of
torsion in the visceral commissure, and the more posterior position of
the anus and pallial organs. Comparative anatomy and embryology prove
that this condition is due, not as formerly supposed to a difference in
the relations of the visceral commissure which prevented it from being
included in the torsion of the visceral hump, but to an actual detorsion
which has taken place in evolution and is repeated to a great extent in
individual development. In several of the more primitive forms the same
torsion occurs as in Streptoneura, viz. in _Actaeon_ and _Limacina_
among Opisthobranchia, and _Chilina_ among Pulmonata. _Actaeon_ is
proso-branchiate, the visceral commissure is twisted in _Actaeon_ and
_Chilina_, and even slightly still in _Bulla_ and _Scaphander_; in
_Actaeon_ and _Limacina_ the osphradium is to the left, innervated by
the supra-intestinal ganglion. But in the other members of the sub-class
the detorsion of the visceral mass has carried back the anus and
circumanal complex from the anterior dorsal region to the right side, as
in _Bulla_ and _Aplysia_, or even to the posterior end of the body, as
in _Philine, Oncidium, Doris_, &c. Different degrees of the same
process of detorsion are, as we have seen, exhibited by the Heteropoda
among the Streptoneura, and both in them and in the Euthyneura the
detorsion is associated with degeneration of the shell. Where the
modification is carried to its extreme degree, not only the shell but
the pallial cavity, ctenidium and visceral hump disappear, and the body
acquires a simple elongated form and a secondary external symmetry, as
in _Pterotrachaea_ and in _Doris, Eolis_, and other Nudibranchia. These
facts afford strong support to the hypothesis that the weight of the
shell is the original cause of the torsion of the dorsal visceral mass
in Gastropods. But this hypothesis leaves the elevation of the visceral
mass and the exogastric coiling of the shell in the ancestral form
unexplained. In those Euthyneura in which the shell is entirely absent
in the adult, it is, except in the three genera _Cenia, Runcina_ and
_Vaginula_, developed in the larva and then falls off. In other cases
(Tectibranchs) the reduced shell is enclosed by upgrowths of the edge of
the mantle and becomes internal, as in many Cephalopods. A few
Euthyneura in which the shell is not much reduced retain an operculum in
the adult state, e.g. _Actaeon, Limacina_, and the marine Pulmonate,
_Amphibola_. The detorted visceral commissure shows a tendency to the
concentration of all its elements round the oesophagus, so that except
in the Bullomorpha and in _Aplysia_ the whole nervous system is
aggregated in the cephalic region, either dorsally or ventrally. The
radula has a number of uniform teeth on each side of the median tooth in
each transverse row. The head in most cases bears two pairs of
tentacles. All the Euthyneura are hermaphrodite.
[Illustration: FIG. 35.--_Acera bullata._ A single row of teeth of the
Radula. (Formula, x.l.x.)]
In the most primitive condition the genital duct is single throughout
its length and has a single external aperture; it is therefore said to
be monaulic. The hermaphrodite aperture is on the right side near the
opening of the pallial cavity, and a ciliated groove conducts the
spermatozoa to the penis, which is situated more anteriorly. This is the
condition in the Bullomorpha, the Aplysiomorpha, and in one Pulmonate,
_Pythia_. In some cases while the original aperture remains undivided,
the seminal groove is closed and so converted into a canal. This is the
modification found in _Cavolinia longirostris_ among the Bullomorpha,
and in all the _Auriculidae_ except _Pythia_. A further degree of
modification occurs when the male duct takes its origin from the
hermaphrodite duct above the external opening, so that there are two
distinct apertures, one male and one female, the latter being the
original opening. The genital duct is now said to be diaulic, as in
_Valvata, Oncidiopsis, Actaeon_, and _Lobiger_ among the Bullomorpha, in
the _Pleurobranchidae_, in the Nudibranchia, except the Doridomorpha and
most of the Elysiomorpha, and in the Pulmonata. Originally in this
condition the female aperture is at some distance from the male, as in
the Basommatophora and in other cases; but in some forms the female
aperture itself has shifted and come to be contiguous with the male
opening and penis as in the Stylommatophora. In all these cases the
female duct bears a bursa copulatrix or receptaculum seminis. In some
forms this receptacle acquires a separate external opening remaining
connected with the oviduct internally. There are thus two female
openings, one for copulation, the other for oviposition, as well as a
male opening. The genital duct is now trifurcated or triaulic, a
condition which is confined to certain Nudibranchs, viz. the
Doridomorpha and most of the Elysiomorpha.
The Pteropoda, formerly regarded as a distinct class of the Mollusca,
were interpreted by E.R. Lankester as a branch of the Cephalopoda,
chiefly on account of the protrusible sucker-bearing processes at the
anterior end of _Pneumonoderma_. These he considered to be homologous
with the arms of Cephalopods. He fully recognized, however, the
similarity of Pteropods to Gastropods in their general asymmetry and in
the torsion of the visceral mass in _Limacinidae_. It is now understood
that they are Euthyneurous Gastropods adapted to natatory locomotion and
pelagic life. The sucker-bearing processes of _Pneumonoderma_ are
outgrowths of the proboscis. The fins of Pteropods are now interpreted
as the expanded lateral margins of the foot, termed parapodia, not
homologous with the siphon of Cephalopods which is formed from epipodia.
The Thecosomatous Pteropoda are allied to _Bulla_, the Gymnosomatous
forms to _Aplysia_. The Euthyneura comprises two orders, Opisthobranchia
and Pulmonata.
[Illustration: FIG. 36.
A, Veliger-larva of an Opisthobranch (_Polycera_). f, Foot; op,
operculum; mn, anal papilla; ry, dry, two portions of unabsorbed
nutritive yolk on either side of the intestine. The right otocyst is
seen at the root of the foot.
B, Trochosphere of an Opisthobranch (_Pleurobranchidium_)
showing--shgr, the shell-gland or primitive shell-sac; v, the cilia
of the velum; ph, the commencing stomodaeum or oral invagination;
ot, the left otocyst; pg, red-coloured pigment spot.
C, Diblastula of an Opisthobranch (_Polycera_) with elongated
blastopore oi.
(All from Lankester.)]
Order 1.--OPISTHOBRANCHIA. Marine Euthyneura, the more archaic forms
of which have a relatively large foot and a small visceral hump, from
the base of which projects on the right side a short mantle-skirt. The
anus is placed in such forms far back beyond the mantle-skirt. In
front of the anus, and only partially covered by the mantle-skirt, is
the ctenidium with its free end turned backwards. The heart lies in
front of, instead of to the side of, the attachment of the
ctenidium--hence Opisthobranchia as opposed to "Prosobranchia," which
correspond to the Streptoneura. A shell is possessed in the adult
state by but few Opisthobranchia, but all pass through a veliger
larval stage with a nautiloid shell (fig. 36). Many Opisthobranchia
have by a process of atrophy lost the typical ctenidium and the
mantle-skirt, and have developed other organs in their place. As in
some Pectinibranchia, the free margin of the mantle-skirt is
frequently reflected over the shell when a shell exists; and, as in
some Pectinibranchia, broad lateral outgrowths of the foot (parapodia)
are often developed which may be thrown over the shell or naked dorsal
surface of the body.
[Illustration: FIG. 37.--_Phyllirhoë bucephala_, twice the natural
size, a transparent pisciform pelagic Opisthobranch. The internal
organs are shown as seen by transmitted light. (After W. Keferstein.)
a, Mouth.
b, Radular sac.
c, Oesophagus.
d, Stomach.
c', Intestine.
f', Anus.
g, g', g", g"', The four lobes of the liver.
h, The heart (auricle and ventricle).
l, The renal sac (nephridium).
l', The ciliated communication of the renal sac with the pericardium.
m, The external opening of the renal sac.
n, The cerebral ganglion.
o, The cephalic tentacles.
f, The genital pore.
y, The ovo-testes.
w, The parasitic hydromedusa Mnestra, usually found attached in this
position by the aboral pole of its umbrella.]
The variety of special developments of structure accompanying the
atrophy of typical organs in the Opisthobranchia and general
degeneration of organization is very great. The members of the order
present the same wide range of superficial appearance as do the
Pectinibranchiate Streptoneura, forms carrying well-developed spiral
shells and large mantle-skirts being included in the group, together
with flattened or cylindrical slug-like forms. But in respect of the
substitution of other parts for the mantle-skirt and for the gill
which the more degenerate Opisthobranchia exhibit, this order stands
alone. Some Opisthobranchia are striking examples of degeneration
(some Nudibranchia), having none of those regions or processes of the
body developed which distinguish the archaic Mollusca from such
flat-worms as the Dendrocoel Planarians. Indeed, were it not for
their retention of the characteristic odontophore we should have
little or no indication that such forms as _Phyllirhoë_ and
_Limapontia_ really belong to the Mollusca at all. The interesting
little _Rhodope veranyii_, which has no odontophore, has been
associated by systematists both with these simplified Opisthobranchs
and with Rhabdocoel Planarians.
[Illustration: FIG. 38.--Three views of _Aplysia sp._, in various
conditions of expansion and retraction. (After Cuvier.)
t, Anterior cephalic tentacles.
t², Posterior cephalic tentacles.
e, Eyes.
f, Metapodium.
ep, Epipodium.
g, Gill-plume (ctenidium).
m, Mantle-flap reflected over the thin oval shell.
os, s, Orifice formed by the unclosed border of the reflected
mantle-skirt, allowing the shell to show.
pe, The spermatic groove.]
In many respects the sea-hare (_Aplysia_), of which several species
are known (some occurring on the English coast), serves as a
convenient example of the fullest development of the organization
characteristic of Opisthobranchia. The woodcut (fig. 38) gives a
faithful representation of the great mobility of the various parts of
the body. The head is well marked and joined to the body by a somewhat
constricted neck. It carries two pairs of cephalic tentacles and a
pair of sessile eyes. The visceral hump is low and not drawn out into
a spire. The foot is long, carrying the oblong visceral mass upon it,
and projecting (as metapodium) a little beyond it (f). Laterally the
foot gives rise to a pair of mobile fleshy lobes, the parapodia (ep),
which can be thrown up so as to cover in the dorsal surface of the
animal. Such parapodia are common, though by no means universal, among
Opisthobranchia. The torsion of the visceral hump is not carried out
very fully, the consequence being that the anus has a posterior
position a little to the right of the median line above the
metapodium, whilst the branchial chamber formed by the overhanging
mantle-skirt faces the right side of the body instead of lying well to
the front as in Streptoneura and as in Pulmonate Euthyneura. The
gill-plume, which in _Aplysia_ is the typical Molluscan ctenidium, is
seen in fig. 39 projecting from the branchial sub-pallial space. The
relation of the delicate shell to the mantle is peculiar, since it
occupies an oval area upon the visceral hump, the extent of which is
indicated in fig. 38, C, but may be better understood by a glance at
the figures of the allied genus _Umbrella_ (fig. 40), in which the
margin of the mantle-skirt coincides, just as it does in the limpet,
with the margin of the shell. But in _Aplysia_ the mantle is reflected
over the edge of the shell, and grows over its upper surface so as to
completely enclose it, excepting at the small central area s where the
naked shell is exposed. This enclosure of the shell is a permanent
development of the arrangement seen in many Streptoneura (e.g.
_Pyrula, Ovula_, see figs. 18 and 32), where the border of the mantle
can be, and usually is, drawn over the shell, though it is withdrawn
(as it cannot be in _Aplysia_) when they are irritated. From the fact
that _Aplysia_ commences its life as a free-swimming veliger with a
nautiloid shell not enclosed in any way by the border of the mantle,
it is clear that the enclosure of the shell in the adult is a
secondary process. Accordingly, the shell of _Aplysia_ must not be
confounded with a primitive shell in its shell-sac, such as we find
realized in the shells of _Chiton_ and in the plugs which form in the
remarkable transitory "shell-sac" or "shell-gland" of Molluscan
embryos (see figs. 26, 60). _Aplysia_, like other Mollusca, develops a
primitive shell-sac in its trochosphere stage of development, which
disappears and is succeeded by a nautiloid shell (fig. 36). This forms
the nucleus of the adult shell, and, as the animal grows, becomes
enclosed by a reflection of the mantle-skirt. When the shell of an
_Aplysia_ enclosed in its mantle is pushed well to the left, the
sub-pallial space is fully exposed as in fig. 39, and the various
apertures of the body are seen. Posteriorly we have the anus, in
front of this the lobate gill-plume, between the two (hence
corresponding in position to that of the Pectinibranchia) we have the
aperture of the renal organ. In front, near the anterior attachment of
the gill-plume, is the osphradium (olfactory organ) discovered by J.W.
Spengel, yellowish in colour, in the typical position, and overlying
an olfactory ganglion with typical nerve-connexion (see fig. 43). To
the right of Spengel's osphradium is the opening of a peculiar gland
which has, when dissected out, the form of a bunch of grapes; its
secretion is said to be poisonous. On the under side of the free edge
of the mantle are situated the numerous small cutaneous glands which,
in the large _Aplysia camelus_ (not in other species), form the purple
secretion which was known to the ancients. In front of the osphradium
is the single genital pore, the aperture of the common or
hermaphrodite duct. From this point there passes forward to the right
side of the head a groove--the spermatic groove--down which the
spermatic fluid passes. In other Euthyneura this groove may close up
and form a canal. At its termination by the side of the head is the
muscular introverted penis. In the hinder part of the foot (not shown
in any of the diagrams) is the opening of a large mucus-forming gland
very often found in the Molluscan foot.
[Illustration: FIG. 39.--_Aplysia leporina_ (_camelus_, Cuv.), with
epipodia and mantle reflected away from the mid-line. (Lankester.)
a, Anterior cephalic tentacle.
b, Posterior cephalic tentacle; between a and b, the eyes.
c, Right epipodium.
d, Left epipodium.
e, Hinder part of visceral hump.
fp, Posterior extremity of the foot.
fa, Anterior part of the foot underlying the head.
g, The ctenidium (branchial plume).
h, The mantle-skirt tightly spread over the horny shell and pushed
with it towards the left side.
i, The spermatic groove.
k, The common genital pore (male and female).
l, Orifice of the grape-shaped (supposed poisonous) gland.
m, The osphradium (olfactory organ of Spengel).
n, Outline of part of the renal sac (nephridium) below the surface.
o, External aperture of the nephridium.
p, Anus.]
With regard to internal organization we may commence with the
disposition of the renal organ (nephridium), the external opening of
which has already been noted. The position of this opening and other
features of the renal organ were determined by J.T. Cunningham.
[Illustration: FIG. 40.--_Umbrella mediterranea_. a, mouth; b,
cephalic tentacle; h, gill (ctenidium). The free edge of the mantle is
seen just below the margin of the shell (compare with _Aplysia_, fig.
39). (From Owen.)]
There is considerable uncertainty with respect to the names of the
species of _Aplysia_. There are two forms which are very common in the
Gulf of Naples. One is quite black in colour, and measures when
outstretched 8 or 9 in. in length. The other is light brown and
somewhat smaller, its length usually not exceeding 7 in. The first is
flaccid and sluggish in its movements, and has not much power of
contraction; its epipodial lobes are enormously developed and extend
far forward along the body; it gives out when handled an abundance of
purple liquid, which is derived from cutaneous glands situated on the
under side of the free edge of the mantle. According to F. Blochmann
it is identical with _A. camelus_ of Cuvier. The other species is _A.
depilans_; it is firm to the touch, and contracts forcibly when
irritated; the secretion of the mantle-glands is not abundant, and is
milky white in appearance. The kidney has similar relations in both
species, and is identical with the organ spoken of by many authors as
the triangular gland. Its superficial extent is seen when the folds
covering the shell are cut away and the shell removed; the external
surface forms a triangle with its base bordering the pericardium, and
its apex directed posteriorly and reaching the the left-hand posterior
corner of the shell-chamber. The dorsal surface of the kidney extends
to the left beyond the shell-chamber beneath the skin in the space
between the shell-chamber and the left parapodium.
When the animal is turned on its left-hand side and the mantle-chamber
widely opened, the gill being turned over to the left, a part of the
kidney is seen beneath the skin between the attachment of the gill and
the right parapodium (fig. 39). On examination this is found to be the
under surface of the posterior limb of the gland, the upper surface of
which has just been described as lying beneath the shell. In the
posterior third of this portion, close to that edge which is adjacent
to the base of the gill, is the external opening (fig. 39, o).
When the pericardium is cut open from above in an animal otherwise
entire, the anterior face of the kidney is seen forming the posterior
wall of the pericardial chamber; on the deep edge of this face, a
little to the left of the attachment of the auricle to the floor of
the pericardium, is seen a depression; this depression contains the
opening from the pericardium into the kidney.
To complete the account of the relations of the organ: the right
anterior corner can be seen superficially in the wall of the
mantle-chamber above the gill. Thus the base of the gill passes in a
slanting direction across the right-hand side of the kidney, the
posterior end being dorsal to the apex of the gland, and the anterior
end ventral to the right-hand corner.
As so great a part of the whole surface of the kidney lies adjacent to
external surfaces of the body, the remaining part which faces the
internal organs is small; it consists of the left part of the under
surface; it is level with the floor of the pericardium, and lies over
the globular mass formed by the liver and convoluted intestine.
[Illustration: FIG. 41.--Gonad, and accessory glands and ducts of
_Aplysia_. (Lankester.)
i, Ovo-testis.
h, Hermaphrodite duct.
g, Albuminiparous gland.
f, Vesicula seminalis.
k, Opening of the albuminiparous gland into the hermaphrodite duct.
e, Hermaphrodite duct (uterine portion).
b, Vaginal portion of the uterine duct.
c, Spermatheca.
d, Its duct.
a, Genital pore.]
Thus the renal organ of _Aplysia_ is shown to conform to the Molluscan
type. The heart lying within the adjacent pericardium has the usual
form, a single auricle and ventricle. The vascular system is not
extensive, the arteries soon ending in the well-marked spongy tissue
which builds up the muscular foot, parapodia, and dorsal body-wall.
The alimentary canal commences with the usual buccal mass; the lips
are cartilaginous, but not armed with horny jaws, though these are
common in other Opisthobranchs; the lingual ribbon is
multidenticulate, and a pair of salivary glands pour in their
secretion. The oesophagus expands into a curious gizzard, which is
armed internally with large horny processes, some broad and thick,
others spinous, fitted to act as crushing instruments. From this we
pass to a stomach and a coil of intestine embedded in the lobes of a
voluminous liver; a caecum of large size is given off near the
commencement of the intestine. The liver opens by two ducts into the
digestive tract.
The generative organs lie close to the coil of intestine and liver, a
little to the left side. When dissected out they appear as represented
in fig. 41. The essential reproductive organ or gonad consists of both
ovarian and testicular cells (see fig. 42). It is an ovo-testis. From
it passes a common or hermaphrodite duct, which very soon becomes
entwined in the spire of a gland--the albuminiparous gland. The latter
opens into the common duct at the point k, and here also is a small
diverticulum of the duct f. Passing on, we find not far from the
genital pore a glandular spherical body (the spermatheca c) opening by
means of a longish duct into the common duct, and then we reach the
pore (fig. 39, k). Here the female apparatus terminates. But when the
male secretion of the ovo-testis is active, the seminal fluid passes
from the genital pore along the spermatic groove (fig. 39) to the
penis, and is by the aid of that eversible muscular organ introduced
into the genital pore of a second _Aplysia_, whence it passes into the
spermatheca, there to await the activity of the female element of the
ovo-testis of this second _Aplysia_. After an interval of some
days--possibly weeks--the ova of the second _Aplysia_ commence to
descend the hermaphrodite duct; they become enclosed in a viscid
secretion at the point where the albuminiparous gland opens into the
duct intertwined with it; and on reaching the point where the
spermathecal duct debouches they are impregnated by the spermatozoa
which escape now from the spermatheca and meet the ova.
[Illustration: FIG. 42.--Follicles of the hermaphrodite gonads of
Euthyneurous Gastropods. A, of _Helix_; B, of _Eolis_; a, ova; b,
developing spermatozoa; c, common efferent duct.]
The development of _Aplysia_ from the egg presents many points of
interest from the point of view of comparative embryology, but in
relation to the morphology of the Opisthobranchia it is sufficient to
point to the occurrence of a trochosphere and a veliger stage (fig.
36), and of a shell-gland or primitive shell-sac (fig. 36, _shgr_),
which is succeeded by a nautiloid shell.
[Illustration: FIG. 43.--Nervous system of _Aplysia_, as a type of the
long-looped Euthyneurous condition. The untwisted visceral loop is
lightly shaded. (After Spengel.)
ce, Cerebral ganglion.
pl, Pleural ganglion.
pe, Pedal ganglion.
ab.sp, Abdominal ganglion which represents also the
supra-intestinal ganglion of Streptoneura and gives off the nerve
to the osphradium (olfactory organ) o, and another to an unlettered
so-called "genital" ganglion. The buccal nerves and ganglia are
omitted.]
In the nervous system of _Aplysia_ the great ganglion-pairs are well
developed and distinct. The euthyneurous visceral loop is long, and
presents only one ganglion (in _Aplysia camelus_, but two distinct
ganglia joined to one another in _Aplysia hybrida_ of the English
coast), placed at its extreme limit, representing both the right and
left visceral ganglia and the third or abdominal ganglion, which are
so often separately present. The diagram (fig. 43) shows the nerve
connecting this abdomino-visceral ganglion with the olfactory ganglion
of Spengel. It is also seen to be connected with a more remote
ganglion--the genital. Such special irregularities in the development
of ganglia upon the visceral loop, and on one or more of the main
nerves connected with it, are very frequent. Our figure of the nervous
system of _Aplysia_ does not give the small pair of buccal ganglia
which are, as in all glossophorous Molluscs, present upon the nerves
passing from the cerebral region to the odontophore.
For a comparison of various Opisthobranchs, _Aplysia_ will be found to
present a convenient starting-point. It is one of the more typical
Opisthobranchs, that is to say, it belongs to the section
Tectibranchia, but other members of the suborder, namely, _Bulla_ and
_Actaeon_ (figs. 44 and 45), are less abnormal than _Aplysia_ in
regard to their shells and the form of the visceral hump. They have
naked spirally twisted shells which may be concealed from view in the
living animal by the expansion and reflection of the parapodia, but
are not enclosed by the mantle, whilst _Actaeon_ is remarkable for
possessing an operculum like that of so many Streptoneura.
The great development of the parapodia seen in _Aplysia_ is usual in
Tectibranchiate Opisthobranchs. The whole surface of the body becomes
greatly modified in those Nudibranchiate forms which have lost, not
only the shell, but also the ctenidium. Many of these have peculiar
processes developed on the dorsal surface (fig. 46, A, B), or retain
purely negative characters (fig. 46, D). The chief modification of
internal organization presented by these forms, as compared with
_Aplysia_, is found in the condition of the alimentary canal. The
liver is no longer a compact organ opening by a pair of ducts into the
median digestive tract, but we find very numerous hepatic diverticula
on a shortened axial tract (fig. 47). These diverticula extend usually
one into each of the dorsal papillae or "cerata" when these are
present. They are not merely digestive glands, but are sufficiently
wide to act as receptacles of food, and in them the digestion of food
proceeds just as in the axial portion of the canal. A precisely
similar modification of the liver or great digestive gland is found in
the scorpions, where the axial portion of the digestive canal is short
and straight, and the lateral ducts sufficiently wide to admit food
into the ramifications of the gland there to be digested; whilst in
the spiders the gland is reduced to a series of simple caeca.
[Illustration: FIG. 44.--_Bulla vexillum_ (Chemnitz), as seen
crawling. a', oral hood (compare with Tethys, fig. 46, B), possibly a
continuation of the epipodia; b, b', cephalic tentacles. (From Owen.)]
The typical character is retained by the heart, pericardium, and the
communicating nephridium or renal organ in all Opisthobranchs. An
interesting example of this is furnished by the fish-like transparent
_Phyllirhoë_ (fig. 37), in which it is possible most satisfactorily to
study in the living animal, by means of the microscope, the course of
the blood-stream, and also the reno-pericardial communication. In many
of the Nudibranchiate Opisthobranchs the nervous system presents a
concentration of the ganglia (fig. 48), contrasting greatly with what
we have seen in _Aplysia_. Not only are the pleural ganglia fused to
the cerebral, but also the visceral to these (see in further
illustration the condition attained by the Pulmonate _Limnaeus_, fig.
59), and the visceral loop is astonishingly short and insignificant
(fig. 48, e'). That the parts are rightly thus identified is
probable from J.W. Spengel's observation of the osphradium and its
nerve-supply in these forms; the nerve to that organ, which is placed
somewhat anteriorly--on the dorsal surface--being given off from the
hinder part (visceral) of the right compound ganglion--the fellow to
that marked A in fig. 48. The Eolid-like Nudibranchs, amongst other
specialities of structure, possess (in some cases at any rate)
apertures at the apices of the "cerata" or dorsal papillae, which lead
from the exterior into the hepatic caeca. Some amongst them
(_Tergipes, Eolis_) are also remarkable for possessing peculiarly
modified cells placed in sacs (cnidosacs) at the apices of these same
papillae, which resemble the "thread-cells" of the Coelentera.
According to T.S. Wright and J.H. Grosvenor these nematocysts are
derived from the hydroids on which the animals feed.
[Illustration: FIG. 45.--_Actaeon._ h, shell; b, oral hood; d, foot;
f, operculum.]
The development of many Opisthobranchia has been examined--e.g.
_Aplysia, Pleurobranchidium, Elysia, Polycera, Doris, Tergipes_. All
pass through trochosphere and veliger stages, and in all a nautiloid
or boat-like shell is developed, preceded by a well-marked
"shell-gland" (see fig. 36). The transition from the free-swimming
veliger larva with its nautiloid shell (fig. 36) to the adult form has
not been properly observed, and many interesting points as to the true
nature of folds (whether parapodia or mantle or velum) have yet to be
cleared up by a knowledge of such development in forms like _Tethys,
Doris, Phyllidia_, &c. As in other Molluscan groups, we find even in
closely-allied genera (for instance, in _Aplysia_ and
_Pleurobranchidium_, and other genera), the greatest differences as to
the _amount_ of food-material by which the egg-shell is encumbered.
Some form their diblastula by emboly, others by epiboly; and in the
later history of the further development of the enclosed cells
(arch-enteron) very marked variations occur in closely-allied forms,
due to the influence of a greater or less abundance of food-material
mixed with the protoplasm of the egg.
Sub-order 1.--TECTIBRANCHIA. Opisthobranchs provided in the adult
state with a shell and a mantle, except _Runcina, Pleurobranchaea,
Cymbuliidae_, and some Aplysiomorpha. There is a ctenidium, except in
some Thecosomata and Gymnosomata, and an osphradium.
Tribe 1.--BULLOMORPHA. The shell is usually well developed, except in
_Runcina_ and _Cymbuliidae_, and may be external or internal. No
operculum, except in _Actaeonidae_ and _Limacinidae_. The pallial
cavity is always well developed, and contains the ctenidium, at least
in part; ctenidium, except in _Lophocercidae_, of folded type. With
the exception of the _Aplustridae, Lophocercidae_ and _Thecosomata_,
the head is devoid of tentacles, and its dorsal surface forms a
digging disk or shield. The edges of the foot form parapodia, often
transformed into fins. Posteriorly the mantle forms a large pallial
lobe under the pallial aperture. Stomach generally provided with
chitinous or calcified masticatory plates. Visceral commissure fairly
long, except in _Runcina, Lobiger_ and _Thecosomata_. Hermaphrodite
genital aperture, connected with the penis by a ciliated groove,
except in _Actaeon, Lobiger_ and _Cavolinia longirostris_, in which
the spermiduct is a closed tube. Animals either swim or burrow.
[Illustration: FIG. 46.
A, _Eolis papillosa_ (Lin.), dorsal view.
a, b, Posterior and anterior cephalic tentacles.
c, The dorsal "cerata."
B, _Tethys leporina_, dorsal view.
a, The cephalic hood.
b, Cephalic tentacles.
c, Neck.
d, Genital pore.
e, Anus.
f, Large cerata.
g, Smaller cerata.
h, Margin of the foot.
C, _Doris (Actinocyclus) tuberculatus_ (Cuv.), seen from the pedal
surface.
m, Mouth.
b, Margin of the head.
f, Sole of the foot.
sp, The mantle-like epipodium.
D, E, Dorsal and lateral view of _Elysia (Actaeon) viridis_.
ep, epipodial outgrowths. (After Keferstein.)]
[Illustration: FIG. 47.--Enteric Canal of _Eolis papillosa_. (From
Gegenbaur, after Alder and Hancock.)
ph, Pharynx.
m, Midgut, with its hepatic appendages h, all of which are not
figured.
e, Hind gut.
an, Anus.]
[Illustration: FIG. 48.--Central Nervous System of _Fiona_ (one of the
Nudibranchia), showing a tendency to fusion of the great ganglia.
(From Gegenbaur, after Bergh.)
A, Cerebral, pleural and visceral ganglia united.
B, Pedal ganglion.
C, Buccal ganglion.
D, Oesophageal ganglion connected with, the Buccal.
a, Nerve to superior cephalic tentacle.
b, Nerves to inferior cephalic tentacles.
c, Nerve to generative organs.
d, Pedal nerve.
e, Pedal commissure.
e', Visceral loop or commissure (?).]
[Illustration: Fig. 49.--_Cavolinia tridentata_, Forsk. from the
Mediterranean, magnified two diameters. (From Owen.)
a, Mouth.
b, Pair of cephalic tentacles.
C, C, Pteropodial lobes of the foot.
d, Median web connecting these.
e, e, Processes of the mantle-skirt reflected over the surface of
the shell.
g, The shell enclosing the visceral hump.
h. The median spine of the shell.]
Fam. 1.--_Actaeonidae._ Cephalic shield bifid posteriorly; margins
of foot slightly developed; genital duct diaulic; visceral
commissure streptoneurous; shell thick, with prominent spire and
elongated aperture; a horny operculum. _Actaeon_, British.
_Solidula. Tornatellaea_, extinct. _Adelactaeon. Bullina.
Bullinula._
Fam. 2.--_Ringiculidae._ Cephalic disk enlarged anteriorly, forming
an open tube posteriorly; shell external, thick, with prominent
spire; no operculum. _Ringicula. Pugnus._
Fam. 3.--_Tornatinidae._ Margins of foot not prominent; no radula;
shell external, with inconspicuous spire. _Tornatina_, British.
_Retusa. Volvula._
Fam. 4.--_Scaphandridae._ Cephalic shield short, truncated
posteriorly; eyes deeply embedded; three calcareous stomachal
plates; shell external, with reduced spire. _Scaphander_, British.
_Atys. Smaragdinella. Cylichna_, British. _Amphisphyra_, British.
Fam. 5.--_Bullidae._ Margins of foot well developed; eyes
superficial; three chitinous stomachal plates; shell external, with
reduced spire. Bulla, British. _Haminea_, British.
Fam. 6.--_Aceratidae._ Cephalic shield continuous with neck; twelve
to fourteen stomachal plates; a posterior pallial filament passing
through a notch in shell. _Acera_, British. _Cylindrobulla.
Volutella._
Fam. 7.--_Aplustridae._ Foot very broad; cephalic shield with four
tentacles; shell external, thin, without prominent spire.
_Aplustrum. Hydatina. Micromelo._
Fam. 8.--_Philinidae._ Cephalic shield broad, thick and simple;
shell wholly internal, thin, spire much reduced, aperture very
large. _Philine_, British. _Cryptophthalmus. Chelinodura.
Phanerophthalmus. Colpodaspis_, British. _Colobocephalus._
Fam. 9.--_Doridiidae._ Cephalic shield ending posteriorly in a
median point; shell internal, largely membranous; no radula or
stomachal plates. _Doridium. Navarchus._
Fam. 10.--_Gastropteridae._ Cephalic shield pointed behind; shell
internal, chiefly membranous, with calcified nucleus, nautiloid;
parapodia forming fins. _Gastropteron._
Fam. 11.--_Runcinidae._ Cephalic shield continuous with dorsal
integument; no shell; ctenidium projecting from mantle cavity.
_Runcina._
Fam. 12.--_Lophocercidae._ Shell external, globular or ovoid; foot
elongated, parapodia separate from ventral surface; genital duct
diaulic. _Lobiger. Lophocercus._
[Illustration: FIG. 50.--Shell of _Cavolinia tridentata_, seen from
the side.
f, Postero-dorsal surface.
g, Antero-ventral surface.
h, Median dorsal spine.
i, Mouth of the shell.]
The next three families form the group formerly known as Thecosomatous
Pteropods. They are all pelagic, the foot being entirely transformed
into a pair of anterior fins; eyes are absent, and the nerve centres
are concentrated on the ventral side of the oesophagus.
Fam. 13.--_Limacinidae._ Dextral animals, with shell coiled
pseudo-sinistrally; operculum with sinistral spiral; pallial cavity
dorsal. _Limacina_, British. _Peraclis_, ctenidium present.
Fam. 14.--_Cymbuliidae._ Adult without shell; a sub-epithelial
pseudoconch formed by connective tissue; pallial cavity ventral.
_Cymbulia. Cymbuliopsis. Gleba. Desmopterus._
Fam. 15.--_Cavoliniidae._ Shell not coiled, symmetrical; pallial
cavity ventral. _Cavolinia. Clio. Cuvierina._
Tribe 2.--APLYSIOMORPHA. Shell more or less internal, much reduced or
absent. Head bears two pairs of tentacles. Parapodia separate from
ventral surface, and generally transformed into swimming lobes.
Visceral commissure much shortened, except in _Aplysia_. Genital duct
monaulic; hermaphrodite duct connected with penis by a ciliated
groove. Animals either swim or crawl.
Fam. 1.--_Aplysiidae_. Shell partly or wholly internal, or absent;
foot long, with well-developed ventral surface. _Aplysia. Dolabella.
Dolabrifer. Aplysiella. Phyllaplysia. Notarchus_.
The next six families include the animals formerly known as
Gymnosomatous Pteropods, characterized by the absence of mantle and
shell, the reduction of the ventral surface of the foot, and the
parapodial fins at the anterior end of the body. They are all pelagic.
Fam. 2.--_Pneumonodermatidae_. Pharynx evaginable, with suckers.
_Pneumonoderma. Dexiobranchaea. Spongiobranchaea. Schizobrachium_.
Fam. 3.--_Clionopsidae_. No buccal appendages or suckers; a very
long evaginable proboscis; a quadriradiate terminal branchia.
_Clionopsis_.
Fam. 4.--_Notobranchaeidae_. Posterior branchia triradiate.
Notobranchaea.
Fam. 5.--_Thliptodontidae_. Head very large, not marked off from the
body; neither branchia nor suckers; fins situated near the middle of
the body. _Thliptodon_.
[Illustration: FIG. 51.--Embryo of _Cavolinia tridentata_. (From
Balfour, after Fol.)
a, Anus.
f, Median portion of the foot.
pn, Pteropodial lobe of the foot.
h, Heart.
i, Intestine.
m. Mouth.
ot, Otocyst.
q, Shell.
r, Nephridium.
s, Oesophagus.
[sigma], Sac containing nutritive yolk.
mb, Mantle-skirt.
mc, Sub-pallial chamber.
Kn, Contractile sinus.]
[Illustration: FIG. 52.--_Styliola acicula_, Rang. sp. enlarged. (From
Owen.)
C, C, The wing-like lobes of the foot.
d, Median fold of same.
e, Copulatory organ.
h, Pointed extremity of the shell.
i, Anterior margin of the shell.
n, Stomach.
o, Liver.
u. Hermaphrodite gonad.]
Fam. 6.--_Clionidae_. No branchia of any kind; a short evaginable
pharynx, bearing paired conical buccal appendages or "cephalocones."
_Clione. Paraclione. Fowlerina_.
Fam. 7.--_Halopsychidae_. No branchia; two long and branched buccal
appendages. _Halopsyche_.
Tribe 3.--PLEUROBRANCHOMORPHA. Two pairs of tentacles. Foot without
parapodia; no pallial cavity, but always a single ctenidium situated
on the right side between mantle and foot. Genital duct diaulic,
without open seminal groove; male and female apertures contiguous.
Visceral commissure short, tendency to concentration of all ganglia in
dorsal side of oesophagus.
Fam. 1.--_Tylodinidae_. Shell external and conical; anterior
tentacles form a frontal veil; ctenidium extending only over right
side; a distinct osphradium. _Tylodina_.
Fam. 2.--_Umbrellidae_. Shell external, conical, much flattened;
anterior tentacles very small, and situated with the mouth in a
notch of the foot below the head; ctenidium very large. _Umbrella_.
Fam. 3.--_Pleurobranchidae_. Shell covered by mantle, or absent;
anterior tentacles form a frontal veil; mantle contains spicules.
_Pleurobranchus. Berthella. Haliotinella. Oscanius_, British.
_Oscaniella. Oscaniopsis. Pleurobranchaea._
Sub-order 2.--NUDIBRANCHIA. Shell absent in the adult; no ctenidium or
osphradium. Body generally slug-like, and externally symmetrical.
Visceral mass not marked off from the foot, except in _Hedylidae._
Dorsal respiratory appendages frequently present. Visceral commissure
reduced; nervous system concentrated on dorsal side of oesophagus.
Marine; generally carnivorous, and brightly coloured, affording many
instances of protective resemblance.
Tribe 1.--TRITONIOMORPHA. Liver wholly or partially contained in the
visceral mass. Anus lateral, on the right side. Usually two rows of
ramified dorsal appendages. Genital duct diaulic; male and female
apertures contiguous.
Fam. 1.--_Tritoniidae._ Anterior tentacles form a frontal veil; foot
rather broad. _Tritonia_, British. _Marionia._
Fam. 2.--_Scyllaeidae._ No anterior tentacles; dorsal appendages
broad and foliaceous; foot very narrow; stomach with horny plates.
_Scyllaea_, pelagic.
Fam. 3.--_Phyllirhoidae._ No anterior tentacles, and no dorsal
appendages; body laterally compressed, transparent; pelagic.
_Phyllirhoë._
Fam. 4.--_Tethyidae._ Head broad, surrounded by a funnel-shaped
velum or hood; no radula; dorsal appendages foliaceous. _Tethys.
Melibe._
Fam. 5.--_Dendronotidae._ Anterior tentacles forming a scalloped
frontal veil; dorsal appendages and tentacles similarly ramified.
_Dendronotus. Campaspe._
Fam. 6.--_Bornellidae._ Dorsum furnished on either side with
papillae, at the base of which are ramified appendages. _Bornella._
Fam. 7.--_Lomanotidae._ Body flattened, the two dorsal borders
prominent and foliaceous. _Lomanotus_, British.
Tribe 2.--DORIDOMORPHA. Body externally symmetrical; anus median,
posterior, and generally dorsal, surrounded by ramified pallial
appendages, constituting a secondary branchia. Liver not ramified in
the integuments. Genital duct triaulic. Spicules present in the
mantle.
[Illustration: FIG. 53.--_Halopsyche gaudichaudii_, Soul. (From Owen.)
Much enlarged; the body-wall removed.
a, The mouth.
c, The pteropodial lobes of the foot.
f, The centrally-placed hind-foot.
d, l, e, Three pairs of tentacle-like processes placed at the sides
of the mouth, and developed (in all probability) from the
fore-foot.
o', Anus.
y, Genital pore.
k, Retractor muscles.
o and p, The liver.
u, v, w, Genitalia.]
[Illustration: FIG. 54.--_Ancula cristata_, one of the pygobranchiate
Opisthobranchs (dorsal view). (From Gegenbaur, after Alder and
Hancock.)
a, Anus.
br, Secondary branchia surrounding the anus.
t, Cephalic tentacles.
External to the branchia are seen ten club-like processes of the
dorsal wall, these are the "cerata" which are characteristically
developed in another suborder of Opisthobranchs.]
Fam. 1.--_Polyceratidae._ A more or less prominent frontal veil;
branchiae non-retractile. _Euplocamus. Polycera_, British.
_Thecacera_, British. _Aegirus_, British. _Plocamopherus. Palio.
Crimora. Triopa_, British. _Triopella._
Fam. 2.--_Goniodorididae._ Mantle-border projecting; frontal veil
reduced, and often covered by the anterior border of the mantle.
_Goniodoris_, British. _Acanthodoris_, British. _Idalia_, British.
_Ancula_, British. _Doridunculus_. _Lamellidoris. Ancylodoris_,
the only fresh-water Nudibranch, from Lake Baikal.
Fam. 3.--_Heterodorididae_. No branchia. _Heterodoris_.
Fam. 4.--_Dorididae_. Mantle oval, covering the head and the greater
part of the body; anterior tentacles, ill-developed; branchiae
generally retractile. _Doris_, British. _Hexabranchus_.
_Chromodoris_.
Fam. 5.--_Doridopsidae_. Pharynx suctorial; no radula; branchial
rosette on the dorsal surface, above the mantle-border.
_Doridopsis_.
Fam. 6.--_Corambidae_. Anus and branchia posterior, below the
mantle-border. _Corambe_.
Fam. 7.-_-Phyllidiidae_. Pharynx suctorial; branchiae surrounding
the body, between the mantle and foot. _Phyllidia. Fryeria_.
The last three families constitute the sub-tribe Porostomata,
characterized by the reduction of the buccal mass, which is modified
into a suctorial apparatus.
Tribe 3.--EOLIDOMORPHA (_Cladohepatica_). The whole of the liver
contained in the integuments and tegumentary papillae. Genital duct
diaulic; male and female apertures contiguous. The anus is
antero-lateral, except in the _Proctonotidae_, in which it is median.
Tegumentary papillae not ramified, and containing cnidosacs with
nematocysts.
Fam. 1.--_Eolididae_. Dorsal papillae spindle-shaped or club-shaped.
_Eolis_, British. _Facelina_, British. _Tergipes_, British.
_Gonieolis. Cuthona. Embletonia. Galvina. Calma. Hero_.
Fam. 2.--_Glaucidae_. Body furnished with three pairs of lateral
lobes, bearing the tegumentary papillae; foot very narrow; pelagic.
_Glaucus_.
Fam. 3.--_Hedylidae_. Body elongated; visceral mass marked off from
foot posteriorly; dorsal appendages absent, or reduced to a single
pair; spicules in the integument. _Hedyle_.
Fam. 4.--_Pseudovermidae_. Head without tentacles; body elongated;
anus on right side. _Pseudovermis_.
Fam. 5.--_Proctonotidae_. Anus posterior, median; anterior
tentacles, atrophied; foot broad. _Janus_, British. _Proctonotus_,
British.
Fam. 6.--_Dotonidae_. Bases of the rhinophores surrounded by a
sheath; dorsal papillae tuberculated and club-shaped, in a single
row on either side of the dorsum; no cnidosacs. _Doto_, British.
_Gellina. Heromorpha_.
Fam. 7.--_Fionidae_. Dorsal papillae with a membranous expansion;
male and female apertures at some distance from each other; pelagic.
_Fiona_.
Fam. 8.--_Pleurophyllidae_. Anterior tentacles in the form of a
digging shield; mantle without appendages, but respiratory papillae
beneath the mantle-border. _Pleurophyllidia_.
Fam. 9.--_Dermatobranchidae_. Like the last, but wholly without
branchiae. _Dermatobranchus_.
Tribe 4.--ELYSIOMORPHA. Liver ramifies in integuments and extends into
dorsal papillae, but there are no cnidosacs. Genital duct always
triaulic, and male and female apertures distant from each other. No
mandibles, and radula uniserial. Never more than one pair of
tentacles, and these are absent in _Alderia_ and some species of
_Limapontia_.
[Illustration: FIG. 55.--Dorsal and Ventral View of _Pleurophyllidia
lineata_ (Otto), one of the Eolidomorph Nudibranchs. (After
Keferstein.)
b, The mouth.
l, The lamelliform sub-pallial gills, which (as in Patella) replace
the typical Molluscan ctenidium.]
Fam. 1.--_Hermaeidae_. Foot narrow; dorsal papillae linear or
fusiform, in several series. _Hermaea_, British. _Stiliger_.
_Alderia_, British.
Fam. 2.--_Phyllobranchidae_. Foot broad; dorsal papillae flattened
and foliaceous. _Phyllobranchus. Cyerce_.
Fam. 3.--_Plakobranchidae_. Body depressed, without dorsal papillae,
but with two very large lateral expansions, with dorsal plications.
_Plakobranchus_.
Fam. 4.--_Elysiidae_. Body elongated, with lateral expansions;
tentacles large; foot narrow. _Elysia_, British. _Tridachia_.
Fam. 5.--_Limapontiidae_. No lateral expansions, and no dorsal
papillae; body planariform; anus dorsal, median and posterior.
_Limapontia_, British. _Actaeonia_, British. _Cenia_.
Order 2 (of the Euthyneura).--PULMONATA. Euthyneurous Gastropoda,
probably derived from ancestral forms similar to the Tectibranchiate
Opisthobranchia by adaptation to a terrestrial life. The ctenidium is
atrophied, and the edge of the mantle-skirt is fused to the dorsal
integument by concrescence, except at one point which forms the
aperture of the mantle-chamber, thus converted into a nearly closed
sac. Air is admitted to this sac for respiratory and hydrostatic
purposes, and it thus becomes a lung. An operculum is present only in
_Amphibola_; a contrast being thus afforded with the operculate
pulmonate Streptoneura (_Cyclostoma_, &c.), which differ in other
essential features of structure from the Pulmonata. The Pulmonata are,
like the other Euthyneura, hermaphrodite, with elaborately developed
copulatory organs and accessory glands. Like other Euthyneura, they
have very numerous small denticles on the lingual ribbon. In aquatic
Pulmonata the osphradium is retained.
In some Pulmonata (snails) the foot is extended at right angles to the
visceral hump, which rises from it in the form of a coil as in
Streptoneura; in others the visceral hump is not elevated, but is
extended with the foot, and the shell is small or absent (slugs).
[Illustration: FIG. 56.--A Series of Stylommatophorous Pulmonata,
showing transitional forms between snail and slug.
A, _Helix pomatia_. (From Keferstein.)
B, _Helicophanta brevipes_. (From Keferstein, after Pfeiffer.)
C, _Testacella haliotidea_. (From Keferstein.)
D, _Arion ater_, the great black slug. (From Keferstein.)
a, Shell in A, B, C, shell-sac (closed) in D; b, orifice leading
into the sub-pallial chamber (lung).]
Pulmonata are widely distinguished from a small number of Streptoneura
at one time associated with them on account of their mantle-chamber
being converted, as in Pulmonata, into a lung, and the ctenidium or
branchial plume aborted. The terrestrial Streptoneura (represented in
England by the common genus _Cyclostoma_) have a twisted visceral
nerve-loop, an operculum on the foot, a complex rhipidoglossate or
taenio-glossate radula, and are of distinct sexes. The Pulmonata have
a straight visceral nerve-loop, usually no operculum even in the
embryo, and a multidenticulate radula, the teeth being equi-formal;
and they are hermaphrodite. Some Pulmonata (_Limnaea_, &c.) live in
fresh waters although breathing air. The remarkable discovery has been
made that in deep lakes such _Limnaei_ do not breathe air, but admit
water to the lung-sac and live at the bottom. The lung-sac serves
undoubtedly as a hydrostatic apparatus in the aquatic Pulmonata, as
well as assisting respiration.
[Illustration: FIG. 57.--_Ancylus fluviatilis_, a patelliform aquatic
Pulmonate.]
The same general range of body-form is shown in Pulmonata as in the
Heteropoda and in the Opisthobranchia; at one extreme we have snails
with coiled visceral hump, at the other cylindrical or flattened slugs
(see fig. 56). Limpet-like forms are also found (fig. 57, _Ancylus_).
The foot is always simple, with its flat crawling surface extending
from end to end, but in the embryo _Limnaea_ it shows a bilobed
character, which leads on to the condition characteristic of
Pteropoda.
The adaptation of the Pulmonata to terrestrial life has entailed
little modification of the internal organization. In one genus
(_Planorbis_) the plasma of the blood is coloured red by haemoglobin,
this being the only instance of the presence of this body in the blood
of Glossophorous Mollusca, though it occurs in corpuscles in the blood
of the bivalves _Arca_ and _Solen_ (Lankester).
[Illustration: _Fig. 58._--Hermaphrodite Reproductive Apparatus of the
Garden Snail (_Helix hortensis_).
[tau], Ovo-testis.
ve, Hermaphrodite duct.
Ed, Albuminiparous gland.
u, Uterine dilatation of the hermaphrodite duct.
d, Digitate accessory glands on the female duct.
ps, Calciferous gland or dart-sac on the female duct.
Rf, Spermatheca or receptacle of the sperm in copulation, opening
into the female duct.
vd, Male duct (vas deferens).
p, Penis.
fl, Flagellum.]
The generative apparatus of the snail (_Helix_) may serve as an
example of the hermaphrodite apparatus common to the Pulmonata and
Opisthobranchia (fig. 58). From the ovo-testis, which lies near the
apex of the visceral coil, a common hermaphrodite duct ve proceeds,
which receives the duct of the compact white albuminiparous gland, Ed,
and then becomes much enlarged, the additional width being due to the
development of glandular folds, which are regarded as forming a uterus
u. Where these folds cease the common duct splits into two portions, a
male and a female. The male duct vd becomes fleshy and muscular near
its termination at the genital pore, forming the penis p. Attached to
it is a diverticulum fl, in which the spermatozoa which have descended
from the ovo-testis are stored and modelled into sperm ropes or
spermatophores. The female portion of the duct is more complex. Soon
after quitting the uterus it is joined by a long duct leading from a
glandular sac, the spermatheca (Rf). In this duct and sac the
spermatophores received in copulation from another snail are lodged.
In _Helix hortensis_ the spermatheca is simple. In other species of
_Helix_ a second duct (as large in _Helix aspersa_ as the chief one)
is given off from the spermathecal duct, and in the natural state is
closely adherent to the wall of the uterus. This second duct has
normally no spermathecal gland at its termination, which is simple and
blunt. But in rare cases in _Helix aspersa_ a second spermatheca is
found at the end of this second duct. Tracing the widening female duct
onwards we now come to the openings of the digitate accessory glands
d, d, which probably assist in the formation of the egg-capsule. Close
to them is the remarkable dart-sac ps, a thick-walled sac, in the
lumen of which a crystalline four-fluted rod or dart consisting of
carbonate of lime is found. It is supposed to act in some way as a
stimulant in copulation, but possibly has to do with the calcareous
covering of the egg-capsule. Other Pulmonata exhibit variations of
secondary importance in the details of this hermaphrodite apparatus.
The nervous system of _Helix_ is not favourable as an example on
account of the fusion of the ganglia to form an almost uniform ring of
nervous matter around the oesophagus. The pond-snail (_Limnaeus_)
furnishes, on the other hand, a very beautiful case of distinct
ganglia and connecting cords (fig. 59). The demonstration which it
affords of the extreme shortening of the Euthyneurous visceral
nerve-loop is most instructive and valuable for comparison with and
explanation of the condition of the nervous centres in Cephalopoda, as
also of some Opisthobranchia. The figure (fig. 59) is sufficiently
described in the letterpress attached to it; the pair of buccal
ganglia joined by the connectives to the cerebrals are, as in most of
our figures, omitted. Here we need only further draw attention to the
osphradium, discovered by Lacaze-Duthiers, and shown by Spengel to
agree in its innervation with that organ in all other Gastropoda. On
account of the shortness of the visceral loop and the proximity of the
right visceral ganglion to the oesophageal nerve-ring, the nerve to
the osphradium and olfactory ganglion is very long. The position of
the osphradium corresponds more or less closely with that of the
vanished right ctenidium, with which it is normally associated. In
_Helix_ and _Limax_ the osphradium has not been described, and
possibly its discovery might clear up the doubts which have been
raised as to the nature of the mantle-chamber of those genera. In
_Planorbis_, which is sinistral (as are a few other genera or
exceptional varieties of various Anisopleurous Gastropods), instead of
being dextral, the osphradium is on the left side, and receives its
nerve from the left visceral ganglion, the whole series of unilateral
organs being reversed. This is, as might be expected, what is found
to be the case in all "reversed" Gastropods.
The shell of the Pulmonata, though always light and delicate, is in
many cases a well-developed spiral "house" into which the creature can
withdraw itself; and, although the foot possesses no operculum, yet in
_Helix_ the aperture of the shell is closed in the winter by a
complete lid, the "hybernaculum" more or less calcareous in nature,
which is secreted by the foot. In _Clausilia_ a peculiar modification
of this lid exists permanently in the adult, attached by an elastic
stalk to the mouth of the shell, and known as the "clausilium." In
_Limnaeus_ the permanent shell is preceded in the embryo by a
well-marked shell-gland or primitive shell-sac (fig. 60), at one time
supposed to be the developing anus, but shown by Lankester to be
identical with the "shell-gland" discovered by him in other Mollusca
(_Pisidium, Pleurobranchidium, Neritina_, &c.). As in other Gastropoda
Anisopleura, this shell-sac may abnormally develop a plug of chitinous
matter, but normally it flattens out and disappears, whilst the
cap-like rudiment of the permanent shell is shed out from the
dome-like surface of the visceral hump, in the centre of which the
shell-sac existed for a brief period.
[Illustration: FIG. 59.--Nervous System of the Pond-Snail, _Limnaeus
stagnalis_, as a type of the short-looped euthyneurous condition. The
short visceral "loop" with its three ganglia is lightly-shaded.
ce, Cerebral ganglion.
pe, Pedal ganglion.
pl, Pleural ganglion.
ab, Abdominal ganglion.
sp, Visceral ganglion of the left side; opposite to it is the
visceral ganglion of the right side, which gives off the long nerve
to the olfactory ganglion and osphradium o.
In _Planorbis_ and in _Auricula_ (Pulmonata, allied to _Limnaeus_)
the olfactory organ is on the _left_ side and receives its nerve
from the _left_ visceral ganglion. (After Spengel.)]
In _Clausilia_, according to the observations of C. Gegenbaur, the
primitive shell-sac does not flatten out and disappear, but takes the
form of a flattened closed sac. Within this closed sac a plate of
calcareous matter is developed, and after a time the upper wall of the
sac disappears, and the calcareous plate continues to grow as the
nucleus of the permanent shell. In the slug _Testacella_ (fig. 56, C)
the shell-plate never attains a large size, though naked. In other
slugs, namely, _Limax_ and _Arion_, the shell-sac remains permanently
closed over the shell-plate, which in the latter genus consists of a
granular mass of carbonate of lime. The permanence of the primitive
shell-sac in these slugs is a point of considerable interest. It is
clear enough that the sac is of a different origin from that of
_Aplysia_ (described in the section treating of Opisthobranchia),
being primitive instead of secondary. It seems probable that it is
identical with one of the open sacs in which each shell-plate of a
_Chiton_ is formed, and the series of plate-like imbrications which
are placed behind the single shell-sac on the dorsum of the curious
slug, _Plectrophorus_, suggest the possibility of the formation of a
series of shell-sacs on the back of that animal similar to those which
we find in _Chiton_. Whether the closed primitive shell-sac of the
slugs (and with it the transient embryonic shell-gland of all other
Mollusca) is precisely the same thing as the closed sac in which the
calcareous pen or shell of the Cephalopod _Sepia_ and its allies is
formed, is a further question which we shall consider when dealing
with the Cephalopoda. It is important here to note that _Clausilia_
furnishes us with an exceptional instance of the _continuity_ of the
shell or secreted product of the primitive shell-sac with the adult
shell. In most other Mollusca (Anisopleurous Gastropods, Pteropods and
Conchifera) there is a want of such continuity; the primitive
shell-sac contributes no factor to the permanent shell, or only a very
minute knob-like particle (_Neritina_ and _Paludina_). It flattens out
and disappears before the work of forming the permanent shell
commences. And just as there is a break at this stage, so (as observed
by A. Krohn in _Marsenia_ = _Echinospira_) there _may_ be a break at a
later stage, the nautiloid shell formed on the larva being cast, and a
new shell of a different form being formed afresh on the surface of
the visceral hump. It is, then, in this sense that we may speak of
primary, secondary and tertiary shells in Mollusca recognizing the
fact that they _may_ be merely phases fused by continuity of growth so
as to form but one shell, or that in other cases they _may_ be
presented to us as separate individual things, in virtue of the
non-development of the later phases, or in virtue of sudden changes in
the activity of the mantle-surface causing the shedding or
disappearance of one phase of shell-formation before a later one is
entered upon.
The development of the aquatic Pulmonata from the egg offers
considerable facilities for study, and that of _Limnaeus_ has been
elucidated by E.R. Lankester, whilst H. Rabl has with remarkable skill
applied the method of sections to the study of the minute embryos of
_Planorbis_. The chief features in the development of _Limnaeus_ are
exhibited in fig. 60. There is not a very large amount of
food-material present in the egg of this snail, and accordingly the
cells resulting from division are not so unequal as in many other
cases. The four cells first formed are of equal size, and then four
smaller cells are formed by division of these four so as to lie at one
end of the first four (the pole corresponding to that at which the
"directive corpuscles" are extruded and remain). The smaller cells now
divide and spread over the four larger cells; at the same time a
space--the cleavage cavity or blastocoel--forms in the centre of the
mulberry-like mass. Then the large cells recommence the process of
division and sink into the hollow of the sphere, leaving an elongated
groove, the blastopore, on the surface. The invaginated cells (derived
from the division of the four big cells) form the endoderm or
arch-enteron; the outer cells are the ectoderm. The blastopore now
closes along the middle part of its course, which coincides in
position with the future "foot." One end of the blastopore becomes
nearly closed, and an ingrowth of ectoderm takes place around it to
form the stomodaeum or fore-gut and mouth. The other extreme end
closes, but the invaginated endoderm cells remain in continuity with
this extremity of the blastopore, and form the "rectal peduncle" or
"pedicle of invagination" of Lankester, although the endoderm cells
retain no contact with the middle region of the now closed-up
blastopore. The anal opening forms at a late period by a very short
ingrowth or proctodaeum coinciding with the blind termination of the
rectal peduncle (fig. 60, pi).
[Illustration: FIG. 60.--Embryo of _Limnaeus stagnalis_, at a stage
when the Trochosphere is developing foot and shell-gland and becoming
a Veliger, seen as a transparent object under slight pressure.
(Lankester.)
ph, Pharynx (stomodaeal invagination).
v, v, The ciliated band marking out the velum.
ng, Cerebral nerve-ganglion.
re, Stiebel's canal (left side), probably an evanescent embryonic
nephridium.
sh, The primitive shell-sac or shell-gland.
pi, The rectal peduncle or pedicle of invagination; its attachment
to the ectoderm is coincident with the hindmost extremity of the
elongated blastopore of fig. 3, C.
tge, Mesoblastic (skeleto-trophic and muscular) cells investing
gs, the bilobed arch-enteron or lateral vesicles of invaginated
endoderm, which will develop into liver.
f, The foot.]
The body-cavity and the muscular, fibrous and vascular tissues are
traced partly to two symmetrically disposed "mesoblasts," which bud
off from the invaginated arch-enteron, partly to cells derived from
the ectoderm, which at a very early stage is connected by long
processes with the invaginated endoderm. The external form of the
embryo goes through the same changes as in other Gastropods, and is
not, as was held previously to Lankester's observations, exceptional.
When the middle and hinder regions of the blastopore are closing in,
an equatorial ridge of ciliated cells is formed, converting the embryo
into a typical trochosphere.
The foot now protrudes below the mouth, and the post-oral hemisphere
of the trochosphere grows more rapidly then the anterior or velar
area. The young foot shows a bilobed form. Within the velar area the
eyes and the cephalic tentacles commence to rise up, and on the
surface of the post-oral region is formed a cap-like shell and an
encircling ridge, which gradually increases in prominence and becomes
the freely depending mantle-skirt. The outline of the velar area
becomes strongly emarginated and can be traced through the more mature
embryos to the cephalic lobes or labial processes of the adult
_Limnaeus_ (fig. 61).
[Illustration: FIG. 61.--A, B, C. Three views of _Limnaeus stagnalis_,
in order to show the persistence of the larval velar area v, as the
circum-oral lobes of the adult. m, Mouth; f, foot; v, velar area, the
margin v corresponding with the ciliated band which demarcates the
velar area or velum of the embryo Gastropod (see fig. 4, D, E, F, H,
I, v). (Original.)]
The increase of the visceral dome, its spiral twisting, and the
gradual closure of the space overhung by the mantle-skirt so as to
convert it into a lung-sac with a small contractile aperture, belong
to stages in the development later than any represented in our
figures.
We may now revert briefly to the internal organization at a period
when the trochosphere is beginning to show a prominent foot growing
out from the area where the mid-region of the elongated blastopore was
situated, and having therefore at one end of it the mouth and at the
other the anus. Fig. 60 represents such an embryo under slight
compression as seen by transmitted light. The ciliated band of the
left side of the velar area is indicated by a line extending from v to
v; the foot f is seen between the pharynx ph and the pedicle of
invagination pi. The mass of the arch-enteron or invaginated
endodermal sac has taken on a bilobed form, and its cells are swollen
(gs and tge). This bilobed sac becomes _entirely_ the liver in the
adult; the intestine and stomach are formed from the pedicle of
invagination, whilst the pharynx, oesophagus and crop form from the
stomodaeal invagination ph. To the right (in the figure) of the rectal
peduncle is seen the deeply invaginated shell-gland ss, with a
secretion sh protruding from it. The shell-gland is destined in
_Limnaeus_ to become very rapidly stretched out, and to disappear.
Farther up, within the velar area, the rudiments of the cerebral
nerve-ganglion ng are seen separating from the ectoderm. A remarkable
cord of cells having a position just below the integument occurs on
each side of the head. In the figure the cord of the left side is
seen, marked re. This paired organ consists of a string of cells which
are perforated by a duct opening to the exterior and ending internally
in a flame-cell. Such cannulated cells are characteristic of the
nephridia of many worms, and the organs thus formed in the embryo
_Limnaeus_ are embryonic nephridia. The most important fact about them
is that they disappear, and are in no way connected with the typical
nephridium of the adult. In reference to their first observer they
were formerly called "Stiebel's canals." Other Pulmonata possess, when
embryos, Stiebel's canals in a more fully developed state, for
instance, the common slug _Limax_. Here too they disappear during
embryonic life. Similar larval nephridia occur in other Gastropoda. In
the marine Streptoneura they are ectodermic projections which
ultimately fall off; in the Opisthobranchs they are closed pouches; in
_Paludina_ and _Bithynia_ they are canals as in Pulmonata.
[Illustration: FIG. 62.--_Oncidium tonganum_, a littoral Pulmonate,
found on the shores of the Indian and Pacific Oceans (Mauritius,
Japan).]
_Marine Pulmonata._--Whilst the Pulmonata are essentially a
terrestrial and fresh-water group, there is one genus of slug-like
Pulmonates which frequent the sea-coast (_Oncidium_, fig. 62). Karl
Semper has shown that these slugs have, in addition to the usual pair
of cephalic eyes, a number of eyes developed upon the dorsal
integument. These dorsal eyes are very perfect in elaboration,
possessing lens, retinal nerve-end cells, retinal pigment and optic
nerve. Curiously enough, however, they differ from the cephalic
Molluscan eye in the fact that, as in the vertebrate eye, the
filaments of the optic nerve penetrate the retina, and are connected
with the surfaces of the nerve-end cells nearer the lens instead of
with the opposite end. The significance of this arrangement is not
known, but it is important to note, as shown by V. Henson, S.J.
Hickson and others, that in the bivalves _Pecten_ and _Spondylus_,
which also have eyes upon the mantle quite distinct from typical
cephalic eyes, there is the same relationship as in Oncidiidae of the
optic nerve to the retinal cells. In both Oncidiidae and _Pecten_ the
pallial eyes have probably been developed by the modification of
tentacles, such as coexist in an unmodified form with the eyes. The
Oncidiidae are, according to K. Semper, pursued as food by the leaping
fish _Periophthalmus_, and the dorsal eyes are of especial value to
them in aiding them to escape from this enemy.
Sub-order 1.--BASOMMATOPHORA. Pulmonata with an external shell. The
head bears a single pair of contractile but not invaginable tentacles,
at the base of which are the eyes. Penis at some distance from the
female aperture, except in _Amphibola_ and _Siphonaria_. All have an
osphradium, except the _Auriculidae_, which are terrestrial, and it is
situated outside the pallial cavity in those forms in which water is
not admitted into the lung. There is a veliger stage in development,
but the velum is reduced.
Fam. 1.--_Auriculidae_. Terrestrial and usually littoral; genital
duct monaulic, the penis being connected with the aperture by an
open or closed groove; shell with a prominent spire, the internal
partitions often absorbed and the aperture denticulated. _Auricula.
Cassidula. Alexia. Melampus. Carychium_, terrestrial, British.
_Scarabus. Leuconia_, British. _Blauneria. Pedipes_.
Fam. 2.--_Otinidae_. Shell with short spire, and wide oval aperture;
tentacles short. _Otina_, British. _Camptonyx_, terrestrial.
Fam. 3.--_Amphibolidae_. Shell spirally coiled; head broad, without
prominent tentacles; foot short, operculated; marine. _Amphibola_.
Fam. 4.--_Siphonariidae_. Visceral mass and shell conical; tentacles
atrophied; head expanded; genital apertures contiguous; marine
animals, with an aquatic pallial cavity containing secondary
branchial laminae. _Siphonaria_.
Fam. 5.--_Gadiniidae_. Visceral mass and shell conical; head
flattened; pallial cavity aquatic, but without a branchia; genital
apertures separated. _Gadinia_.
Fam. 6.--_Chilinidae_. Shell ovoid, with short spire, wide aperture
and folded columella; inferior pallial lobe thick; visceral
commissure still twisted. _Chilina_.
Fam. 7.--_Limnaeidae_. Shell thin, dextral, with prominent spire and
oval aperture; no inferior pallial lobe. _Limnaea_, British.
_Amphipeplea_, British.
Fam. 8.--_Pompholygidae_. Shell dextral, hyperstrophic, animal
sinistral. _Pompholyx. Choanomphalus_.
Fam. 9.--_Planorbidae_. Visceral mass and shell sinistral; inferior
pallial lobe very prominent, and transformed into a branchia.
_Planorbis_, British. _Bulinus. Miratesta_.
Fam. 10.--_Ancylidae_. Shell conical, not spiral; inferior pallial
lobe transformed into a branchia. _Ancylus_, British. _Latia.
Grundlachia_.
Fam. 11.--_Physidae_. Visceral mass and shell sinistrally coiled;
shell thin, with narrow aperture; no inferior pallial lobe. _Physa_,
British. _Aplexa_, British.
Sub-order 2.--STYLOMMATOPHORA. Pulmonata with two pairs of tentacles,
except _Janellidae_ and _Vertigo_; these tentacles are invaginable,
and the eyes are borne on the summits of the posterior pair. Male and
female genital apertures open into a common vestibule, except in
_Vaginulidae_ and _Oncidiidae_. Except in _Oncidium_, there is no
longer a veliger stage in development.
Tribe 1.--HOLOGNATHA. Jaw simple, without a superior appendage.
Fam. 1.--_Selenitidae_. Radula with elongated and pointed teeth,
like those of the Agnatha; a jaw present. _Plutonia.
Trigonochlamys_.
Fam. 2.--_Zonitidae_. Shell external, smooth, heliciform or
flattened; radula with pointed marginal teeth. _Zonites_, British.
_Ariophanta. Orpiella. Vitrina. Helicarion_.
Fam. 3.--_Limacidae_. Shell internal. _Limax_, British. _Parmacella.
Urocyclus. Parmarion. Amalia. Agriolimax. Mesolimax. Monochroma.
Paralimax. Metalimax_.
Fam. 4.--_Philomycidae_. No shell; mantle covers the whole surface
of the body; radula with squarish teeth. _Philomycus_.
Fam. 5.--_Ostracolethidae_. Shell largely chitinous, not spiral, its
calcareous apex projecting through a small hole in the mantle.
_Ostracolethe_.
Fam. 6.--_Arionidae_. Shell internal, or absent; mantle restricted
to the anterior and middle part of the body; radula with squarish
teeth. _Arion_, British. _Geomalacus. Ariolimax. Anadenus_.
Fam. 7.--_Helicidae_. Shell with medium spire, external or partly
covered by the mantle; genital aperture below the right posterior
tentacle; genital apparatus generally provided with a dart-sac and
multifid vesicles. _Helix_, British. _Bulimus. Hemphillia.
Berendtia. Cochlostyla. Rhodea_.
Fam. 8.--_Endodontidae_. Shell external, spiral, generally
ornamented with ribs; borders of aperture thin and not reflected;
radula with square teeth; genital ducts without accessory organs.
_Endodonta. Punctum. Sphyradium. Laoma. Pyramidula._
Fam. 9.--_Orthalicidae._ Shell external, ovoid, the last whorl
swollen, aperture oval with a simple border; radular teeth in
oblique rows. _Orthalicus._
Fam. 10.--_Bulimulidae._ Jaw formed of folds imbricated externally
and meeting at an acute angle near the base. _Bulimulus. Peltella.
Amphibulimus._
Fam. 11.--_Cylindrellidae._ Shell turriculated, with numerous
whorls, the last more or less detached. _Cylindrella._
Fam. 12.--_Pupidae._ Shell external, with elongated spire and
numerous whorls, aperture generally narrow; male genital duct
without multifid vesicles. _Pupa_, British. _Eucalodium. Vertigo_,
British. _Buliminus_, British. _Clausilia_, British. _Balea.
Zospeum. Megaspira. Strophia. Anostoma._
Fam. 13.--_Stenogyridae._ Shell elongated, with a more or less
obtuse summit; aperture with a simple border. _Achatina. Stenogyra.
Ferussacia_, British. _Cionella. Caecilianella. Azeca. Opeas._
Fam. 14.--_Helicteridae._ Shell bulimoid, dextral or sinistral;
radular teeth, expanded at their extremities and multicuspidate.
_Helicter. Tornatellina._
Tribe 2.--AGNATHA. No jaws; teeth narrow and pointed; carnivorous.
Fam. 1.--_Oleacinidae._ Shell oval, elongated, with narrow aperture;
neck very long; labial palps prominent. _Oleacina (Glandina).
Streptostyla._
Fam. 2.--_Testacellidae._ Shell globular or auriform, external or
partly covered by the mantle. _Streptaxis. Gibbulina. Aerope.
Rhytida. Daudebardia. Testacella. Chlamydophorus. Schizoglossa._
Fam. 3.--_Rathouisiidae._ No shell, a carinated mantle covering the
whole body; male and female apertures distant, the female near the
anus. _Rathouisia. Atopos._
Tribe 3.--ELASMOGNATHA. Jaw with a well-developed dorsal appendage.
Fam. 1.--_Succineidae._ Anterior tentacles much reduced; male and
female apertures contiguous but distinct; shell thin, spiral, with
short spire. _Succinea_, British. _Homalonyx. Hyalimax.
Neohyalimax._
Fam. 2.--_Janellidae._ Limaciform, with internal rounded shell;
mantle very small and triangular; pulmonary chamber with tracheae;
no anterior tentacles. _Janella. Aneitella. Aneitea.
Triboniophorus._
Tribe 4.--DITREMATA. Male and female apertures distant.
Fam. 1.--_Vaginulidae._ No shell; limaciform; terrestrial; female
aperture on right side in middle of body; anus posterior.
_Vaginula._
Fam. 2.--_Oncidiidae._ No shell; limaciform; littoral; female
aperture posterior, near anus; a reduced pulmonary cavity with a
distinct aperture. _Oncidium. Oncidiella_, British. _Peronia._
AUTHORITIES.--L. Boutan, "La Cause principale de l'asymétrie des
mollusques gastéropodes," _Arch. de zool. expér._ (3), vii. (1899); A.
Lang, "Versuch einer Erklärung der Asymmetrie der Gastropoder,"
_Vierteljahrsschr. naturforsch. Gesellschaft_, Zürich, 36 (1892); A.
Robert, "Recherches sur le développement des Troques," _Arch. de zool.
expér._ (3), x. (1903); P. Pelseneer, "Report on the Pteropoda,"
_Zool. "Challenger" Expedit._ pts. lviii., lxv., lxvi. (1887, 1888);
P. Pelseneer, "Protobranches aériens et Pulmonés branchifères," _Arch.
de biol._ xiv. (1895); W.A. Herdman, "On the Structure and Functions
of the Cerata or Dorsal Papillae in some Nudibranchiate Mollusca,"
_Quart. Journ. Mic. Sci._ (1892); J.T. Cunningham, "On the Structure
and Relations of the Kidney in Aplysia," _Mitt. Zool. Stat. Neapel_,
iv. (1883); Böhmig, "Zur feineren Anatomie von _Rhodope veranyi_,
Kölliker," _Zeitschr. f. wiss. Zool._ vol. lvi. (1893).
TREATISES.--S.P. Woodward, _Manual of the Mollusca_ (2nd ed., with
appendix, London, 1869); E. Forbes and S. Hanley, _History of British
Mollusca_ (4 vols., London, 1853); Alder and Hancock, _Monograph of
British Nudibranchiate Mollusca_ (London, Roy. Society, 1845); P.
Pelseneer, _Mollusca. Treatise on Zool._, edited by E.
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