A Monograph on the Sub-class Cirripedia (Volume 2 of 2)
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Title: A Monograph on the Sub-class Cirripedia (Volume 2 of 2)
Author: Charles Darwin
Release date: July 25, 2014 [eBook #46408]
Most recently updated: October 24, 2024
Language: English
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Transcriber's Note
Obvious misprints have been fixed. Archaic and unusual words, spellings
and styling have been maintained. Details of the changes are in the
Detailed Transcriber's Notes at the end of the book.
This book was published in two volumes, of which this is the second.
The first volume was released as Project Gutenberg ebook #31558,
available at https://www.gutenberg.org/ebooks/31558.
Transcriber Added
Table of Contents.
Page
Dedication v
Preface vii
_Monograph on the Cirripedia_ 1
Introduction 1
On the Names given to the different parts of Cirripedes 3
Class Crustacea, Sub-Class Cirripedia 9
On the Sexual Relation of Cirripedes 23
Order I.--Thoracica 30
Family Balanidæ 33
Table of Contents 33
Structure of Shell 34
Structure of the Individual Compartments 43
Structure of the Radii 45
Structure of the Alæ 47
Structure of the Sheath 48
Structure of the Basis 49
Structure of the Opercular Valves (Scuta and Terga) 51
Growth of the Whole Shell, and Its Microscopical Structure
54
Muscles of Sack 61
Branchiæ 63
Thorax and Body 65
Muscular System 68
Movements and Muscles of the Cirri 71
Mouth 74
Cirri 81
Caudal Appendages 85
Alimentary Canal 85
Circulatory System 87
Nervous System 88
Eyes and Vision 93
Acoustic Organs 95
Olfactory Sacks 97
Male Organs of Generation 97
Female Organs of Generation 100
Metamorphoses and Homologies 102
Larva, First Stage 103
Larva, Second Stage 109
Larva in the Last or Pupal Stage 110
Act of Metamorphosis 126
On the Homologies of the Carapace and Shelly Valves 131
Cementing Apparatus 133
Affinities, Classification, Variation 152
Rate of Growth, Exuviation, Powers of Repairing Injuries
156
Geographical Range and Habits 159
Geological History 172
Sub-Family Balaninæ 175
1. Genus Balanus 177
Sections of the Genus 193
Section A 194
1. Balanus tintinnabulum 194
Var. communis, vesiculosus, validus, zebra, crispatus,
spinosus, coccopoma, concinnus, intermedius, occator,
d'Orbignii
Varieties 201
2. Balanus tulipiformis 204
3. Balanus psittacus 206
4. Balanus Capensis 209
5. Balanus Nigrescens 210
6. Balanus decorus 212
7. Balanus vinaceus 213
8. Balanus Ajax 214
Section B 216
9. Balanus stultus 216
10. Balanus calceolus 218
11. Balanus galeatus 220
12. Balanus cymbiformis 221
13. Balanus navicula 221
Section C 223
14. Balanus trigonus 223
15. Balanus spongicola 225
16. Balanus lævis 227
Var. nitidus, Coquimbensis
17. Balanus perforatus 231
Var. angustus, Cranchii, fistulosus, mirabilis
18. Balanus concavus 235
19. Balanus amphitrite 240
Var. communis, venustus, pallidus, niveus, modestus,
Stutsburi, obscurus, variegatus, cirratus
Varieties 245
20. Balanus pœcilus 245
21. Balanus eburneus 248
22. Balanus improvisus 250
Var. assimilis
23. Balanus nubilus 253
24. Balanus corrugatus 254
Section D 256
25. Balanus porcatus 256
26. Balanus patellaris 259
27. Balanus crenatus 261
28. Balanus glandula 265
Section E 267
29. Balanus balanoides 267
Varieties 270
30. Balanus cariosus 273
31. Balanus declivis 275
Section F 277
32. Balanus Hameri 277
33. Balanus amaryllis 279
34. Balanus allium 281
35. Balanus cepa 283
36. Balanus quadrivittatus 284
37. Balanus terebratus 285
38. Balanus vestitus 286
39. Balanus Imperator 288
40. Balanus flosculus 290
Var. sordidus
41. Balanus bisulcatus 293
Var. plicatus
42. Balanus dolosus 295
43. Balanus unguiformis 296
Var. erisma
44. Balanus varians 298
45. Balanus inclusus 299
2. Sub-Genus Acasta 302
1. Acasta spongites 308
2. Acasta sulcata 310
3. Acasta cyathus 312
4. Acasta undulata 313
5. Acasta glans 314
6. Acasta lævigata 315
7. Acasta fenestrata 316
8. Acasta purpurata 318
9. Acasta sporillus 319
3. Genus Tetraclita 321
1. Tetraclita porosa 329
Var. communis, nigrescens, viridis, rubescens, elegans,
communis (young), patellaris
2. Tetraclita serrata 334
3. Tetraclita rosea 335
4. Tetraclita purpurascens 337
5. Tetraclita costata 339
6. Tetraclita vitiata 340
7. Tetraclita cœrulescens 342
8. Tetraclita radiata 343
4. Genus Elminius 345
1. Elminius Kingii 348
2. Elminius modestus 350
3. Elminius plicatus 351
4. Elminius simplex 353
5. Genus Pyrgoma 355
1. Pyrgoma anglicum 360
2. Pyrgoma Stokesii 361
3. Pyrgoma cancellatum 362
4. Pyrgoma conjugatum 364
5. Pyrgoma grande 365
6. Pyrgoma milleporæ 367
7. Pyrgoma dentatum 369
8. Pyrgoma crenatum 370
9. Pyrgoma monticulariæ 372
Species dubiæ 374
6. Sub-Genus Creusia 375
1. Creusia spinulosa 376
Varieties with the Scuta and Terga calcified together 380
7. Genus Chelonobia 382
1. Chelonobia testudinaria 392
2. Chelonobia caretta 394
3. Chelonobia patula 396
Second Section of the Sub-Family of Balaninæ 397
8. Genus Coronula 397
1. Coronula balænaris 415
2. Coronula diadema 417
3. Coronula reginæ 419
4. Coronula barbara 421
Species Dubiæ 423
9. Genus Platylepas 424
1. Platylepas bissexlobata 428
2. Platylepas decorata 429
Species Dubiæ 430
10. Genus Tubicinella 430
1. Tubicinella trachealis 431
11. Genus Xenobalanus 438
1. Xenobalanus globicipitis 440
Sub-Family Chthamalinæ 446
12. Genus Chthamalus 447
1. Chthamalus stellatus 455
Var. communis, fragilis, depressus
2. Chthamalus antennatus 460
3. Chthamalus cirratus 461
4. Chthamalus fissus 462
5. Chthamalus dentatus 463
6. Chthamalus Hembeli 465
7. Chthamalus intertextus 467
8. Chthamalus scabrosus 468
13. Nov. Genus Chamæsipho 470
1. Chamæsipho columna 470
2. Chamæsipho scutelliformis 472
14. Nov. Genus Pachylasma 475
1. Pachylasma giganteum 477
2. Pachylasma aurantiacum 480
15. Genus Octomeris 482
1. Octomeris angulosa 483
2. Octomeris brunnea 484
16. Genus Catophragmus 485
1. Catophragmus polymerus 487
2. Catophragmus imbricatus 490
Remarks on Bronn's List of Fossil Balaninæ and Chthamalinæ 492
Family Verrucidæ 495
Genus Verruca 496
Powers of Excavation 512
1. Verruca Strömia 518
2. Verruca lævigata 520
3. Verruca Spengleri 521
4. Verruca nexa 522
5. Verruca prisca 525
Family Lepadidæ 526
Genus Alcippe 529
Alcippe lampas 530
Female 530
Male 555
Order II.--Abdominalia 563
Cryptophialus minutus 566
Female 566
Male 584
Order III.--Apoda 587
Proteolepas bivincta 589
Synopsis et Index Systematicus 606
Synopsis et Index Systematicus Specierum, et recentium, et fossilum
611
Description of plates 641
Plate 1. Balanus tintinnabulum 641
Plate 2. Genus Balanus 641
Plate 3. Genus Balanus 642
Plate 4. Genus Balanus 642
Plate 5. Genus Balanus 642
Plate 6. Genus Balanus 643
Plate 7. Genus Balanus 643
Plate 8. Genus Balanus 644
Plate 9. Sub-Genus Acasta 644
Plate 10. Genus Tetraclita 645
Plate 11. Genera Tetraclita and Elminius 645
Plate 12. Genera Elminius and Pyrgoma 646
Plate 13. Genera Pyrgoma and Creusia 646
Plate 14. Genera Creusia and Chelonobia 647
Plate 15. Genera Chelonobia and Coronula 648
Plate 16. Genus Coronula 649
Plate 17. Genera Platylepas, Tubicinella, and Xenobalanus 651
Plate 18. Genus Chthamalus 652
Plate 19. Genera Chthamalus, Chamæsipho, and Pachylasma 652
Plate 20. Genera Pachylasma, Octomeris, and Catophragmus 653
Plate 21. Genus Verruca 654
Plate 22. Alcippe lampas 655
Plate 23. Genera Alcippe and Cryptophialus 658
Plate 24. Genera Cryptophialus and Proteolepas 660
Plate 25. Genera Proteolepas and Balanus 662
Plate 26. Structure of the Mouth and Thorax 664
Plate 27. Nervous System and Senses 666
Plate 28. Cementing Apparatus 667
Plate 29. Cirri and Larvæ, first stages 669
Plate 30. Larvæ of Lepas: second and last stages of development
671
Errata 674
Index 675
Plates 685
THE
RAY SOCIETY.
INSTITUTED MDCCCXLIV.
[Illustration]
LONDON.
MDCCCLIV.
A MONOGRAPH
ON THE SUB-CLASS
CIRRIPEDIA,
WITH
FIGURES OF ALL THE SPECIES.
BY
CHARLES DARWIN, F.R.S., F.G.S.
THE BALANIDÆ,
(OR SESSILE CIRRIPEDES);
THE VERRUCIDÆ,
ETC., ETC., ETC.
LONDON:
PRINTED FOR THE RAY SOCIETY.
MDCCCLIV.
Reprinted with the permission of the Ray Society
JOHNSON REPRINT CORPORATION
111 Fifth Avenue, New York, N. Y. 10003
JOHNSON REPRINT COMPANY LTD.
Berkeley Square House, London, W. 1
First reprinting, 1968, Johnson Reprint Corporation
Printed in the United States of America
TO
PROFESSOR H. MILNE EDWARDS,
DEAN OF THE FACULTY OF SCIENCES OF PARIS;
PROFESSOR AT THE MUSEUM OF NATURAL HISTORY;
MEMBER OF THE INSTITUTE OF FRANCE;
FOREIGN MEMBER OF THE ROYAL SOCIETY OF LONDON,
OF THE ACADEMIES OF BERLIN, STOCKHOLM, ST. PETERSBURG, VIENNA,
KONIGSBERG, MOSCOW, BRUSSELS, HAARLEM, BOSTON, PHILADELPHIA, ETC.
THIS WORK IS DEDICATED,
WITH THE MOST SINCERE RESPECT,
AS THE ONLY, THOUGH VERY INADEQUATE ACKNOWLEDGMENT
WHICH THE AUTHOR CAN MAKE OF HIS GREAT AND
CONTINUED OBLIGATIONS TO THE
'HISTOIRE NATURELLE DES CRUSTACÉS,'
AND TO
THE OTHER MEMOIRS AND WORKS ON NATURAL HISTORY PUBLISHED BY
THIS ILLUSTRIOUS NATURALIST.
PREFACE.
Having so lately, in my volumes on the Recent and Fossil Lepadidæ,
expressed as strongly as I could, and with the utmost sincerity, the
obligations under which I lie to very many naturalists, I will not
here repeat my thanks, and will only say that the assistance formerly
rendered me from so many quarters has been most kindly continued
without intermission. The references under the Habitats, in which I
may remark the names of Mr. Cuming and of Mr. Stutchbury, and of the
British Museum, so often recur, show my deep obligations to these
gentlemen and to Dr. Gray, and indeed to most of the British and
several Foreign[1] collectors of recent and fossil shells. At the
period when the Introduction to this volume was printed, I stated that
I did not know whether the Palæontographical Society would publish
the few British fossil Balanidæ; the Council has now honoured me by
determining on this publication, so that these species will hereafter
be more fully illustrated than they could be in the present volume. I
cannot conclude this short preface, without again tendering my most
grateful thanks to the Council of the Ray Society for the publication
of my two volumes, and for the very kind manner in which they have
acceded to all my requests.
DOWN, KENT; _July, 1854_.
[1] I feel under special obligation to Mr. Dana for several very
interesting communications connected with the present subject, and
for information derived from his magnificent work on the Crustacea,
collected during the United States Exploring Expedition. Also
to M. Bosquet, of Maestricht, for the loan and gift of several
interesting fossils, described and illustrated with the utmost
fidelity, in his beautiful "Monographie des Crustacés fossiles du
terrain Crétacé du D. de Limbourg."
MONOGRAPH
ON
THE CIRRIPEDIA.
INTRODUCTION.
My former volume, published by the Ray Society, treated only of the
Lepadidæ, one family of the Cirripedia: I was induced to print it
from having the materials ready, though this partial publication has
been in some respects inconvenient. The Palæontographical Society has
done me the honour to publish, with ample illustrations, the fossil
species of this same family of Lepadidæ. This present volume completes
my work on the sub-class Cirripedia.[2] I had originally intended to
have published a small volume on my anatomical observations; but the
full abstract given in my former volume, which will be illustrated to a
certain extent in the plates appended to this volume, together with the
observations here given under the Balanidæ, appear to me sufficient,
and I am unwilling to spend more time on the subject. In the volume on
the Lepadidæ, I gave the specific or diagnostic characters in English
and Latin: I have here left out the latter, inasmuch as I have appended
at the end of this volume a Latin Synopsis of all the species, recent
and fossil, of the whole class. To each species is added a reference
to the pages and plates of my three volumes, so that the Synopsis
will serve as a systematic index to the three: an alphabetical index
to the present volume is also given. In the Lepadidæ, I gave an
additional specific character, derived from the softer parts of the
animal's body: in the Balanidæ, these parts are more alike in the
different species, and I have found it impossible to give a diagnostic
character thus derived. In those cases in which a Family contains but
one genus, or a Genus but one species, I have assigned my reasons for
the institution of such groups, but have given, as heretofore, only a
single description in full: it would have been easy to have separated,
by analogy, this description into one for the species, another for
the genus or for the family; but as I believe such separation and
subordination of the characters would have been largely conjectural,
I have thought it best to act as I have done, and give, thus saving
useless repetitions, only a single description, and leave it for my
successors, when more genera or species are known, to separate, with
such certainty as is ever possible, the generic from the specific
characters.
[2] The number of the British fossil species of the Balanidæ and
Verrucidæ in a recognisable state is so small, that I do not
know whether it will be considered worth while to publish in the
Palæontographical series more detailed illustrations than are given
in this volume.
In nomenclature, I have endeavoured rigorously to follow the rules
of the British Association, and have never, at least intentionally,
broken through the great law of priority. In accordance with the rules,
I have rejected, that is, as compulsory, all names given before the
introduction of the binomial system in 1758. I have given much fewer
synonyms than is usual in conchological works; for it is impossible to
recognise with any approach to certainty, several even of the common
European forms, in the short descriptions given by most authors; this
holds good in many cases in which figures, in appearance excellent,
have been added. I assert this the more confidently, from having had
the advantage of having gone over some of the Linnean synonyms with Mr.
S. Hanley. I may further venture to express my conviction, that giving
references to works, in which there is not any original matter, or in
which the plates are not of a high order of excellence, is absolutely
injurious to the progress of natural history.
NOMENCLATURE OF THE SHELL OF A SESSILE CIRRIPEDE.
[Illustration: SHELL. Fig. 1.
_Orifice_ of shell, surrounded by the _sheath_. _Sheath_ formed by the
_alæ_ (_a_--_a_.) and by portions of the upper and inner surfaces of
the _parietes_ (_p_--_p_.)
N.B. In Balanus, and many other genera, the Rostrum and Rostro-lateral
compartments are confluent, and hence the Rostrum has the structure of
Fig. 2.]
COMPARTMENTS.
[Illustration: Fig. 2.
Fig. 2. Compartment with two radii, serving either as a
Rostrum or Rostro-lateral compartment.]
[Illustration: Fig. 3.
Fig. 3. serves as a Lateral and Carino-lateral
Compartment.]
[Illustration: Fig. 4.
Fig. 4. serves as a Carina or Rostrum.]
OPERCULAR VALVES.
[Illustration: Fig. 5. SCUTUM (internal view of).]
[Illustration: Fig. 6. TERGUM (external view).]
[Illustration: Fig. 7. TERGUM (internal view).]
Sessile Cirripedes, partly from being attached to surfaces having
very different characters, partly from undergoing a varying amount of
disintegration, and partly from unknown innate causes, are extremely
variable. Under the head of _Variation_, in the Family Balanidæ and
under the Genus Balanus, I have enlarged on this subject, and have
shown that there is scarcely a single _external_ character which is not
highly variable in most of the species. As whole groups of specimens
often vary in exactly the same manner, it is not easy to exaggerate the
difficulty of discriminating species and varieties. It is absolutely
necessary in most cases, for mere identification, that the valves of at
least one specimen in a group should be disarticulated and well cleaned
(for which purpose caustic potash is most useful), so that the internal
characters may be examined. Whoever attempts to make out from external
characters alone, without disarticulating the valves, the species,
(even those inhabiting one very confined region, for instance the
shores of Great Britain,) will almost certainly fall into many errors:
hence it is, and can thus only be accounted for, that I have not seen
one collection of British specimens with _all_ the species, though so
few in number, rightly discriminated; and in the large majority of
cases, either two or three species, certainly distinct, were confounded
together, or two or three varieties, as certainly not distinct, were
separated from each other.
_On the Names Given to the Different Parts of Cirripedes._
In my former volume I have stated that I found it indispensable, in
part owing to the extreme confusion of the nomenclature previously
used, to attach new names to several of the external parts of
Cirripedes. Almost all these names are applicable to the Balanidæ,
or sessile Cirripedes, and to the Verrucidæ; but a few additional
names are requisite, which, together with the old names, will, I
hope, be rendered clear by the accompanying woodcuts. In sessile
Cirripedes, the whole of that which is externally visible, may for
convenience sake be divided into the _operculum_ or _opercular valves_
(_valvæ operculares_), and the _shell_ (_testa_), though these parts
homologically present no real difference. The operculum is seated
generally some little way down within the _orifice_ of the shell; but
in very young specimens and in Verruca, the operculum is attached
to the summit of the shell, and the shell, without the operculum being
removed, can hardly be said to have any orifice; though, of course, the
opercular valves themselves have an aperture for the protrusion of the
cirri.
The shell consists of the _basis_ (called the support by some authors),
which is membranous or shelly, and flat or cup-formed, and of the
_compartments_ (_testæ valvæ_), which vary from eight to four in
number, and occasionally are all calcified together.
The compartment, at that end of the shell where the cirri are exserted
through the aperture or lips of the operculum, is called the _carina_
(fig. 1); the compartment opposite to it is the _rostrum_,--these two
lying at the ends of the longitudinal axis of the shell. Those on the
sides are the _lateral compartments_; that nearest the carina, being
the _carino-lateral_ (_testæ valva carino-lateralis_), that nearest the
rostrum, the _rostro-lateral_, and the middle one, simply the _lateral_
compartment; but these three compartments are rarely present together.
The _rostro-lateral_ compartment, which always resembles fig. 2, and
may be always known by having radii on both sides, is often absent; and
not rarely the lateral and carino-lateral compartments are confounded
together, or one is absent; in such cases the compartment that is left
is simply called the lateral one. The compartments are separated from
each other by _sutures_, which are often so fine and close as to be
distinguished with difficulty. The edge of a compartment, which can
only be seen when disarticulated from its neighbour, I have called the
sutural edge (_acies suturalis_).
Each separate _compartment_ consists of a _wall_ (_paries_), or
_parietal portion_ (_pp_ in woodcuts), which always grows downwards,
and forms the basal margin; and is furnished on the two sides either
with _alæ_ (fig. 4), or with _radii_ (fig. 2), or with an ala on one
side and a radius (fig. 3) on the other.
The _radius_[3] (adopting the name used by Bruguière, Lamarck, and
others) differs remarkably in appearance (though not in essence) from
the walls or parietal portion, owing to the direction of the lines
of growth and the state of its usually depressed surface. In the
upper part the radii overlie the alæ of the adjoining compartments:
in outline (_r_, fig. 1, 2, 3), they are wedge-formed, with their
points downwards; their summits (and this is often a useful specific
character) are either parallel to the basis or as in fig. 1 and 2,
oblique. The radii are sometimes not developed.
[3] The radii have been called by Ranzani and De Blainville
"areæ depressæ" (the parietal portions of the compartments being
the "areæ prominentes"); by Poli, "areæ interjectæ;" by Gray,
"sutures;" by Coldstream, "compartments of the second order," (the
parietal portions being those of the first order); by some authors
as "intersticia." I may here add that the scuta are the "ventral
valves" of Gray, the "anterior" of Ranzani, and the "inferior
opercular" of De Blainville: the terga are the "posterior valves"
of Gray and Ranzani, but the "superior opercular" of De Blainville:
the rostrum, on the other hand, is the "anterior valve" of Ferussac
and the "ventral" of De Blainville; the carina being the "dorsal
valve" of the latter author.
The _alæ_ (so called by Dr. Gray) are overlapped by the radii and by
part of the walls; they usually extend only about half way down the
compartment (_a_ fig. 3, 4, 1); their summits are either parallel to
the basis or oblique. The alæ of the several compartments, together
with the internal, upper, thickened surfaces of the walls, against a
shoulder of which the sutural edges of the alæ abut, have been called
(by Dr. Gray) the _sheath_ (_vagina_). The upper and greater portion of
the sheath is marked by transverse lines, caused by the exuviation of
the _opercular membrane_, as that membrane may be called, which unites
the operculum all round to the sheath, or upper internal surface of the
shell.
The _carina_ has always two _alæ_, as in fig. 4.
The _carino-lateral_ and _lateral compartments_ have always an _ala_ on
one (the rostral) side, and a _radius_ on the other (the carinal) side,
as in fig. 3.
The _rostro-lateral compartment_ (when present) has always _radii_ on
both sides, as in fig. 2.
The _rostrum_ has normally _alæ_ on both sides, as in fig. 4, but very
often from fusion with the rostro-lateral compartments on both sides,
it has _radii_ on both sides, as in fig. 2.
The walls of the shell, the basis, and the radii, are in very many
cases composed of an _outer_ and _inner lamina_, united together by
_longitudinal septa_; a set of tubes or pores being thus formed. The
points of the longitudinal septa generally project beyond the laminæ,
and are denticulated on both sides (see woodcut, further on;) the septa
are sometimes branched, several irregular rows of pores between the two
laminæ being then formed (see Pl. 7, fig. 3 _b_, and Pl. 10, fig. 1
_g_, 1 _h_).
_Operculum_, or _opercular valves_.--These consist of a pair of scuta
and a pair of terga. They are joined to the sheath of the shell by the
_opercular membrane_.
_Scutum_ (woodcut 5): this valve is generally sub-triangular, and its
three margins are the _basal_, the _tergal_, so called from being
articulated with the tergum, and the _occludent_, so called from
opening and shutting against the opposed valve. The angles are called
from the adjoining margins, as _basi-tergal_, &c.; the upper angle
being the apex. The scutum is ordinarily articulated to the tergum by
an _articular ridge_ (_crista articularis_), running up to the apex
of the valve, and by an _articular furrow_, which latter receives the
scutal margin of the tergum. The articular ridge, instead of projecting
straight up from the valve, when laid flat on its external surface,
often bends over to the tergal side, and is then said to be _reflexed_.
On the internal surface of the valve, there is almost always an
_adductor pit_ or _cavity_ (_fossa adductoris_), for the attachment of
the adductor scutorum muscle: this pit is often bounded on its tergal
and basal sides, by a ridge, called the _adductor ridge_ (_crista
adductoris_), which, in its upper part, is often confluent with the
articular ridge. Beneath the adductor ridge, in the basi-tergal corner
of the valve, there is often a _lateral-depressor pit_ (_fossa musculi
lateralis depressoris_), for the attachment of the so-called muscle;
and this pit is sometimes furnished with crests.
_Tergum_, (woodcut 6 and 7):--this valve, also, has three margins,
the _scutal_, _basal_, and _carinal_; its upper end, or _apex_, is
sometimes _beaked_; on the basal margin a _spur_ (_calcar_) depends;
the outer surface of the valve is depressed or longitudinally
_furrowed_ (_sulcus longitudinalis_) in the line of the spur. The
part called the spur is often so broad, that the name becomes not very
appropriate. The angles are denominated, from the adjoining margins,
as _basi-carinal_, or _basi-scutal_ angle, &c. On the under side, in
the upper part, there is an _articular ridge_, and on its scutal side,
an _articular furrow_, receiving the articular ridge of the scutum. In
the basi-carinal corner of the valve there are often crests for the
attachment of the tergal depressor muscle.
_Sack_, _Body_, _Cirri_, _Mouth_.--A slit-like orifice between the
opercular valves leads into the _sack_, in which the body is lodged.
The body consists of the six (perhaps the seven) posterior thoracic
segments of the archetype Crustacean; the first of these six segments
(or first two, if there be seven segments) is developed on its dorsal
aspect into a part, which I have called the _prosoma_[4] (see fig. 1,
_c_, Pl. 25). There is no abdomen. The thoracic segments support six
pairs of _cirri_. Each cirrus consists of a two-jointed _pedicel_,
carrying two multiarticulated _rami_. Rarely there are articulated
_caudal appendages_ (_appendices caudales_) on each side of the anus.
The prominent mouth consists of a _labrum_, _palpi_, _mandibles_,
_maxillæ_, and _outer maxillæ_, the latter resembling a lower lip:
these organs may be conveniently spoken of, after Milne Edwards, as
_gnathites_. Within the sack, attached to its carino-lateral end, a
folded membrane forms the _branchiæ_. The sheets of ova lying within
the sack are called the _ovigerous lamellæ_.
[4] A discussion on the homologies of the different parts is given
under the head of the Metamorphoses of the Balanidæ.
I have often found it convenient to designate the membrane investing
the body, lining the sack, &c., by its proper chemical name of
_chitine_, instead of by horny, or other such equivalents; but when
covering parts of the shell, for brevity's sake I have often spoken of
it as an _epidermis_, but I do not believe that such is its nature.
When this membrane sends into the body of the animal rigid projections
or crests, for the attachment of muscles or any other purpose, I call
them, after Audouin, _apodemes_. For the underlying true skin, I use
the term _corium_.
_Relative position of parts._--The centre of the generally flat
basis, which is cemented to the supporting surface, is properly the
_anterior_ end, and the tips of the terga, often hidden within the
shell, are properly the _posterior_ end of the external covering; but
I have found it more convenient to speak of the _upper_ and _basal_
surfaces and aspects, which hardly admit of any mistake. A line drawn
from the centre of the basis, along the middle of the rostrum to the
tips of the scuta, shows the strictly _medio-ventral_ surface of the
shell; and another line drawn from the centre of the basis, along the
carina, to the tips of the terga, shows the strictly _medio-dorsal_
line; but from the crooked course of these lines, I have found it far
more convenient to speak of the _rostral_ and _carinal_ end or aspect
of the different parts of the shell; this is the more necessary with
respect to the internal parts of the animal, owing to their remarkable
changes of position during the metamorphosis, whence it comes that the
dorsal surface of the thorax faces partly dorsally, partly anteriorly
or downwards, and partly even ventrally; and the ventral surface of the
whole posterior part of the thorax faces upwards or posteriorly; but
when we refer these parts to the _rostral_, _carinal_, _basal_, and
_upper_ ends of the shell, there can be no mistake. There has moreover
been great confusion in these relative terms, as applied by different
authors.
When a sessile Cirripede is held in the position in which they have
generally been figured, namely with the basis downwards and the
scuta towards the beholder, then the _right_ and _left_ sides of the
Cirripede correspond with those of the holder.
I have followed the example of Botanists, and added the interjection
(!) to synonyms, when I have seen an authentic specimen bearing the
name in question.
Every locality, under each species, is given from specimens ticketed
in a manner and under circumstances appearing to me worthy of
confidence,--the specific determination being in each case made by
myself.
CLASS--CRUSTACEA.
Sub-Class--CIRRIPEDIA.
_Crustacea attached by the anterior end of the head, by cement
proceeding from a modified portion of the ovaria; archetype composed
of seventeen segments, with the three first of large size, and almost
always developed into a carapace, not wholly exuviated, and capable of
various movements; antennæ none; eyes rudimentary; mouth prominent,
formed by the partial confluence of the labrum, palpi, mandibles, and
two pairs of maxillæ; thorax attached to the internal sternal surface
of the carapace, generally bearing six pairs of captorial, biramous,
multiarticulated limbs; abdomen generally rudimentary; branchiæ,
when present, attached to the under sides of the carapace; generally
bisexual, when unisexual, males epizoic on the female; penis single,
generally probosciformed, seated at the posterior end of the abdomen;
oviducts none; metamorphoses complex._
Within the memory of many living naturalists, Cirripedes were
universally looked on as belonging to the Molluscous kingdom; nor was
this surprising, considering the fixed condition of their shells, and
the degree of external resemblance between, on the one hand, Lepas and
Teredo, and on the other hand, between Balanus and a Mollusc compounded
of a patella and chiton. It is remarkable that this external false
appearance overbore, even in the mind of Cuvier, his knowledge of their
internal structure, namely, their lateral jaws, articulated appendages,
and regular ganglionic nervous system, which now strike us as such
conclusive evidence of their position in the great Articulate kingdom.
Straus[5] was, I believe, the first who, in 1819, maintained that
Cirripedes were most closely allied to Crustacea. But this view was
disregarded, until J. Vaughan Thompson's[6] capital discovery, in 1830,
of their metamorphoses, since which time, Cirripedes have been almost
universally admitted amongst the Crustaceans. It is well known, that
it is hardly possible to give a definition of this great class, which
shall include every member of it; nevertheless, even if the mature
Cirripede alone be considered, the following characters, viz. the
slight separation of the head and thorax, the latter generally bearing
six pairs of appendages, and the being enclosed in a carapace--together
with the periodical exuviation of the greater part of the external
membranes, would, perhaps, suffice to show that it should be classed
amongst Crustacea.
[5] Mémoires du Muséum d'Histoire Nat., tom. v, p. 381.
[6] Zoological Researches and Illustrations.
But it still remains undecided what rank in this class Cirripedes
should hold. Before briefly discussing this point, it is indispensable
to indicate their essential characters, which I will immediately
attempt. For as long as it remained doubtful which was their anterior
extremity, which the ventral or dorsal surface; as long as the peduncle
was thought by one naturalist to be the legs, by another the abdomen,
in a modified condition, it was hopeless to compare Cirripedes with
ordinary Crustaceans, and assign to them their due rank.
In the larva in the first stage, an eye and two pairs of antennæ are
in process of formation or are developed; here, then, according to
the analogy of all Crustaceans, we have evidence of the existence of
the first three cephalic segments. The mouth always consists of three
pairs of gnathites, and hence again, from analogy, this part may be
inferred to be formed of, and supported on, three other segments;
making thus far six segments. In two Orders out of the three into
which Cirripedes may be divided, namely, in the Abdominalia and Apoda,
eight quite distinct segments succeed the mouth; of these the first
differs slightly from the seven succeeding segments, and may, I think,
be safely considered as forming the seventh (cephalic) segment. The
next seven segments resemble each other in all essential respects, and
are no doubt the normal, seven thoracic segments. These, in both the
above orders, are succeeded by three smaller segments, which differ
in structure from the thoracic segments, and must be abdominal. Hence
we here have, altogether, seventeen segments. It should, however, be
observed that in the two orders just referred to, each includes only
a single species; but I know of no good reason why, on this account,
their structure should be valued the less. In the third order, the
Thoracica, which includes all common Cirripedes, two segments with
their appendages are missing out of the eight that should succeed
the mouth; from the open interval in the pupa, between the mouth and
first pair of natatory legs, and from some other reasons, I believe
that the two missing segments are the seventh and eighth, or last
cephalic and first thoracic segments, and that they have coalesced
close posteriorly to the mouth.[7] In the order Thoracica, the abdomen
is quite rudimentary, though often still bearing caudal appendages;
in the pupa, however, of this order, as in the mature animal of the
two other orders, it is formed of three segments. Hence I conclude
that, notwithstanding the absence of the above two segments with their
appendages in the Thoracica, the archetype Cirripede may be safely said
to be composed of seventeen segments.
[7] This question and the whole subject of the homologies of the
several parts of a Cirripede, will be discussed under the head of
the Metamorphoses of the Balanidæ.
In the classification of Crustacea, the relation and number of the
segments of the different parts of the body, are viewed both by Prof.
Milne Edwards[8] and Mr. Dana,[9] as of the highest importance.
I may premise that both these authors divide the Crustacea into
Podophthalmia, Edriophthalmia, and Entomostraca; Milne Edwards making
a fourth legion, the Branchiopoda, and another division, including
Limulus, of equal value to the above four legions altogether; whereas
Dana sinks Limulus and the Branchiopoda under his Entomostraca. As
far as concerns our present discussion on Cirripedes, the first
three divisions, as valued by Dana, will best serve as standards
of comparison; but it is not unimportant to our present purpose,
as showing how difficult it is to weigh the value of the higher
divisions of a Class, to observe the wide difference in opinion of two
naturalists, so eminent for their knowledge of the class in question
and for their high abilities.
[8] Annales des Sciences Nat., tom. xviii, p. 120, 1852.
[9] Crustacea: 'United States Exploring Expedition,' p. 1395, 1852.
In the order Thoracica, including all common Cirripedes, the
cephalic and thoracic segments are as much confounded together (but
with coalescence and abortion of two middle segments) as in most
Podophthalmia; but in the two other orders, the cephalic and thoracic
segments are as distinct as in the Edriophthalmia. The number of the
segments, however, which _strictly_ appertain to the anterior part of
the head and mouth, being only six, is an Entomostracan character; on
the other hand, the first pair of cirri in the Thoracica, has some
claim from their position, apparent functions, and separation from the
succeeding pairs, to be said to belong to the mouth; on which view, the
first nine segments would, in function, be cephalic, as in the highest
Crustaceans. The fewness of the segments of the abdomen, and their not
bearing in two of the orders appendages, is an Entomostracan character.
Cirripedes are permanently attached, even before their final
metamorphosis, by a tissue or cement, first debouching through the
second pair of antennæ, and, subsequently, in most cases, through
special orifices, penetrating the anterior part of the head; this
cement proceeds from glands, which certainly are modified portions of
the ovarian system. This fact I consider of the highest classificatory
importance, for it is absolutely the one single character common to all
Cirripedes, besides such as show only that these animals belong to the
articulated kingdom, and are Crustaceans. No structure of this kind has
hitherto been observed in any other member of the class or kingdom.
Even in the Suctorial Entomostracans, which become immoveably attached
to the fish on which they prey, the males are free; and the means of
attachment, as far as known, are quite different.
Both the first and second pairs of antennæ are absent in the mature
animal; for the three terminal segments of the antennæ of the pupa,
which may always be found cemented under the centre of the surface of
attachment, are functionless, after maturity. The eyes are rudimentary,
and are singular from being seated far from the anterior extremity of
the head. In their rudimentary state, and in the absence of antennæ, we
have characters common with certain Suctorial Entomostracans; and this
similarity apparently arises from the fixed condition of the animals
of both groups.
The carapace, which covers the dorsal surface of the larva in the first
stage, in the last larval or pupal stage is developed so as to enclose,
like a bivalve shell, the whole body. In the mature Cirripede, the
whole external covering, whether shell and operculum, or capitulum and
peduncle, can be conclusively shown to be the carapace of the pupa,
modified. In thus enclosing the mouth and whole body, the modified
carapace resembles that of several Entomostracans; but in being
apparently formed (as I hope hereafter to show) by the development
of the third segment of the head, and in consisting generally of
distinct sclerodermic plates, arranged in an imbricated order, there
is, I think, a closer resemblance to the same part in some of the
Podophthalmia. The carapace, or portions of the carapace, being capable
of other movements, besides merely opening and shutting, differs, I
believe, from that of all other Crustaceans; as it likewise does[10] in
the greater part not being periodically moulted.
[10] The carapace, however, of the Isaura, a Branchiopod, according
to M. Joly ('Annales des Sc. Nat.,' 2 ser. vol. xvii, p. 293), is
not moulted.
Moreover, in all Cirripedes there is another striking peculiarity
connected with these parts, namely, the exclusive attachment of the
whole thorax or included body to the internal ventral or sternal
surface of the carapace and head. In the pupa, the thorax, as in all
Crustaceans, opens into, and is continuously united with, the large
anterior part of the head; but from the singular fact that the thorax
of the young Cirripede is developed not within the thorax, but within
the head of the pupa (Pl. 30, fig. 2), with its longitudinal axis
placed at right angles to that which it held in the pupal condition
(the mouth and the whole exterior being developed conformably with
that of the pupa), it comes to pass after the metamorphosis, that the
Cirripede is, as it were, internally cut in twain (compare Pl. 25, fig.
1, and Pl. 30, figs. 2 and 3). Thus it is, as will hereafter be more
fully explained, that the sack originates, and thus the body becomes
attached to the internal ventral surface of the carapace and front of
head.
The thorax in two of the Orders bears no appendages, but in all common
Cirripedes it is furnished with six pairs of biramous, multiarticulated
cirri, which have a peculiar character, different from the limbs of
other Crustaceans, not being natatory, ambulatory, or branchial,
but "captorial" or fitted for sweeping the water, and thus catching
prey.[11] The cirri, at least the anterior pairs, can, besides other
movements, lengthen and shorten themselves; and this Milne Edwards[12]
states is the case with the Podophthalmia, and is considered by him
as an important character. The cirri of the first pair are attached
on each side close to the bases of the mandibles, and, as already
remarked, have some claim to be considered as maxillipeds or mouth
organs. The three or the four posterior pairs of cirri in the Balanidæ,
form a series somewhat distinct from the two or three anterior pairs,
thus recalling a characteristic feature in the Edriophthalmia.
[11] M. A. Hancock, in 'Annals and Magazine of Natural History,' 2d
series, 1849, p. 312, speaks of the cirri acting like a prehensile
net.
[12] 'Annales des Sciences Nat.,' tom. xviii, p. 121, 1852.
The mouth is prominent, and is formed by the partial confluence of the
labrum, palpi, and lower segments of the mandibles, and of two pairs of
maxillæ; it is capable of movement as a whole; in this respect we are
reminded of the Suctorial Entomostracans; but I believe the above type
of structure of the mouth is peculiar to Cirripedes.
The alimentary canal is simple, but can be distinctly divided
into--(1st) an œsophagus, singular from the bell-shaped expansion
of its lower end; (2d) the stomach, which is directed forwards and
then doubled back; and (3d) the rectum. There is no distinct liver.
The circulation is lacunal. In one family there are well-developed
branchiæ, which differ entirely in their homologies and position
from these organs in all other Crustaceans. In the nervous system,
the sub-œsophageal ganglions vary in concentration from that degree
observed in the lower Macroura, to that in the highest Brachyoura; but
the supra-œsophageal ganglions are always much less concentrated, and
are even embryonic in condition; presenting a difference not observed
in other Crustaceans. On the under side of the sub-œsophageal ganglion,
two nerves, apparently splanchnic, arise, and run almost parallel and
under the collar surrounding the œsophagus; they are very remarkable
from their great size, and from forming a plexus together with a
large branch, arising on each side from the collar close behind the
supra-œsophageal ganglion,--a structure unlike anything observed in
other Crustaceans. The eyes, as already remarked, are rudimentary, and
singular from being imbedded at a distance from the anterior end of the
animal. In the basal confluent segments of the outer maxillæ there are
two orifices, leading into little sacks, which I believe are olfactory
organs: again there are two other orifices on each side of the thorax,
beneath the first pair of cirri, leading into sacks, with a curious
elastic vesicle suspended within them; and these I can hardly doubt are
acoustic organs. Of these orifices and organs, there is no trace in the
same relative positions in any known Crustacean.
Cirripedes are ordinarily bisexual, in which they differ from all
Crustaceans: when the sexes are separate, the males are minute,
rudimentary in structure, and permanently epizoic on the females; to
these latter facts we have a partial analogy in some of the Suctorial
Entomostracans; but a far closer analogy in certain Rotifers, which are
considered by many naturalists as Crustaceans; but to the above subject
I shall almost immediately have to recur.
The male excretory organ is probosciformed and capable of the most
varied movements; it is single and medial; it is seated (in the one
instance in which this point can be safely judged of) at the extremity
of the abdomen, and therefore near the normal position of the anus;
in all these respects there is a very great difference from other
Crustaceans, in which the male organs are laterally double, and are not
seated at the extremity of the abdomen. In regard to the female organs,
the ovarian tubes and cæca inosculate together: there are no oviducts;
the ova, connected together by membrane, and so forming the "ovigerous
lamellæ," become exposed by the exuviation of the lining tunic of the
carapace or sack, and by the formation of a new tunic on the under side
of these lamellæ; a process, I believe, unknown in other Crustaceans.
The metamorphoses are highly complex. The larva in its first stage
bears a very close general resemblance, in having three pairs of
natatory appendages, the first being uniramous and the two others
biramous, and in having a single eye on its broad anterior front, to
the larvæ of most Entomostracans; but I cannot avoid the belief, that
this resemblance is only apparent, and not essential; and of false
resemblances, how many instances occur in the animal kingdom! In the
larva, when first freed from the egg, both pairs of antennæ are in
process of formation within envelopes: the mouth is probosciformed
and capable of movement, but is destitute of gnathites; it occupies a
position between the three pairs of natatory limbs; and these limbs I
must believe, for reasons hereafter to be assigned, answer (improbable
as I am well aware it must at first appear) to the second, third,
and fourth thoracic legs of the archetype Crustacean: the two hinder
pairs of limbs apparently soon become captorial, or fitted to secure
prey. Now, I cannot find in the published accounts of the larvæ of
Entomostracans, any that answer to this description.
The larva in the last stage might be included in the vast class of
Entomostracans: the attachment of the eyes to the singular apodemes
produced inwards from the basal segment of the great prehensile
antennæ, and the development of only the posterior six pairs of
thoracic limbs, are its chief peculiarities: but its rudimentary mouth,
owing to its transitional or pupal condition, renders the assignment of
its proper rank difficult.
* * * * *
Having now given this short comparative sketch of the structure of
a Cirripede, I may venture to express strongly my opinion, that the
group is formed on a distinct type; as different from the other
three or four main Crustacean groups, namely, the Podophthalmia,
Edriophthalmia, Branchiopoda, and Entomostraca, as these differ from
each other; the differences, moreover, being of the kind considered
by the highest authorities on this subject, as the most important. It
should be observed that there is no special blending at either end of
the Cirripedial series, towards any one of the other main groups of
Crustacea; it is hardly possible to take some one Cirripede, and say
that it leads, more plainly than some other Cirripede, into ordinary
Crustaceans. Moreover, a great range of structure, as we shall soon
briefly show, is included within the group: I can adduce three or four
undoubted Cirripedes, very considerably more different from each other,
than any two members within the sub-class Podophthalmia, or within the
Edriophthalmia, or the Branchiopoda, and quite as different as within
the Entomostraca.
The opinion here expressed, that Cirripedes form a sub-class of equal
value with the other main Crustacean groups, I am well pleased to find,
accords with Mr. Dana's[13] view, who remarks that this sub-class
"has so many peculiarities of structure, that it should be regarded
as a distinct type, rather than a subordinate division of the third
(or Entomostracan) type." M. Milne Edwards,[14] after dividing all
Crustacea into two groups, divides one of them into four legions;
and of one of these, the Entomostraca, he makes the Cirripedes a
sub-group. I feel so entire a deference for any opinion on affinities
or classification expressed by Milne Edwards, that I differ from
him with the greatest hesitation. He does not give his reasons for
assigning so subordinate a rank to Cirripedes, but I imagine it is from
the nature of their metamorphoses: but if this be the case, I cannot
understand why he should assign to his Branchiopods a rank equal to his
Entomostracans. Moreover, I must repeat, that I do not believe that
the larvæ do resemble the larvæ of Entomostracans and Branchiopods
nearly so closely as at first appears to be the case. I may add, that
Burmeister[15] has assigned to the Cirripedes a place amongst the
Crustacea, almost equally subordinate to that given to them by Milne
Edwards.
[13] 'Crustacea: United States Exploring Expedition,' p. 1407, 1852.
[14] 'Annales des Sciences Nat.,' tom. xviii, p. 120, 1852.
[15] 'Beiträge zur Naturgesehichte der Rankenfüsser,' 1834.
That Cirripedes have some special affinity to the Entomostraca, may
be inferred from the fewness of the cephalic appendages, the biramous
legs, the state of the abdomen, and the form of the carapace. Perhaps
in the peculiar state of confluence of the lower segments of the
gnathites, in the aborted antennæ, the rudimentary eyes, and in the
minute parasitic males (when such exist), there is a more direct
relation to the Suctorial division of the Entomostraca; but some of
these resemblances are probably only analogical, resulting from the
fixed condition of both groups. It should not be overlooked, that
out of the three orders into which Cirripedes may be divided, in
the two latter, the mature animal presents hardly any resemblance
to an Entomostracan. From the distinct presence in either pupa or
mature animal of the fourteen segments of the cephalo-thorax; from
the apparent composition of the carapace, as will be subsequently
explained; and from the concentrated condition of the nervous system,
one is led to glance at the higher Crustacea; and here we shall find
amongst the Podophthalmia, one aberrant group of low organisation,
namely, that including Phyllosoma, Amphion, &c., in which more points
of resemblance to Cirripedes may be detected, than, as I believe, in
any other group whatever; for we here see that remarkable elongation
of the head in front of the mouth, so eminently characteristic of
Cirripedes; we have a carapace overlapping the thorax, which is
sometimes free beneath; we have the abdomen sometimes almost obsolete;
we have biramous legs: and especially we have the posterior cephalic
and the first thoracic appendages more or less rudimentary and
obsolete; and this, I infer from Mr. Dana, is a very rare phenomenon,
though characteristic of all _ordinary_ Cirripedes, in which the
seventh and eighth segments with their appendages have disappeared.
In the order including Phyllosoma, &c., namely, in the Macroura, the
ganglions which give nerves to the five posterior thoracic limbs, are
distinct from the great sub-œsophageal ganglion which supplies the
several anterior appendages; this is the case with those Cirripedes in
which all the infra-œsophageal ganglions are not concentrated into one.
In the Macroura and Brachyoura, the first pair of legs almost always
differs in structure from the others, so does the homologous or second
cirrus in Cirripedes differ from the four succeeding pairs; in some
few Macroura, the second leg is antenniformed, so in some few cases is
the homologous (or third) cirrus; J. Vaughan Thompson was even struck
by the resemblance in the curious, doubly pectinated spines on the
anterior limbs of Mysis (allied to Phyllosoma[16]), and on those of
many Cirripedes: these several latter resemblances may be small, but
certainly I do not believe that they are accidental. Now the little
group of Crustaceans, which includes Phyllosoma, &c., has lately
been placed, by Milne Edwards, as a satellite amongst the Macrourous
Podophthalmia; it leads into the Stomopoda, and likewise, as has been
noticed by many authors, into the sub-class Branchiopoda, which latter
sub-class is considered by Mr. Dana as only a part of the Entomostraca;
this group, therefore, exhibits affinities radiating in several
directions, and amongst these lines of relationship, one more must, I
believe, be added, plainly directed towards the Cirripedia.
[16] M. Martin St. Ange ('Mémoire sur l'Organ. des Cirripèdes,'
1835, extrait des 'Savans Etrangers,' tom. vi) has compared the
mouth of Lepas with that of Phyllosoma, and has given comparative
figures; but the resemblance is founded, I believe, on quite false
homologies.
One naturally wishes to ascertain how far Cirripedia are highly or
lowly organised and developed; but in all cases this, as it seems to
me, is a very obscure enquiry. Mr. Dana considers that, in Crustacea,
the greater or less centralisation of all the appendages round the
mouth is the main sign of high development; on this view, the anterior
part of a Cirripede, from being so much elongated, must be considered
as very low in the scale; the whole posterior part of the body, on the
other hand, is, in ordinary Cirripedes, brought close to the mouth; but
this is effected by the abortion of the seventh and eighth segments of
the cephalo-thorax and of the whole abdomen, and so, I presume, would
not, in Mr. Dana's estimation, raise the class much in the scale. Von
Baer[17] considers that the perfection of the type of any animal is in
relation to the amount of "morphological differentiation" which it has
undergone; on this view, Cirripedes ought to stand high in the scale,
for they differ much morphologically from the type of the class to
which they belong; as indeed is shown by the long time that elapsed
before their true position, namely amongst the Crustacea, was even
suspected; but something more must, I think, be added to Von Baer's
definition; for, to take as an example the eyes of a Cirripede,--as
seen in the first larval stage, there is only one eye, and that most
simple; in the pupa there are two, both compound, and furnished with
complicated muscles; lastly, in the mature animal there are still
two, but of very minute size, often almost confluent, and of the
simplest structure; hence, then, there has been much morphological
differentiation, but it is almost a contradiction in terms to speak, in
relation to such a case, of _perfection_ of type; and what has happened
to one organ, might happen to other organs, and so to the whole animal.
Lastly, under a physiological point of view, and taking the Balanidæ as
the most perfect type of the class, the sub-œsophageal portion of the
nervous system is highly concentrated; the organs of sense, excepting
the eyes, seem more largely developed than in ordinary Crustaceans; the
circulating system is of the simplest kind, being only lacunal; special
Branchiæ, however, are developed by the metamorphosis of, as I believe,
a special organ, occurring only in the Lepadidæ; the digestive organs
are very simple, from not having any distinct liver; the generative
system is very low, for both sexes are generally united in the same
individual; and the testes and ovaria closely resemble each other.
On the other hand, the thoracic limbs are, to a considerable extent,
specialised in their structure and functions; only the three posterior
pairs strictly resembling each other. Lastly, the dermal and muscular
systems are complicated, and not, to use Professor Owen's term, by mere
vegetative repetition, as will be obvious to any one who will study the
beautifully constructed and modified carapace--that is the operculum,
shell and basis--of a Balanus. On the whole, I look at a Cirripede
as a being of a low type, which has undergone much morphological
differentiation, and which has, in some few lines of structure, arrived
at considerable perfection,--meaning, by the terms perfection and
lowness, some vague resemblance to animals universally considered of a
higher rank.
[17] English Translation, in 'Scientific Memoirs,' 1853, vol. i, p.
228.
* * * * *
It has been seen that I divide the Cirripedia into three orders,--the
Thoracica, Abdominalia, and Apoda; between which the fundamental
difference consists in the limbs or cirri being thoracic in the first,
abdominal in the second; and entirely absent in the third. For the sake
of showing the range of character in Cirripedes, to which allusion has
been made, I will briefly indicate the leading differences in each
order. In the Thoracica, three families are included,--the Balanidæ,
or sessile Cirripedes, the Verrucidæ, remarkable from their quite
asymmetrical shell, and the Lepadidæ, or pedunculated Cirripedes. The
great difference in external appearance between these three families is
known to all naturalists. Even within the one family of Lepadidæ there
are great differences in external appearance, as will be admitted on
comparison of Lepas, Pollicipes, Conchoderma, &c.; but we have also
important internal differences, as in the case of Anelasma, in which
the cirri are barely articulated, and are not capable of seizing prey,
whilst the mouth is almost probosciformed, with the outer maxillæ
and palpi rudimentary: still more important are the differences in
Alcippe, in which the cirri of the first pair act as brushes; the
second, third, and fourth pairs being quite aborted; and the fifth and
sixth pairs consist only of four segments, with one of the two normal
rami converted into a crenated, button-like projection, for the sake
apparently of triturating food; Alcippe, also, is very remarkable in
being destitute of a rectum and anus. In this same genus Alcippe, in
Ibla and Scalpellum, there are either separate males or Complemental
males, some of which are so utterly abnormal in their characters, that
by no definition which I could frame, could they be included even in
their proper Order, much less in their proper Family.
In the second order of Abdominalia (Pl. 23 and 24) the seventh or last
cephalic segment is quite distinct, and bears rudimentary organs,
answering to the first pair of maxillipeds of ordinary Crustaceans, of
which organs, and of the segment supporting them, there is no trace in
the Thoracica: the seven succeeding thoracic segments are destitute
of any appendages; but the three segments of the abdomen bear three
pairs of cirri. The mouth is peculiar in the labrum being developed
into very large, moveable, lancet-formed organ; and the lower end of
the œsophagus is armed with beautiful discs of teeth, and brushes of
hairs,--a structure confined to this order. The male resembles the
male of Alcippe; and the latter genus seems to be the connecting link
between the Thoracica and Abdominalia. But the most important character
of this latter order, in which it differs from Alcippe, and all other
known Cirripedes, is in its metamorphoses; all the first changes are
merely indicated by changes in form in an egg-like larva, without the
development of distinct organs; and the last, or pupal condition, which
is attained within the sack of the parent, is very peculiar, by the
entire absence of natatory limbs.
The third order of Apoda is the most peculiar of all; it contains,
like the last, only one known species: the most acute naturalist, I
am convinced, if he had not made the class his special study, would
never even have suspected that this animal was a Cirripede. We see
much magnified in Pl. 25, fig. 7 a naked, plainly-articulated animal,
resembling the larva or maggot of a fly, attached by two threads;
and these threads, on analysis, can be clearly shown to be the last
rudiment of the carapace, specially modified. The last cephalic, the
seven thoracic, and the three abdominal segments, are all equally
destitute of appendages. The mouth is suctorial, and constructed on a
plan unlike, I believe, anything known in the articulate kingdom; for
the mandibles and maxillæ have rotated on their axes, and stand back
to back; they can act only by tearing open a slit, and this action is
performed in a hood, formed by the confluence of the broad palpi and
labrum. Although the œsophagus is distinct, there is no stomach or
anus. Lastly, owing to there being no carapace, the ova are developed,
differently from in all other Cirripedes, within the thorax.
I will close this preliminary discussion on the confines and type of
the sub-class, by recalling attention, now that a sketch has been given
of the three Orders, to the remark before made, that a wide range of
structure is included within it, and by reurging that the Cirripedia
should be ranked, not as one of the subordinate groups, but as one of
the main divisions of the Crustacea.
_On the Sexual Relation of Cirripedes._
Cirripedes are commonly bisexual or hermaphrodite, but in Ibla,
Scalpellum, and Alcippe, members of the Lepadidæ in the order
Thoracica, and in Cryptophialus in the order Abdominalia, the sexes
are separate. As two of these genera were described in my former
volume, and two others (Alcippe and Cryptophialus) are described in
this volume, I may as well here give a brief summary of the facts as
yet known on this very curious subject. The Males, in the above four
genera, present a wonderful range of structure; they are attached in
the usual way by cement proceeding from the not-moulted antennæ of the
pupa, to different parts, in the different species, of the female.
These males are minute, often exceedingly minute, and consequently
generally more than one is attached to a single female; and I have
seen as many as fourteen adhering on one female! In several species
the males are short-lived, for they cannot feed, being destitute of a
mouth and stomach. As the females are longer lived, successive crops
of males, at each period of propagation, become attached to her.
It is the females in the above genera which retain the characters
of the genus, family, and order to which they belong; the males
often departing widely from the normal type. Some of the males are
rudimentary to a degree, which I believe can hardly be equalled in the
whole animal kingdom; they may, in fact, be said to exist as mere bags
of spermatozoa. So widely do some of them depart in every character
from their class, that twice it has happened to me to examine specimens
with a little care, and not even to _suspect_, until a long period
afterwards, that these males were Cirripedes.[18]
[18] In my volume on the Lepadidæ (p. 200) in searching for
analogies for the permanently epizoic and rudimentary condition
of the male Cirripedes, I quoted two cases, which I believe are
now known not to be analogous; namely, the _Syngamus trachealis_
of Von Siebold, and the worm-like Hectocotyle, which latter was
quite lately supposed to be a male Cephalopod, but has now been
ascertained to be only one of the arms of the male wonderfully
adapted and organised as a sperm-receptacle. The Asplanchna, the
mouthless male of a Rotifer, (p. 292) alone remains for me.
In _Scalpellum Peronii_, and _villosum_, the males are but little
abnormal, for if classified independently of their sexual relations,
they would be considered as immature specimens of a new genus, standing
next to Scalpellum; in _Scalpellum rostratum_, the male would form
another and rather more distinct genus. The males, in the latter, are
attached to the other sex, between the basal edge of the labrum and
the adductor scutorum muscle; but in _S. Peronii_ and _villosum_ they
are attached lower down, in the furrow between the two scuta, and are
thus protected: in these three species, the internal parts of the male
present nothing particular. In Ibla, the males are attached low down
within the sack of the female; they may be said to consist of a mouth
surmounted on a long peduncle, for there is no capitulum or general
covering, and the whole thorax is in a rudimentary condition, the cirri
being reduced to two distorted pairs. As these males certainly moult
several times and grow a little, they must feed; and as they have no
cirri fit for action, they must seize their food by the contortions of
their peduncle, which we know homologically consists of the three first
segments of the head. The movements of the peduncle must, also, supply
those of the probosciformed penis, almost invariably present with other
Cirripedes, but here absent. If compelled to class these males without
regard to the female, great difficulty would be experienced; we could
hardly place in the family of the Lepadidæ, a Cirripede without a
capitulum, and without cirri, those very organs which give their name
to the class, and with a thorax reduced to the dimensions of a lower
lip; yet, if the presence of a peduncle did determine the classifier to
place these males amongst the Lepadidæ, then undoubtedly the character
of the mouth, &c. would fix their position next to Ibla.
The males of _Scalpellum vulgare_, _ornatum_, and _rutilum_, resemble
each other in all essential points, and differ wonderfully in
appearance and structure from all ordinary Cirripedes. They consist of
a minute flattened bag with a small orifice at the summit, and at the
lower end attached by the cemented pupal antennæ. On each side of the
orifice, there is a pair of calcareous beads, representing the two
scuta and two terga of ordinary Cirripedes; and between the scuta a
minute black eye is generally conspicuous. In _S. ornatum_ the beads, I
may remark, on the two sides are not equal; those either on the right
or on the left side, being larger than those on the opposite side, so
that the animal externally is asymmetrical. Inside, within a tubular
sack, the thorax is lodged, supporting four (instead of six) pairs
of limbs; and these, instead of forming biramous, multiarticulated,
captorial cirri, are reduced to almost a rudiment, supporting a few
long sharp spines, which apparently act only as defensive organs. At
the end of the thorax there is seated a large abdominal lobe, which
does not occur in the other sex. Hence the thorax, though rudimental,
has been specially modified. Of the mouth and stomach there is not a
vestige. Constructed as these males are, assuredly they have no claim
to be ranked amongst the Lepadidæ or pedunculated Cirripedes; nor is it
possible to class them in any group whatever of ordinary Cirripedes.
In _S. vulgare_ the males are attached, often several together, to the
extreme edges of the two scuta, and therefore immediately over the
orifice leading into the sack; in _S. rutilum_ and _ornatum_, they are
attached in concavities on the under side of both scuta, just above the
depression for the adductor scutorum muscle. In the former of these
species, the pit for the reception of the male is formed by shelly
matter not having been deposited over a certain space on the under side
of the valve; and the pit is converted by a covering of membrane into
a pouch. As there are two scuta so there are two pouches, in each of
which a male is lodged; hence, according to the Linnean nomenclature,
_Scalpellum ornatum_ may be said to belong to Diandria monogynia. As
these males, from being mouthless, soon die, they are succeeded by
successive pairs; the pupa being led by a wonderful instinct to crawl
into the pouch, and there undergo its metamorphosis.
Lastly, the males of Alcippe and Cryptophialus (Pl. 23, fig. 19, and
Pl. 24, fig. 19) are remarkable for their similarity to each other,
considering the essential dissimilarity of the two females. The females
live in cavities which they excavate in the shells of Molluscs, and
within which they are attached by a horny disc; this disc is the only
part of the outer integument which is not frequently moulted, and,
apparently in consequence, the males are attached to its edges. It
results from this position, that the males are protected by being
enclosed within the cavity excavated by the female; and it further
results, that the males are attached at a considerable distance from
the orifice of the sack of the female, into which the spermatozoa
have to be conveyed; and to effect this, the probosciformed penis is
wonderfully developed, so that in Cryptophialus, when fully extended,
it must equal between eight and nine times the entire length of the
animal! These males, like those last mentioned of Scalpellum, consist
of a mere bag, lined by a few muscles, enclosing an eye, and attached
at the lower end by the pupal antennæ; it has an orifice at its upper
end, and within it there lies coiled up, like a great worm, the
probosciformed penis, and beneath it a single testis, with a single
vesicula seminalis. These organs complete the whole organisation of the
male; for there is no mouth, no stomach, no thorax, no abdomen, and
no appendages or limbs of any kind. Yet all these parts are present
in the female. I know of no other instance in the animal kingdom
of such an amount of abortion. The whole exterior of these males
evidently is composed, as in all ordinary Cirripedes, of the three
first cephalic segments; of the fourteen succeeding segments of the
archetype Cirripede we have not a vestige, excepting the probosciformed
penis, which, from analogy, should arise from the ventral apex of
the seventeenth segment, the first three segments of the head being
counted in the seventeen. Here, then, fourteen out of seventeen
segments have aborted, the tip of the seventeenth having coalesced with
the third cephalic segment! I am tempted just to notice the case of
Proteolepas, in the order Apoda, as showing, within the limits of the
same sub-class, a wonderful amount and diversity in abortion; for in
Proteolepas, the three anterior cephalic segments are reduced to the
merest rudiment, encasing the cement-ducts, the fourteen succeeding
segments being unusually well developed; whereas in the above described
males, we have just seen the three anterior segments fully developed,
whilst the fourteen succeeding segments are lost or have coalesced
with the others; so that within the same sub-class all seventeen
segments of the archetype have almost disappeared.
It may be asked how I know that the several above described rudimentary
epizoons are really the males of the Cirripedes to which they are
attached. Even if the whole course of the metamorphoses had not been
known in three of the cases, the mere fact of these epizoons being
cemented by the three terminal segments of their peculiar, pupal
antennæ, would have been sufficient to have shown that they belonged
to the class of Cirripedes. In nearly every case, I was able to
demonstrate, and not in one or two but in many specimens, that these
epizoons were males; and as in several instances the spermatozoa
were developed, and as, notwithstanding, in no instance was there a
vestige of ova or ovaria, it may safely be concluded that they were not
hermaphrodites, and therefore required females of some kind. If these
epizoic Cirripedes had been independent animals, as they all belong
to the same sub-class, and all have such peculiar habits, it might
have been expected that they would have shown some special affinity
towards each other; but this is not the case; the epizoon of Ibla is
more nearly related to Ibla, and the epizoon of Scalpellum more nearly
related to Scalpellum, than are these epizoons to each other. If the
several epizoons were classed by themselves, they would be grouped in
divisions, corresponding with those of the Cirripedes on which they are
attached, which is just what might have been expected if these latter
were their females. There are, also, many special relations between
the male epizoons and the Cirripedes to which they are attached; thus,
the mouth of the epizoon of Ibla, is so like the mouth of Ibla, which
is peculiar in several respects, that I should easily have recognised
it as belonging to a member of that genus. _Scalpellum villosum_ is
remarkable as one out of only two or three members of the whole Family,
which is destitute of caudal appendages, so is its male epizoon; again,
_S. villosum_ is unusually spinose, so is its male epizoon; on the
other hand, _Scalpellum ornatum_ is remarkably smooth, so is its male
epizoon; and I could give other similar instances. Will it be believed
that these coincidences are accidental, and that the epizoons have no
special or sexual relation to the Cirripedes to which they are attached?
One other instance of coincident structure is so important, that it
must, even in this sketch, be noticed; the prehensile antennæ of
the pupa are most important and complicated organs, and differ in
the different genera of the same family; they are preserved in a
functionless condition throughout life, and in two instances I was able
accurately to compare these organs in the epizoon and in the Cirripede
to which it was attached, and they were identical in every particular.
The full force of the excessive improbability of this resemblance,
and of the above coincidences in structure, on the supposition of the
epizoon and its support not being sexually related, will hardly be
perceived without referring to the facts given in detail in my former
volume.
Lastly, in the case of Cryptophialus (and indirectly in that of
Alcippe) the nature of the male epizoon is, I think, actually
demonstrated; for I traced both it and the female or ordinary form of
Cryptophialus, through the same several larval stages, from the egg,
enclosed within the sack of the female, to the pupa and mature animal.
Moreover, if the male nature and sexual relation to the supporting
Cirripede, be admitted in any one of these epizoons, then so close
is the agreement in habits, and to a certain extent in structure,
in all the foregoing epizoons, that probably no one admitting one
instance would dispute the others, and further evidence would even be
superfluous. Indeed, had it not been for the following facts, I should
not have brought forward, either here in abstract, or in other places
in detail, so many arguments and so much evidence.
I have as yet not entered in detail on the sex of the supporting
Cirripede: in Cryptophialus, Alcippe, and in one species of Ibla, I
was able to demonstrate in many specimens, that all the male organs,
internal and external, were entirely absent; and consequently that
these Cirripedes were _exclusively_ female. In _Scalpellum ornatum_,
also, there is no trace of external male organs (the state of the four
dried specimens not allowing the internal organs to be examined), and
there cannot be any reasonable doubt that this species likewise is
exclusively female. It should be borne in mind that the male organs,
external and internal, are most easily discovered, and that in the
above cases I had an abundant supply of excellent specimens. On the
other hand, in _Ibla Cumingii_, and in four species of Scalpellum, I
was able to demonstrate in the supporting Cirripede the presence of
all the male organs, as well as of the female; and in the vesiculæ
seminales of several specimens, both in the Ibla and in _Scalpellum
vulgare_, spermatozoa were contained; the male organs, however, not
being very amply developed. These species, consequently, are not
exclusively female, but are hermaphrodite, though having male epizoons
attached to them. This statement, I am well aware, is enough, at first,
to cast a doubt on all that I have said; but let any one reflect on
the evidence, of which I have here given a summary, and which has
been elsewhere given in full, and I think he must admit that at least
those epizoons which are exclusively male, and which are attached
to Cirripedes exclusively female, are sexually related and form one
species; but if he admit this, he cannot possibly escape from the
conclusion that some of the other epizoons, for instance that of _Ibla
quadrivalvis_, are the males of the hermaphrodites to which they are
attached,--these epizoons not exclusively impregnating the ova of a
female, but aiding the self-impregnation of an hermaphrodite. Hence I
have called these males _Complemental Males_, to show that they do not
pair with a female, but with a bisexual individual. Nothing strictly
analogous is known in the animal kingdom, but amongst plants, in the
Linnean class, Polygamia, closely similar instances abound.
In the series of facts now given, we have one curious illustration
more to the many already known, how gradually nature changes from one
condition to the other,--in this case from bisexuality to unisexuality.
Finally, in the four genera so often named, we meet the following
several cases, some of them even the most diverse, occurring in closely
allied species. (1st), a female, with a single male (rarely with two)
permanently attached to her, protected by her, and capable of seizing,
by the movements of its pedunculated body, any minute animals or
substances found within her sack; (2d), a female with successive pairs
of short-lived, mouthless males, inhabiting pouches on each side under
her scutal valves; (3d), a female with many, in one instance fourteen,
short-lived males, destitute of mouth, thorax, and appendages, but
furnished with a stupendously long male organ, attached to a thickened
portion of her outer integuments, but lying within the cavity which
she has excavated; (4th), an hermaphrodite with a male attached within
the sack, capable of feeding itself, as in the first case; (5th), an
hermaphrodite with from one to three males, organised like ordinary
Cirripedes, and apparently capable of seizing prey in the common way;
and attached between the scuta, and thus protected; (6th and lastly),
an hermaphrodite with from one or two up to five or six, short-lived,
mouthless males, like those in the second case, attached in one
particular spot, on each side of the orifice leading into the sack.
ORDER I.--THORACICA.
_Cirripedia having a carapace, consisting either of a capitulum on a
peduncle, or of an operculated shell with a basis. Body formed of six
thoracic segments, generally furnished with six pairs of cirri; abdomen
rudimentary, but often bearing caudal appendages; mouth with the labrum
not capable of independent movements; larva firstly uniocular, with
three pairs of legs, lastly, binocular, with six pairs of thoracic
legs._
In the sketch of the three Orders given in the Introduction, it will
have been seen that the differences in their structure are so great,
that it would have been hardly possible to have given a single blended
account of the whole Class. But as all common Cirripedes are included
in the present Order, here would have been the natural place for a
full description of their external and internal structure. Having,
however, been necessarily, yet perhaps unfortunately, led to give, in
my former volume, a description of this kind of the Lepadidæ; and as it
is necessary to give a similar account of the other great family of the
Order, namely, the Balanidæ, I have found it more convenient to make
this latter account comparative and supplemental to the former one on
the Lepadidæ, and so serve for the Order, rather than attempt to give
a separate description in full of it. For this latter plan would have
involved much useless repetition, as, on account of the many exceptions
and limitations necessary to almost every statement, there is little
choice between a description of great length and a mere diagnostic
character of the Order, such as I have given above.
The Thoracica may be divided into three very natural Families, of
nearly equal value; firstly, the Balanidæ, or sessile Cirripedes,
which may be subdivided into two sub-families, also very natural, the
Balaninæ and Chthamalinæ; secondly, the Verrucidæ, containing only
one genus; and thirdly, the Lepadidæ, or pedunculated Cirripedes.
These three families differ from each other, besides in mere external
appearance, almost exclusively in the relation of the different
portions of their external covering or carapace, and of the muscles
moving such portions. In the Balanidæ, the four opercular valves
surrounding the orifice leading into the sack, are capable of other
movements, besides being opened and shut; whereas all the other valves
are immoveably united together. In the Lepadidæ, the valves answering
to the opercular valves, are furnished with a muscle only for shutting
them; whereas the peduncle answering to the basis in the Balanidæ is
capable of various movements. In the Verrucidæ the shell is singularly
asymmetrical; only half of the operculum (either the right or the left
side, this varying even in the same species) being moveable; the other
half being immoveably united to the remaining valves; and the whole
shell has only one muscle serving to shut the moveable half of the
operculum. All the internal parts and organs are very similar in the
above three Families. If, however, the internal structure of one of the
two sub-families, into which the Balanidæ may be divided, namely, of
the Balaninæ, be compared with that of the Lepadidæ, several important
differences may be detected;--on the one hand, in the Balaninæ, the
presence of branchiæ, the extremely complicated cementing apparatus,
the difference in structure between the third and succeeding pairs of
cirri, the large palpi, the notched labrum, and the laterally double
teeth of the mandibles;--and on the other hand, in the Lepadidæ, the
presence of ovigerous fræna, caudal appendages, bullate labrum, and
often prominent olfactory orifices. But if the Lepadidæ be compared in
these several respects with the other sub-family, or Chthamalinæ, which
cannot possibly be removed out of the family of Balanidæ, many of these
differences break down and disappear, in some or all of the species.
The Lepadidæ include, as has previously been noticed, a much greater
range of difference than the Balanidæ; and this is what might have
been expected, for it is the most ancient family, and extinction has
done its work, separating genera, which, in accordance to analogy, we
may suppose were once more nearly connected by intermediate forms. The
Lepadidæ, in one sense, may be taken as the type of their order; for
they have undergone less "morphological differentiation;" that is, they
differ the least from the last larval stage, and seem to give the most
_general_ idea of a Thoracic Cirripede. On the other hand, if we mean,
as some authors do, by the word type, that form which, in the group in
question, has been most modified, and illustrates every peculiarity of
its class in the strongest manner, then we must look to the Balaninæ,
and to its typical genus, Balanus, for the most Cirripedial form. In
this genus the different portions of the carapace differ most, and
subserve to a certain extent different ends, and in minute structure
are most complicated; here the cementing apparatus, which offers the
main characteristic of the whole sub-class, is most complex; here the
several pairs of cirri differ most from each other in structure and
action; here the peculiar branchiæ (organs apparently derived from the
modification of another organ, itself confined to Cirripedes, viz., the
ovigerous fræna) are best developed; here the nervous system is most
highly concentrated; and, lastly, here we meet with the largest and
most massive species of the whole group.
1. Family BALANIDÆ, (or Sessile Cirripedes).
_Cirripedia without a peduncle; scuta and terga furnished with
depressor muscles; other valves united immoveably together._
TABLE OF CONTENTS.
Page
Structure of shell 34
" of the individual compartments 43
" of the radii 45
" of the alæ 47
" of the sheath 48
" of the basis 49
" of the opercular valves (scuta and terga) 51
Growth of whole shell and microscopical structure 54
Muscles of sack 61
Branchiæ 63
Thorax and body 65
Muscular system 68
Movements and muscles of the cirri 71
Mouth 74
Cirri 81
Caudal appendages 85
Alimentary canal 85
Circulatory system 87
Nervous system 88
Eyes and vision 93
Acoustic organs 95
Olfactory sacks 97
Male organs of generation 97
Female organs of generation 100
Metamorphoses and homologies 102
Larva, first stage 103
Larva, second stage 109
Larva, last or pupal stage 110
Act of metamorphosis 126
On the homologies of the carapace 131
Cementing apparatus 133
Affinities, classification, variation 152
Rate of growth, exuviation, &c. 156
Geographical range and habitats 159
Geological history 172
Almost every one who has walked over a rocky shore knows that a
barnacle or acorn-shell is an irregular cone, formed generally of six
compartments, with an orifice at the top, closed by a neatly-fitted,
moveable lid, or operculum.[19] Within this shell the animal's body is
lodged; and through a slit in the lid, it has the power of protruding
six pairs of articulated cirri or legs, and of securing by their
means any prey brought by the waters within their reach. The basis is
firmly cemented to the surface of attachment. The whole shell, basis,
and operculum consists, as we have already seen, of the first three
segments of the head, modified into a singularly constructed carapace,
which encloses the mouth and rest of the body. The anterior extremity
of the shell is situated in the centre of the basis, where indeed,
by due care, the antennæ of the pupa may be always detected; and the
posterior extremity is directed vertically upwards.
[19] The best published description of the structure of the
shell of a sessile Cirripede, is given by Dr. Coldstream, in the
'Encyclopædia of Anatomy and Physiology,' article 'Cirrhopoda.'
_Structure of Shell._
When the shell of a sessile Cirripede or barnacle, for instance,
of a Balanus, is first examined, the structure appears extremely
complicated; but this can hardly be considered as really the case. The
structure will, I think, be best understood by recalling to mind that
of Pollicipes,--the oldest known genus, from which, in one sense, all
ordinary Cirripedes, both sessile and pedunculated, seem to radiate.
I must premise, and the fact in itself deserves early notice, that
the homologies of the several parts in the pedunculated and sessile
Cirripedes admits of no doubt,--that is, if amongst the pedunculated,
the genus Pollicipes, or certain species of Scalpellum, be taken as a
standard of comparison.[20] The peduncle corresponds with the basis,
as may be clearly seen, if a Pollicipes with a short peduncle, and a
Balanus, with a deep cup-formed or cylindrical basis be compared, for
the contained parts are similar, and both grow at their upper edges
upwards and outwards. Secondly, the valves round the lower part of
the capitulum of a Pollicipes, though generally much more numerous,
and forming more than one whorl or circle, and not so closely packed
together, answer to the compartments forming the shell of a sessile
Cirripede; this is shown by their lateral and downward growth, by their
upper ends generally projecting freely above the cavity in which the
animal's body is lodged; and in the case _Pollicipes mitella_, by an
actual resemblance in outline, some being triangular, some broad at
the upper end, and some sub-rhomboidal, and, lastly, in the manner in
which they slightly overlap and indent each other: moreover further
resemblances in the relative position and even in the size of the
several valves, will hereafter be pointed out between certain sessile
genera amongst the Chthamalinæ and certain genera of the Lepadidæ.
Thirdly, the scuta and terga in Pollicipes, so strikingly resemble
in manner of growth in position relatively to the animal's body--in
shape--and even in being articulated together, the valves which form
the operculum or lid of sessile Cirripedes, that their identity is at
once obvious.
[20] Dr. J. E. Gray long ago observed these homologies. If Lepas be
taken, the comparison is not quite so simple, owing to the growth
of all the valves in that genus being upwards; but in several
species of Scalpellum we may see the intermediate steps between
the normal downward growth of the valves in Pollicipes, and the
abnormal upward growth in Lepas.
It may be well here further to premise, that apparently none of the
sutures in the shells of Cirripedes correspond with the articulations
between the three archetype cephalic segments, of which the whole shell
is formed; or with the eight elemental pieces, of which each separate
segment in the archetype crustacean is known to consist. But, as I
believe, the several valves in the shell of a Cirripede are homologous,
or at least analogous, with the sclerodermic plates,[21] of which
the carapace of the Podophthalmia is formed; with this difference,
that in the latter they become, after their first formation, united
together into a single piece, and are thus moulted as a whole; whereas
in Cirripedes, the valves or sclerodermic plates are not moulted, but
continue to be added to throughout life.
[21] Milne Edwards, 'Annales des Sciences Naturelles,' tom. xviii,
(1852), p. 236.
In Pollicipes, there is no difficulty in understanding the growth of
the lower valves of the capitulum, especially if a species be taken
in which these valves stand a little way apart: at each period of
growth, they are added to at their basal edges and a little way up
both sides; at the same time, a new membrane connecting them together
is formed, the old membrane disintegrating, or being left hanging in
tatters to the last zone of growth. Now if we look at the shell of
a sessile Cirripede, there is no essential difference in the growth
of the compartments or valves; all grow downwards and laterally; but
they overlap each other much more laterally than in Pollicipes, and
the connecting membrane is in most parts reduced to a mere film jammed
in between the valves; but, in the case of the opercular membrane, it
still remains wide, and is periodically moulted.
In the annexed woodcut (fig. 1), of the rostrum of _Balanus Hameri_,
the downward growth and the lateral growth on both sides is plain.
The modified sides (_rr_) for convenience sake, have been called the
_radii_; they invariably overlap the adjoining compartments. The middle
part (_p_), has been called the wall, or parietal portion: in the
specimen figured, the walls and radii are distinctly separated, but in
some cases, especially amongst the Chthamalinæ, the lines of growth are
absolutely continuous from one to the other. In fig. 2 of a Lateral
compartment of the same Balanus, we have the same essential structure;
but the left side (_a_) is more protuberant, and is hollowed out in its
lower half; it is, also, more distinctly separated from the parietal
portion: this side has for convenience been called the _ala_; it is
invariably overlapped by the adjoining compartment: in some few cases,
as in Pachylasma, the ala is not hollowed out in its lower part, and
from being added to in a straight line along its whole edge, with the
lines of growth continuous with those on the wall, it differs hardly at
all in appearance from a radius. Lastly, in fig. 3 of the carina, or
compartment facing the rostrum, we have alæ (_aa_) on both sides; these
being, as in all cases, overlapped by the adjoining compartments.
[Illustration: Fig. 1.
Fig. 2.
Fig. 3.
_p_, _p_, Parietes; _r_, _r_, Radii; _a_, _a_, Alæ.
Fig. 1, Rostrum with two radii, serving in the Chthamalinæ for
rostro-lateral compartments.
Fig. 2, always serving for lateral and carino-lateral compartments.
Fig. 3, Carina, serving in the Chthamalinæ, also, as a rostrum.]
Now, the compartments in the shell of every sessile Cirripede, are
without exception constructed on the above three simple patterns. In
number, they are 8, 6 or 4, or all confluent together.
Considering this simplicity in growth and form of the separated
compartments, it seems at first surprising that the construction
and enlargement of the whole shell in Balanus, should long have been
viewed as a difficulty. But the radii, from growing against rectangular
indentations, or rather furrows, in the opposed compartments, come to
be set a little inwards; and their external surfaces assume a very
different appearance from the wall-portions of the compartments,
which grow against the surface of attachment. In different species,
the summits of the radii (and of the alæ) grow either very much more
obliquely than in the species figured, or more squarely--that is,
they extend from tip to tip of the adjoining compartments, parallel
to the basis. In this latter case, and when the surfaces of the radii
differ considerably in appearance from the walls, as in _Balanus
tintinnabulum_ (Plate 1), I am not at all surprised that the radii
should have been described as separate elements, and called "areæ
interjectæ," or "compartments of the second order:" for the shell of
this Balanus seems to be composed of six wedges with their points
upwards, namely, the parietal portions of the compartments, and of
six other narrower wedges, the radii, with their points downwards;
and the fact that these latter wedges consist simply of the sides
of the parietal portions, modified by growing against the adjoining
compartments, is completely masked. I should add, that sometimes the
radii are not developed, which simply means that the overlapping
lateral edges of the compartments have not been added to during growth.
The alæ are originally developed at the period of the metamorphosis,
as slight lateral protuberances in the upper part of the compartments;
from being overlapped, and therefore not exposed to external
influences, and from growing (as in the case of the radii) against
rectangular indentations or furrows in the adjoining compartments, they
generally assume an extremely different appearance from the parietes,
and might naturally be thought to have a very different nature. But
the alæ in all cases (as is obvious in Pachylasma) are nothing but
the protuberant lateral edges of the compartments, rendered thin and
modified during growth. In order that the margins of the alæ should be
received in an indentation, the upper internal surfaces of the walls
of the recipient compartments are thickened all round, excepting where
they receive the alæ. This thickened, upper, internal portion of the
walls or shell, together with the alæ themselves, form the part called
the sheath. The sheath sometimes blends insensibly into the lower parts
of the compartments, and then perhaps it would not be thought to be a
distinct element; but often it is abruptly separated by an overhanging
edge (see Pl. 9, fig. 5 _b_, 9 _b_; Pl. 20, fig. 1 _b_; Pl. 25, fig. 1,
K′) from the lower part, and then the sheath greatly complicates the
internal appearance, but not the essential structure of the shell. The
sheath acts beautifully, like an internal hoop, in strengthening the
shell round the orifice, where it is naturally weaker than at the lower
or basal end, where it adheres to the surface of attachment: in the
upper part of the shell, moreover, the sutures between the compartments
do not go straight through, but owing to the alæ projecting and being
overlapped, are extremely oblique; or the joints, in the language of
carpenters, may be said to be broken.
There is one other point of structure in the shells of the Balanidæ,
more especially of species like _Balanus tintinnabulum_, which adds to
their apparent complexity, namely, that the rim or orifice of the shell
formed by the upper ends of the compartments, projects considerably
above the opercular valves. In a young Balanus, immediately after the
metamorphosis, the operculum is attached by the opercular membrane all
round to the summits of the compartments, and there cannot be said to
be any orifice to the shell itself, but only an orifice or slit between
the opercular valves; but during growth, as the compartments are added
to at their basal edges, their upper ends are deserted, and cease to
enclose the sack, within which lies the animal's body. Hence the upper
ends come to project freely, either quite separately as in some species
of Pollicipes, where they cannot be said to form an orifice; or more or
less united into a ring so as to form an orifice, as in the different
species of Balanidæ. It follows, that to understand the real shape of
a Balanus, or rather of the cavity enclosing the animal's body, all
that part of the shell which projects above the opercular membrane,
may, in imagination, be removed as something extraneous, like so many
spines; not that I mean to say that these points of shell are dead; on
the contrary, they are often porose and penetrated by numerous threads
of corium. This upper part of the shell, thus produced so as to form
an orifice, no doubt serves to protect the less strong and moveable
operculum.
[Illustration: Fig. 4. Octomeris.
Fig. 5. Chthamalus.
Fig. 6. Chamæsipho.
Fig. 7. Balanus.
Fig. 8. Tetraclita.
_a_, Rostrum; _b_, Rostro-lateral, _c_, Lateral, _d_, Carino-lateral
compartment; _e_, Carina.
Horizontal sections through the Shells of the principal genera of
Balanidæ, showing the arrangement of the Compartments. Genera 4, 5, and
6 belong to the Chthamalinæ; 7 and 8 to the Balaninæ.]
_Number and Arrangement of the Compartments._--I have already stated
that the shell, in every one of the Balanidæ, consists of eight, six,
or four compartments, or of all fused together into a single piece;
and that the compartments themselves are all constructed on the three
simple patterns of which woodcuts (figs. 1, 2, 3) have been given.
They are arranged in a certain definite order. The type arrangement is
found amongst the Chthamalinæ, as might have been expected, inasmuch
as this sub-family is so closely related to the ancient genera
Pollicipes and Scalpellum, whence all the Thoracic Cirripedia may be
said to radiate. In Octomeris (fig. 4) the type-arrangement of the
compartments, eight in number, is well shown; the rostrum and carina
resemble each other, and have alæ on both sides, and therefore are
overlapped on both sides: the rostro-lateral compartments have radii
on both sides, and therefore overlap the adjoining compartments on both
sides; the lateral and carino-lateral compartments have radii on their
carinal, and alæ on their rostral sides; and therefore overlap on one
side, and are overlapped on the other side. Now the shell of every
other sessile Cirripede differs, I believe, from that of Octomeris,
only in the fusion together or abortion of some of the eight normal
compartments: in one genus, however, Catophragmus, outer whorls of
small compartments, arranged like the lower valves in the capitulum
of Pollicipes, are superadded. The genus Chthamalus (fig. 5) differs
from Octomeris only in the carino-lateral compartments being aborted,
(as will presently be discussed), and hence has six compartments.
Chamæsipho (fig. 6) differs from Chthamalus only in the rostro-lateral
and lateral compartments being fused together; and hence has only four
compartments. In Balanus (fig. 7) and the whole sub-family of the
Balaninæ, the rostrum is compounded of the true rostrum, as seen in the
type Octomeris, and of the two rostro-lateral compartments; hence this
compounded rostrum has radii instead of alæ on both sides, and there
are only six compartments. Tetraclita (fig. 8) and Elminius differ
from Balanus only in having the carino-lateral compartments absent, and
probably aborted; hence there are only four compartments. Lastly, in
Pyrgoma, all the compartments are blended together into a single piece.
In Pollicipes, the old type-form of the whole order, and in Scalpellum,
we have four valves, (answering to the operculum), surrounding the
aperture leading into the sack, and the valves below are arranged in
successive whorls, with a strong tendency to alternation. For, the
rostrum alternates with, that is faces the interval between, the two
scuta; the carina alternates with the two terga; and the upper lateral
valves alternate with the scutum and tergum on each side. These four
valves, namely, the carina and rostrum, which resemble each other
in structure, and the pair of upper latera, which are larger than
the other lateral valves, together form the uppermost whorl, or that
beneath the scuta and terga. In the next whorl we have the rostro-
and carino-lateral valves, alternating with those above them; and
beneath them there are generally other valves, which decrease in size
and display the same tendency to alternation. The valves here just
specified, namely, the rostrum, carina, and three pairs of lateral
valves, in the Lepadidæ, are so much larger, and are so much more
commonly present, than the other valves of the capitulum, that to
them alone I affixed special names. Now if amongst sessile Cirripedes
we look to that genus, viz., Catophragmus, which comes in its whole
structure the nearest to Pollicipes, one of the Lepadidæ, we find (as
in fig. 4), firstly, a rostrum and carina resembling each other, and
a pair of lateral compartments, larger than the other lateral pairs;
these four valves alternating with the opercular valves: and, secondly,
we find, but forming part of the same whorl, a pair of rostro-lateral
and a pair of carino-lateral compartments, which, just as in
Pollicipes, are larger than the exterior and lower valves. These lower
little valves, I may remark, decrease in size in the successive whorls,
and tend to alternate in position, just as in Pollicipes. Observing
these several striking points of correspondence in the valves, (and
indeed in the whole structure), of Catophragmus and Pollicipes, one
is strongly inclined to suspect that in Catophragmus, and therefore
in Octomeris and other sessile Cirripedes, although the rostro- and
carino-lateral compartments appear to lie in the same whorl with the
rostrum, carina, and large lateral compartments, yet that they really
belong, as in Pollicipes and Scalpellum, to a lower whorl. Now if a
very young shell of Balanus, immediately after the metamorphosis, be
examined, the carino-lateral compartments will be found not to have
been developed; they first appear after two or three zones of growth
have been added to the other compartments; bearing in mind that in
Pollicipes and in Catophragmus the lower whorls are added successively
during growth, we find in this fact strong confirmation of the view
that the carino-lateral compartments normally belong to a whorl beneath
that including the rostrum, carina, and lateral compartments. Whether
the rostro-lateral, like the carino-lateral compartments, are developed
subsequently to the others, I have had no opportunity of ascertaining,
and therefore cannot confirm the above analogical conclusion, namely,
that they, also, belong to a lower whorl.
In the sub-family Balaninæ, which includes Balanus (woodcut 7), and
Tetraclita (woodcut 8), the shell is characterised by not having
rostro-lateral compartments, and by the rostrum having radii: now in
_Pachylasma giganteum_, which undoubtedly belongs to the sub-family
Chthamalinæ, at a very early age the rostro-lateral compartments become
blended with the true rostrum, making a compound rostrum, exactly like
the rostrum in the Balaninæ; distinct evidence of a similar fusion
is retained throughout life (Pl. 15, fig. 1) in all three species of
Chelonobia, which is undoubtedly a member of the Balaninæ. Hence, I
think, we may conclude that in all the genera of the Balaninæ the
rostro-lateral compartments are probably not aborted, but are blended
with a normal rostrum (resembling that in woodcuts 4, 5, 6), making
together a compound rostrum furnished with radii: it must, however, be
observed that I could not detect any actual evidence of this fusion
in Balanus, even immediately after the metamorphosis. In Chamæsipho
(woodcut 6), either the rostro-lateral compartments attain a most
unusual breadth, or, as is more probable, they have become confluent
with the lateral compartments, which in the Lepadidæ seem to be the
most persistent of all the lateral valves. In such genera as Tetraclita
and Chthamalus, in which the carino-lateral compartments are absent,
they may be fused with the lateral compartments or with the carina; but
seeing that they are normally developed later than the other valves,
it appears to be the simplest theory to assume, until the contrary
be proved, that they are aborted. Finally, the somewhat unexpected
conclusion that the shell (not including the operculum) of sessile
Cirripedes normally consists of eight valves, four belonging to an
upper whorl, and four to a lower whorl, all forced into a single ring,
and often more or less fused together, though not strictly proved,
is rendered highly probable. I will only further add, that the Basis
perhaps represents several whorls of the small valves or scales on the
peduncle of Pollicipes, fused together; the comparison of the basis
with the calcareous cup, forming the lowest portion of the peduncle in
Lithotrya, which has been made by some authors, I do not think is very
accurate, as the cup in Lithotrya seems to have a special relation to
the boring habits of that genus.
[Illustration: Fig. 9.
Basal edge of wall of compartment in _Balanus tintinnabulum_; _a_, _a_,
outer lamina; _b_, _b_, inner lamina; _c_, _c_, longitudinal septa
uniting the inner and outer laminæ with their ends denticulated.]
_Structure of the Individual Compartments._
If the basal margin of a compartment, for instance, of _Balanus
tintinnabulum_, be examined, it appears sufficiently complicated,
being composed of an outer and inner lamina, separated by longitudinal
septa, which are denticulated at their bases; and the tubes formed
by these longitudinal septa are crossed by transverse septa. On the
other hand, in some cases, as in the genera Chthamalus and Elminius,
each compartment consists of a simple shelly layer. These two extreme
states graduate into each other: we have, firstly, on the internal
surface, quite irregular points and ridges; these become regular,
causing the internal surface to be longitudinally ribbed; then these
ribs themselves become finely furrowed on their sides and at their
lower ends, producing sharp, minute ridges, the ends of which I have
called the denticuli; and, lastly, some of the denticuli on the
adjoining longitudinal septa become united into a solid layer, forming
the internal lamina of the wall. These denticuli do not generally
cover the whole surface of the longitudinal ribs, but leave a portion
near the outer lamina of the compartment smooth. The denticulated ends
of the longitudinal septa project beyond the basal edge of both the
outer and inner laminæ, and enter the mouths of the tubes (where such
occur) in the basis, and thus strengthen the shell. The whole of the
internal lamina generally is more or less striated longitudinally,
thus displaying its origin from the union of the inner edges of the
longitudinal septa. I need only further remark that on the internal
surface of the outer lamina, between the main longitudinal septa, there
are generally (as in the woodcut) smaller longitudinal ridges, which do
not reach the inner lamina, and on this account alone are not called
septa.
Tubes are formed by the longitudinal septa, between the outer and
inner laminæ. These tubes are almost square, and are occupied by
threads of corium, which enter at pores left open between the edge of
the compartment and that of the basis on which it rests. The tubes
extend high up the compartments; but in the uppermost part they are
generally cut off by thin, transverse, calcareous septa, deposited by
the ends of the threads of corium; a cancellated structure being thus
produced. Or the uppermost part of each tube becomes filled up solidly
with compact shelly layers, which are always first thrown down on the
side of the tube facing the outside, and thus greatly strengthen the
shell: in several instances, as in _Balanus perforatus_ and _Tetraclita
porosa_, in which the disintegration of the upper part of the shell is
a necessary element in its growth for the enlargement of the orifice,
these filled up tubes become exposed. In Coronula and Tubicinella, the
tubes in their upper parts are, I believe, crossed only by transverse
membranous septa.
_Anomalies and exceptions._--In Tetraclita (Pl. 10, fig. 1 _g_, 1 _h_)
from the branching of the longitudinal septa, several irregular rows
of tubes are formed. In certain varieties of _Balanus balanoides_
(Pl. 7, fig. 2 _b_), and in _B. cariosus_ (Pl. 7, fig. 3 _b_), slight
branching ridges on the internal surface of the walls, seem to
answer to the longitudinal septa, and produce, during the downward
growth of the shell, extremely irregular cells, and short tubes. In
_Balanus vinaceus_ (Pl. 2, fig. 7 _d_), the internal lamina, instead
of being solid, as in every other species, is left cancellated, and
thus betrays, much more plainly than usual, its origin in the united
denticuli of the adjoining longitudinal septa. In _Balanus porcatus_,
between the main longitudinal septa, there are (Pl. 6, fig. 4 _e_)
what may be called rudimentary and disconnected longitudinal septa. In
Coronula and its allies (Pl. 16, fig. 6, and Pl. 17, fig. 4 _c_) it
is the outer lamina of the compartment which is anomalous; for in the
two or three lower zones of growth, it forms only a ledge on each side
of the longitudinal septa; which ledges, higher up, become confluent,
and so form an ordinary outer lamina. In Coronula, also, the wall of
each compartment (see transverse section, Pl. 16, figs. 5, 7) is very
remarkable from being deeply folded, the folds being on their internal
faces firmly calcified together, and on their external faces closely
pressed together (often with a neatly serrated suture), so that the
whole nature of the shell might be, as has happened, easily quite
misunderstood; and the walls be considered as very thick, instead of
being, as is really the case, very thin. In Chelonobia (Pl. 15, fig.
1), however, the walls are truly of such great thickness, that the
nature of the relative parts might likewise be misunderstood; in this
genus the ovarian tubes enter the walls, extending up between the
longitudinal septa, or, as they may here be more naturally called,
the radiating septa. I will specify a few more peculiarities worthy
of remark:--in some species of the sub-genus Acasta, clefts are left,
covered only by membrane, on the lines of suture (Pl. 9, figs. 7 _a_,
8 _a_), between the compartments, just above the basis; and in other
species the basis is perforated by numerous membrane-covered, minute
orifices. In Platylepas, each compartment has one deep inward fold (Pl.
17, fig. 1), somewhat analogous to the three folds in Coronula; this
fold is produced into an internal midrib, supporting and rendering
convex the membranous basis; in this genus, also, the rostrum, owing
to its midrib, is generally thrust a little on one side, and the shell
thus rendered asymmetrical. In _Chamæsipho scutelliformis_ the shell
is symmetrically perforated (Pl. 19, fig. 4 _a_) by four apertures.
Lastly, in _Chthamalus Hembeli_ and _intertextus_, after a certain age,
the basal edges of the walls become inflected, and continue to grow
inwards till they entirely take the place of the true membranous basis.
_Structure of the Radii._
_Radius._--This term, as we have seen, is applied to that side of the
compartment, the growth of which is modified, by abutting against and
overlapping the adjoining compartment. Hence the structure of the
radius is essentially the same with that of the parietal portion of the
compartment. When best developed, as in _Balanus tintinnabulum_, the
radius consists of an outer and inner lamina, separated by denticulated
septa, extending in horizontal lines parallel to the basis, and is
consequently perforated by minute tubes or pores. The tubes become
filled up solidly much more commonly than do the parietal tubes; and
the inner lamina, in such cases, is hardly distinct from the outer
lamina. The denticuli often fail, or are present only on the lower
sides of the septa; and very frequently the edge of the radius can only
be said to be crenated. Notwithstanding these frequent anomalies, if a
series of species and genera be taken, it is certain that there is, as
might have been expected, a close relationship in internal structure,
between the radii and the parietes. The edge of the radius is received
in a slight furrow (generally marked like a seal, with the impression
of the denticulated septa) in the opposed compartment: sometimes the
outer edge or lid of the recipient furrow, is so broad as to give the
false appearance of a radius having been developed, at least in the
lower part of the shell, on both sides of the suture. A crest of corium
runs into each suture between the edge of the radius and the furrow in
the opposed compartment; and when the radius is permeated by pores (as
in woodcut 10), threads of corium branch off this crest, and enter the
pores. In the lower part of the shell, these crests of corium project
from the corium forming and surrounding the sack; but in the upper
part of the shell, above the opercular membrane, and therefore above
the sack, the corium is produced up each line of suture as a separate
ribbon. In proportion as these ribbons extend more or less near to the
summit of the shell, so do the edges of the radii continue to be added
to, to a greater or less height from the basis; and consequently their
summits become less or more oblique.
[Illustration: Fig. 10.
Edge of the radius of _Balanus tintinnabulum_. _a_, outer lamina; _b_,
inner lamina; _c_, denticulated septa, uniting the two laminæ.]
_Peculiarities in the Structure of the Radii._--In some of the species
of Tetraclita, in which genus the walls consist of several rows of
tubes, the radii are likewise perforated by several rows; and in some
of the other species (Pl. 10, fig. 1 _h_), the edge, or disarticulated
surface of the radius, is marked by irregularly branching ridges; and
these evidently correspond with the branching septa or ridges of the
wall. In Chelonobia, the outer lamina of the radius, as well as of
its recipient furrow, is of extraordinary thickness; and this lamina,
in _C. testudinaria_ (Pl. 14, fig. 1 _a_, 5, _b_, and Pl. 15, fig.
1, _f_), is modelled into sharp transverse ridges and valleys. In
the Chthamalinæ, the radii, like the parietes, are simply solid; and
apparently in consequence, for the sake of strengthening the sutures,
the edges of the radii, and of the recipient furrow in Octomeris
(Pl. 20, fig. 3 _a_) and in _Chthamalus dentatus_ and _Hembeli_ (Pl.
18, fig. 3 _b_, 5 _a_), are neatly dentated. In some other species
of Chthamalus (Pl. 19, fig. 1 _a_), the radii present a slight
modification of this structure, the sutures being formed by oblique
interfolding laminæ. In the radii of Coronula and Tubicinella, there
is a peculiarity, in apparent connection with the fact, that in these
genera the parietal tubes are not crossed by transverse calcareous
septa, namely, that the pores by which the radii are permeated keep
unclosed throughout their length, and open into a special longitudinal
tube (Pl. 16, fig. 7, _d′_), which runs along that margin of the wall,
whence the radius arises. In Coronula the wall is of extreme thinness,
and in conformity so is the true radius, but that the shell might not
thus be rendered very weak, complementary or pseudo-radii are developed
on their inner sides (Pl. 16, fig. 7, adjoining the true radii A _d_,
C _d_, and shaded by distant convex lines). Even in the allied genus
Xenobalanus, in which the whole shell tends to become rudimentary,
traces of these pseudo-radii (Pl. 17, fig. 4 _b_, _d_) can be detected.
In Coronula, though the radii (Pl. 16, fig. 7, A _d_, C _d_) are, by
the above special means, rendered thick, and though the alæ also are
thick (C _a′_, D _a′_), yet together they do not equal in thickness the
folded walls; and consequently, there is left between the radii and alæ
square chambers (_v_), occupied by the branching ovarian tubes.
_Structure of the Alæ._
These project, generally abruptly, from the sides of the upper part of
the compartments; they appear from the first growth of the shell; they
are overlapped by the radius and by part of the wall of the adjoining
compartment; they are thinner, and have, owing apparently to being
overlapped, a very different aspect from the parietal portion; but
they do not differ from it in essential nature. They are solid, that
is, they are never permeated by pores; but their edges are generally
crenated, and there is, in some cases, as in Chelonobia, sufficient
evidence that these crenations answer to the horizontal septa on
the edges of the radii (also often reduced to mere crenations), and
consequently, likewise, to the longitudinal septa of the parietes.
In Coronula the edge of each ala consists of a medial ridge, sending
off denticulated septa on _both_ sides, and is therefore anomalous as
compared with the alæ in other genera, but corresponds in structure
with the similarly anomalous radius of Coronula. In order to allow of
the growth of the edge of the ala, a fine thread of corium runs up the
narrow furrow in which the edge is lodged, proceeding from the corium
of the sack. In proportion as this thread runs up higher or lower,
so are the summits of the alæ rendered, during growth, less or more
oblique.
_Structure of the Sheath._
As the compartments overlap each other, the edges of the alæ would have
projected, and the inner surface of the orifice of the shell would not
have been smooth and rounded, had not that part of each wall, which
does not overlie an ala, been thickened so as to allow of the formation
of a shoulder or indentation, against which the edge of the ala fits
and abuts. The thickened portions, and the alæ themselves, together
form the sheath, of which the use seems to be to strengthen, like a
broad internal hoop, the upper part of the shell round the orifice,
where naturally it is weak. The sheath is composed of successive, fine,
shelly layers, which extend, as the shell is added to at the basal
margin, lower and lower down on the inner surface of the walls. The
lower edge of the sheath either simply projects a little inwards, or
more commonly is formed into a sharp depending ridge, as represented
in fig. 1, K′, Pl. 25. In some species of Pyrgoma (Pl. 13, fig. 2
_b_), the sheath reaches nearly to the bases of the compartments;
and in Chelonobia (Pl. 14, fig. 4 _e_ _c_ _e_), the inner layer of
shell surrounding the sack, which seems to correspond more nearly
to the sheath than to the inner lamina of the walls, actually rests
on the basal membrane. The opercular membrane is generally, but not
invariably, attached only a little way above the lower edge of the
sheath: at each exuviation, a new opercular membrane is formed, and is
attached to the next lower zone of the sheath; the old membrane being
cast off, but a circular slip of it is left, investing the last zone.
Hence the whole upper part of the sheath above the opercular membrane,
comes to be thus invested; and is marked by circular lines, one above
the other, caused by the successive exuviations. This investing
membrane often supports rows of minute bristles, directed upwards.
Generally, a film of shell is deposited, at the period of the formation
of each new opercular membrane, on that part of the sheath which lies
immediately beneath. This innermost film or thin layer of shell, on
the lines of suture between the compartments, breaks joint, at least
in some cases, with the underlying shelly layers,--that is, the suture
in this last-formed film does not lie exactly over the suture in the
subjacent layers of the sheath. In Tubicinella, the sheath extends down
close to the basis; and what is unique in this one genus, the opercular
membrane, gradually thinning out downwards, closely adheres to the
whole inner surface of the shell. In Tubicinella and in Xenobalanus
(Pl. 17, fig. 4 _b_), the sheath separates easily into separate
successive rims of shell; and this structure evidently is for the sake
of facilitating the breakage of the upper end of the shell, which, as
we shall presently see, is necessary to allow of the increase in size
of its orifice.
_Structure of the Basis._
This, in several genera and species, is composed of simple membrane,
and consists of successive, concentric, circular slips, added round the
outside, at each period of growth. In some species of Tetraclita and
Balanus the basis is calcareous, but diaphanous, very thin, smooth, or
somewhat granulated. In other cases it consists of a single calcareous
lamina, either smooth, or with ridges radiating from its centre; it
is formed of two laminæ, (as is most usual in Balanus,) separated by
radiating septa. These septa, as well as the radiating ridges in the
case of the single lamina, are homologous with the longitudinal septa
of the parietes. The denticulated ends of the latter enter the mouths
of the tubes formed by the radiating septa of the basis: threads of
corium pass between the denticuli of the parietal septa, and thus
enter the basal tubes. The ends of these threads of corium generally
deposit transverse calcareous septa, exactly as within the parietal
tubes. When the basis is thick the septa themselves (_ccc_) between
the proper basal tubes, become porose, (or rather cancellated,) and
they sometimes expand into a very thick, cancellated layer, separating
the outer lamina (_a_) of the basis from the proper basal tubes, which
always lie close under the inner lamina (_b_). This structure differs
only slightly from that seen in the parietes of Tetraclita, in which
the branching of the longitudinal parietal septa, produces thick walls,
formed of several rows of tubes or pores. With respect to peculiarities
in structure of the basis, _Balanus lævis_ offers the most remarkable
case; for here, in specimens which have grown crowded together, the
whole interior appears sometimes to have become too much elongated or
too deep for the animal's body, and consequently the lower part of
the deeply-concave basis has been filled up (Pl. 4, fig. 2 _a_) by
thin, irregular, calcareous diaphragms. In elongated specimens, also,
of _Balanus balanoides_, the shell sometimes appears to have grown
too long for the animal's body; but in this case the membranous basis
becomes extremely convex inwards; it still reaches the basal edges of
the parietes all round, but in the middle it is raised high above the
surface of attachment; yet sometimes threads of the cementing tissue
depend from the middle part to the surface of attachment. In _Balanus
terebratus_ (Pl. 8, fig. 2 _a_, 2 _b_), and in some species of Acasta,
the basis is riddled, as previously stated, by numerous, minute,
membrane-covered orifices. In _B. declivis_ the membranous basis is
always extremely oblique, owing to the rostral end of the shell being
twice as high as the carinal and opposite end.
[Illustration: Fig. 11.
Portion of edge of basis of _Balanus tintinnabulum_, _a_, _a_, outer
lamina; _b_, _b_, inner or upper lamina; _c_, _c_, _c_, porose or
cancellated radiating septa.]
Regarding the very remarkable means by which the basis of sessile
Cirripedes is cemented to the surface of attachment, it will be
convenient to defer for a little the description, on account of its
necessary length.
_Structure of the Opercular Valves (Scuta and Terga)._
These are situated on each side of the slit or orifice leading into the
sack; from their shape, their powers of movement, their separation by
flexible membrane from the shell, to which they serve as a lid, they
appear at first as if they constituted an element very distinct from
the shell itself, but this is not the case. They are, together with the
opercular membrane, as essentially as the whole of what is externally
visible, a part of the modified carapace, of which they occupy the
upper or posterior extremity: from tracing the metamorphoses, or
even by comparison of a Balanus with Pollicipes, there can be no
doubt of the truth of this conclusion. The opercular valves are four
in number,--a pair of scuta and a pair of terga; but the latter in
_Coronula diadema_ and _reginæ_, are either aborted or represented by
a mere rudiment; and in Xenobalanus both scuta and terga are quite
absent. In several cases, more especially in the genus Pyrgoma (Pl.
13, fig. 1 _b_), the scutum and tergum on each side are calcified
together, so that sometimes not even a trace of the line of junction
can be discovered. In most cases the scutum is firmly united, being
articulated in a manner presently to be described, to the tergum; but
in Coronula, Tubicinella (Pl. 17, fig. 3 _c_), and Platylepas, the ends
of these valves are simply approximated.
_Scuta._--These valves are important, inasmuch as the animal's body
is attached to them; in Pl. 25, fig. 1, the broken line, surrounding
_a_, _b_, shows where the body has been cut, in removing the scutum
on the near side, the other scutum, S, being left articulated to the
tergum, T. In shape the scuta are generally sub-triangular; but in some
species of Pyrgoma and in Chelonobia, &c. they are much elongated. The
lines of growth are usually prominent; and along the occludent margins
the alternate, or sometimes every third or fourth line, is developed
into a knob, which produces a serrated edge, serving to lock the two
opposed valves together; there is, however, no trace of this structure
in Coronula and Tubicinella. In some species of Pyrgoma, a ledge of
considerable breadth (Pl. 13, fig. 3 _e_, &c.) is developed along
the occludent margins of the two scuta, as well as of the two terga,
giving them an anomalous structure. The _Terga_ differ considerably
in outline in the different genera and species: their shape approaches
more nearly to a triangle than to any other regular form; but there is
generally a projection or spur on the basal margin, on the side towards
the scutum. In some species of Pyrgoma, the tergum is of so irregular a
shape as to defy description. In most cases, a longitudinal depression
or furrow runs down the valve, from the apex to the extremity of the
spur; and it not rarely happens that the sides of this furrow become
folded inwards and almost closed. The spur probably answers to the
basal point of the usually sub-rhomboidal tergum in Pollicipes and
Scalpellum.[22] The tips of the terga in some species of Balanus, &c.,
are specially modified into sharp points or beaks (Pl. 2, fig. 3 _b_,
3 _d_), bowed a little inwards, and projecting considerably above the
tips of the scuta; this is effected by the medial, uppermost part
of the valve being internally thickened and hardened, and then, by
the disintegration of the two margins and the external surface, the
internal modified portion becomes exposed. The whole valve, also, at
least in such cases as in _Balanus psittacus_, appears to be forced
slowly upwards in the articular furrow of the scutum. I am assured, by
a competent observer, that the beaks of the terga in _B. porcatus_ can
give an object placed within the orifice of the shell a sharp tap.
[22] In comparing the Tergum of one of the Balanidæ with that
of a typical member of the Lepadidæ, for instance, that of
Balanus with that of Pollicipes, apex corresponds with apex:
the extremity of the spur in Balanus corresponds with the basal
point of the whole valve in Pollicipes: the scutal margin, (which
in Balanus homologically extends down to the extremity of the
spur), corresponds with the scutal margin of Pollicipes: the
carinal margin in Balanus corresponds with the _upper_ carinal
margin in Pollicipes: the basal margin of Balanus on the carinal
side of the spur, corresponds with the _lower_ carinal margin in
Pollicipes: lastly, (and this is the chief difference), in Balanus
there is no appreciable occludent margin, the apex of the valve
being brought close to the upper angle of the scutal margin; in
Chthamalus, however, there is yet left some remnant of an occludent
margin,--which margin in Pollicipes is conspicuous.
The scutum and tergum, with the few exceptions above stated, are
articulated together at a large or open angle. The articulation (see
Pl. 11, fig. 5 _b_, _c_, _d_, and fig. 6 _b_, _c_) is effected by the
margin of the tergum being a little inflected, and lodged in a furrow
in the margin of the scutum. This furrow in the scutum has its further
border generally prominent and often reflexed or curved over; I have
called it the articular ridge; it, also, is lodged in a furrow in the
upper part of the tergum, which again is bordered by a ridge, viz., the
articular tergal ridge. So that in both scutum and tergum there is an
articular furrow, bordered in each case, on one side by the margin of
the valve, and on the other side by the so-called articular ridge. In
Chelonobia (Pl. 14, fig. 1 _b_) the articular ridge of the scutum is
horny. When, as often happens, the scuta and terga have been much worn,
the manner of their articulation (Pl. 18, fig. 1 _a_) is pretty well
shown even from the outside; in this case their external appearance
is very different from what it is in those individuals (fig. 1 _c_)
of the same species, which have not suffered disintegration. This
articulation of the scuta and terga is prefigured amongst the Lepadidæ,
in _Pollicipes mitella_, and in Lithotrya.
The scuta are brought together by a short, strong, straight, adductor
muscle (Pl. 25, fig. 1, _a_); its attachment leaves (with very few
exceptions, as in Tubicinella) a rounded impression, or even pit, on
the under side of the valve in its upper part. This pit is frequently
bounded, on its lower side, by a sharp ridge, which, though not in
actual connexion with the adductor muscle, I have, for convenience
sake, called the adductor ridge; it serves apparently to give support
to the animal's body; in some few cases (as in _B. psittacus_, Pl. 2,
fig. 3 _c_) it is confluent at its upper end with the articular ridge,
and converts the whole basi-tergal corner of the valve into a deep
cavity. In some of the species of Pyrgoma (Pl. 12, fig. 5 _c_, 7 _b_),
and in some varieties of Creusia, this adductor ridge is enormously
developed, so as to depend far beneath the true basal margin, or
that to which the opercular membrane is attached. At the basi-tergal
corner of the valve, there is generally a small pit or impression, and
sometimes distinct crests, for the attachment of the lateral depressor
muscle. At the rostral end there is, also, a small cavity formed by the
overfolding of the occludent margin (rarely furnished with crests) for
the attachment of the rostral depressor muscle. In the Terga, at the
basi-carinal corner, there are usually crests, though sometimes feebly
developed, for the attachment of the tergal depressor muscle. But in
Chelonobia, Coronula, Tubicinella, Platylepas, and in some other cases,
there are no crests. The crests, when well developed, are furnished
with rectangular sub-crests or denticuli on both sides; in fact they
resemble, and are probably homologous with, the denticulated ribs or
septa in the parietes, radii, and basis. Altogether the scuta and terga
are attached, as far as muscles are concerned, to the shell and sack,
by three longitudinal pairs.
_Growth of the Whole Shell, and Its Microscopical Structure._
The opercular valves are added to along their basal margins alone;[23]
the animal's body, together with the several muscles, becoming attached
at each period of growth lower and lower down to the valves; this
no doubt is effected by the absorption of the upper surfaces of the
muscles, and the formation of new fasciæ on their lower surfaces. The
opercular membrane, which, though thin and flexible, forms part of
the general outer surface of the animal as much as does any portion
of the rigid shell, with which indeed it is strictly homologous, is
periodically moulted, together with the integuments of the whole
included animal. The new opercular membrane is of course each time
formed a little larger than the old one. In Coronula and Tubicinella,
however, several successive opercular membranes are preserved one
over the other, and the outside membrane gradually disintegrates; in
these cases the undermost opercular membrane is formed wrinkled and
considerably too large, so as to allow of being stretched, before it
is finally cast off. In Tubicinella, the opercular membrane runs down,
adhering to the inner surface of the shell, to nearly the basis, and
hence during the diametric growth of the shell, it is longitudinally
split, and is repaired by slips of new membrane, which resemble the
radii in form and in direction of the lines of growth.
[23] In some species of Pyrgoma, the ledge (_limbus occludens_)
which is added along the occludent margin of both scuta and terga,
and in some species of Balanus a narrow rim, or slight protuberance
which is added along the carinal margin of the terga, offer
unimportant exceptions to the rule, that the opercular valves grow
only at their basal margins.
The basis is added to only round the circumference, and hence
increases in diameter, and, when concave, in depth. The compartments
grow at their basal margins, where they are in contact with the basis;
hence the shell is added to in height, and, owing to the outward
inclination of the compartments, also, in basal diameter; but the
compartments likewise, in most cases, grow along both lateral margins,
that is, on the edges of the radii and alæ; and hence the upper part
of the shell, also, increases in diameter. The orifice of the shell,
moreover, thus becomes enlarged. In some cases the shell is destitute
of radii, only sutures being present, that is, the compartments do not
grow laterally; and sometimes, as in the whole genus Pyrgoma, there
are not even sutures, the compartments having been fused together:
in both these cases, the shell can increase in diameter only at the
base; and the orifice, it might have been thought, would necessarily
have remained, to the destruction of the animal, of the same minute
size, as when first formed after the metamorphosis: this certainly
would have been the case had not the upper ends of the compartments,
surrounding and forming the orifice, been nicely adapted always to
yield, in a certain limited degree, to the disintegrating influences
to which every shell is exposed, but which most Cirripedes can resist;
and the disintegration of the narrow end of a conical tube, of course
increases the diameter of its orifice. In Tubicinella, in which the
shell is furnished with narrow radii, and does increase in diameter
from top to bottom, the increase is not sufficient in proportion to
the continued elongation of the shell; to compensate for this, the
orifice is enlarged at short intervals by the breakage of the upper
end of the shell, for which purpose (as explained under the genus) it
is evidently constructed. Hence we see that, in certain Cirripedes,
decay or disintegration, and breakage, are necessary elements in their
growth! It is a remarkable fact, which I cannot explain, that in
some species in which the orifice of the shell is usually increased
by disintegration, if individuals are so situated that they are not
exposed to sufficiently energetic disintegrating influences, as may be
inferred from the well-preserved condition of the whole surface of the
shell, then the radii become developed, and the orifice is increased in
size by the diametric growth of the upper part of the shell: I have
seen instances of this in _Tetraclita porosa_, and _purpurascens_, and
in _Balanus perforatus_: it appeared, but of course erroneously, as if
the lateral growth of the compartments had been subjected to the will
of the animal.
Considering the strength of the shell of sessile Cirripedes, the
separation of their compartments one from another and from the basis,
during growth, has justly been thought a surprising circumstance. In
most Chthamalinæ and in some species of Balanus, however, if the shell
be boiled in caustic potash, the compartments fall apart with a touch;
this shows that their union is due to animal and probably to organised
matter, and the growth of such matter between the opposed edges of
the compartments, and their consequent gradual separation, offers
no particular difficulty. But in many Balani, boiling in potash for
hours does not seem even to weaken, in the least degree, the sutures,
which are wonderfully strong--the shell often breaking rather than
yield on these lines; if, however, the shell be dissolved in acid,
the animalised tissue which is left easily separates on the lines of
suture, and if this tissue be boiled in potash, the remnants of the
compartments fall quite separate. These facts seem to me to show, that
the compartments in such cases are joined along the lines of suture by
tissue, which must be in a calcified state, but which, nevertheless,
continues to grow by intersusception; in other words, I believe that
the tips of the complicated ridges and points interlocking on the lines
of suture, are not separated from each other by films of corium or
simple animal matter, but are actually united by corium in a calcified,
yet still growing condition.
In ordinary Crustaceans, the growth is periodical and sudden; a new
and larger carapace, for instance, is formed under the old one, and
after the exuviation of the latter, the new one soon hardens, and does
not subsequently increase in size; so it is in the case of Cirripedes,
with the membranes of the body, and even with certain parts, as
the opercular membrane, of the external covering. But a Cirripede
cannot, like a crab, crawl into some crevice and remain protected
till its shell becomes hardened; hence, probably, it is that the
shell is never[24] wholly moulted. Even if the margins of the opposed
compartments and of the basis were to grow rapidly, the shell would
necessarily be much weakened on the lines of suture, and unable to
withstand the heavy breakers, to which so many species of sessile
Cirripedes are exposed. On the other hand, although the margins are
thus compelled to grow slowly, they do not grow continuously, as may
be seen in the zones of increment on all the valves, corresponding, I
believe, with the periods of exuviation of the membranes of the body. A
layer of shell, often very thin, seems to be generally deposited over
the whole internal surface of the several valves, at the same time that
the marginal zones are added; so that the only essential difference
in the growth of the external covering, in Cirripedes as compared
with ordinary Crustaceans, is that the old shell is not cast off, but
adheres to the outside of the new shell, and that the margins are
added to (in certain definite directions) slowly yet not continuously,
instead of the whole being formed at a single period.
[24] In the genus Alcippe, and in Cryptophialus, the whole of
the external membranes are moulted, excepting the surface of
attachment; but then these Cirripedes live in cavities which
they form for themselves, and are thus protected. In Lithotrya
the membrane of the peduncle, with its little valves or scales,
is moulted, but here, again, this very part is protected by the
tubular cavity, which the animal forms and inhabits. Neither of
these three genera belong to the Balanidæ, or sessile Cirripedes,
which we are now more especially describing.
If, now, a section of one of the shelly zones of growth be carefully
examined, it can in some cases be distinctly seen to be formed of
successive, excessively fine laminæ; but the animalised tissue (which
differs much in amount in different Cirripedes) left after the shell
has been dissolved in acid, exhibits, in most cases, neither laminæ nor
any other structure whatever. The shell seems to be the actual pulpy
corium, or true skin, in a calcified condition, but generally with its
cellular structure modified and much reduced: I have taken a bit of
recently-formed shell of Tetraclita and of Coronula, with the corium
still adherent to its under surface, and after dissolution in acid, I
could not distinguish the part, which had just before existed as shell,
from the corium itself. In the case of Coronula, immediately prior
to the period of moulting and growth, I found the unaltered corium so
charged, as to effervesce, with carbonate of lime, either in a state of
dissolution, or in granules too minute to be visible under the highest
powers.
The sutures between the several compartments and the basis are covered
by thin membrane, which is continually splitting during the growth
of the opposed edges of the underlying shell; but previously to each
splitting, a new slip of membrane is, I believe, already formed under
the old one; so that the corium is not even momentarily exposed. Owing
to this manner of growth, the slips of membrane consist of successive
rims united together; in most cases, these soon become abraded from the
older parts of the shell, but are sometimes preserved. The last-formed
slip of membrane over a suture is homologous with the opercular
membrane; and both are strictly analogous with the ring of flexible
membrane, forming the joint of the leg of a crab. In the latter case,
the flexible membrane and hardened crust are both moulted together:
in the opercular membrane, there is a double line of splitting, one
close round the opercular valves, and the other at the basal edge of
the sheath, and the intermediate portion is moulted, but with a zone of
membrane left adherent to the non-moulted valves and sheath: lastly,
in the slips of membrane covering the sutures, there is only a single
line of splitting, and no portion, I believe, is moulted; the rims of
membrane on each side remaining adherent on the compartments and basis,
until worn away.
The opercular membrane, when closely examined, exhibits no structure,
except that it can sometimes be plainly seen to be composed of
successive, numerous, excessively thin laminæ. Occasionally, however,
it presents the false appearance of being permeated by parallel and
anastomosing vessels: this appearance is clue to one or more of the
component laminæ having been wrinkled before a succeeding lamina was
thrown down and attached to its under side. If a small piece of an
opercular valve of Tubicinella, with the opercular membrane adhering
to it, and with the corium under both, be dissolved in acid, it may
be clearly made out that the corium under the valve has gone on
being converted into shell, whereas under the opercular membrane it
has been converted and condensed into fine constituent laminæ of
chitine. Inasmuch as the successive layers of shell, during each period
of growth, go on encroaching on those of the membrane, the line of
junction between the shell and chitine becomes oblique or bevelled. The
membrane on this bevelled line of junction assumes a slightly different
aspect to what it has elsewhere; it becomes yellowish or brown, thicker
and very much tougher. In many genera it is also furnished with a row
of small bristles. At the period of exuviation the opercular membrane
separates just outside this modified portion, leaving the latter
adherent, as a rim or slip, on the valves. If, however, the opercular
membrane be rudely torn off before its proper period of exuviation,
it carries with it the as yet continuous, but already modified, slip.
A slightly indented line may sometimes be traced before the period of
exuviation, showing where the separation will take place: what produces
this line I know not. The coloured, thickened, and modified slips of
opercular membrane, which are thus retained adhering to the valves,
and which together form an investing membrane, have been considered
by most authors as the epidermis; but they have no more right to be
thus called than has any other part of the opercular membrane. Exactly
similar slips of membrane are left investing the sheath. So, again, the
membrane which, when well preserved, invests the walls of the shell,
is made up, as already stated, of successively adherent slips, which
originally covered the lines of suture.[25]
[25] In the case of Coronula there is a peculiarity, described in
the last section of this Introduction, (under the head of Cementing
Apparatus), namely, that the two or three last-formed, exterior
zones of the Basal membrane continue for a period to increase in
width; being, as I believe, dragged one from over the other, with
fresh laminæ of membrane continually thrown down. In this same
genus, and in Tubicinella, the walls of the shell are invested
by membrane, which is doubled inwards under their basal edges;
and as the latter grow, the investing parietal membrane splits
and separates from the basal membrane, and is pulled outwards and
downwards. This inflected, often broad border of membrane, seems to
me more strictly comparable with the opercular membrane, than with
those narrow, thickened rims of yellowish membrane which in other
Cirripedes cover the suture between the basal edges of the walls
and the basis.
The little bristles above alluded to, which arise from the slips of
membrane left adherent on the opercular valves, sheath, and walls,
stand in rows; a row corresponding to each period of exuviation of
the opercular membrane. The bristles are generally largest on the
opercular valves and sheath; in _Balanus tintinnabulum_, they are
from 1 to 2/1000ths of an inch in length, but they are longer in some
other species. I may here mention, as showing the connexion of these
bristles with the opercular membrane, that similar bristles occur in
_B. perforatus_, scattered over the surface of that membrane, and
are necessarily moulted with it. In the imbedded genera Coronula and
Tubicinella, none of these bristles exist. When a portion of valve or
shell, furnished with bristles, is dissolved in acid, tough, sinuous,
and apparently hollow, threads are seen to run from their bulb-like
bases, into and up the corresponding layer, which, before dissolution,
existed as shell; and they terminate internally in very fine points,
which I believe are united to the underlying corium. These threads, or
_tubuli_,[26] as I have called them in my volume on the Lepadidæ, are,
in _Tetraclita porosa_, about 1/5000ths of an inch in diameter, but
only half that size in _B. tintinnabulum_. On parts of the shell where
there are no bristles, similar tubuli penetrate the shelly layers, and
come to the surface. The tubuli running to the lowest and last-formed
row of bristles, just after a period of exuviation, are so delicate as
hardly, or not at all, to be distinguished; in the row above, they are
plain and longer, and for the next two or three upper rows they are, in
some cases, as in _Tetraclita porosa_, longer and longer, having been
added to during each successive thickening of the valve. These tubuli
consist of chitine, and no doubt first existed as threads of corium;
they are so tough that they must serve to strengthen the successive
layers of shell, but I imagine their chief function is to keep up the
vitality of the newly-formed layers of shell. May we not, also, venture
to suppose that by their means, some degree of sensibility is given
to the bristles? I need only further remark, that in some species of
Balanus and of Chthamalus, the under side of the shell is penetrated
by irregular pores, large enough to be visible to the naked eye, into
which threads of corium penetrate; but these can hardly be said to
appertain to the microscopical structure; and are more nearly related
to those pores and furrows, formed by the greater or less development
of the longitudinal septa, and in which the threads of corium deposit,
or rather become changed into, transverse septa, or solid shelly
matter, as previously described.
[26] I regret that I have used this term "tubuli"; for the threads
thus designated, I believe, are not the same with the _tubuli_ of
Dr. Carpenter, which are not left after dissolution in acid. I have
seen tubuli, as called by me, in the shell from the leg of a crab,
after having been placed in acid.
_Muscles of Sack._
In the pupa, the thorax, as we shall hereafter more fully see, is
continuous with, and opens into the large anterior end or front part of
the head; but during the metamorphosis (Pl. 30, fig. 2), the thorax of
the Cirripede becomes, owing to the almost transverse position occupied
by the young animal within the pupa, to a great extent internally
separated from the anterior end,--which anterior end forms, as we know,
either the peduncle or the basis. Hence it comes to pass that the
body or Thorax (Pl. 25, fig. 1) is lodged within a sack (_f_) within
the shell. The chitine membrane lining this sack is excessively thin
and transparent, but less so in Xenobalanus and Tubicinella; it is
obviously continuous with that investing the body of the animal; it is
also essentially continuous with the opercular valves and membrane,
and consequently with the whole shell. It is periodically moulted. It
is lined by corium, as is likewise the surrounding shell; hence the
corium is double round the sack, as indeed might have been expected
from the shell and opercular valves (at least their upper parts) being
formed by the prolongation, as is obvious in the pupa, of the posterior
edges of the carapace. Between the two folds of corium, which are
united together by transverse ligamentous fibres, branching out at
both extremities, like the roots and branches of a tree, we have the
longitudinal muscles, which go to the opercular valves; and likewise a
layer-like mass of branching ovarian tubes (Pl. 25, fig. 1, _g_): the
ovarian tubes, however, are often confined to the base of the sack.
In Xenobalanus, the two folds of corium are united by longitudinal
membranous septa, making a series of quite peculiar, square tubes.
The above-mentioned muscles are attached at their upper ends to the
opercular valves, and at their lower ends to the basis. There are, in
fact, three pairs, but the pair attached to the basi-carinal angles of
the two terga (Pl. 25, fig. 1, _i_), are almost invariably confluent,
forming one great bundle; the second pair is attached to the lateral
or basi-tergal corners of the two scuta, and are hidden in the figure;
the third pair (_h_) is attached also to the scuta, to their rostral
angles. These muscles can only act as depressores; they are often
extremely powerful; they belong to the voluntary class, for they
are transversely striped. By their action, the opercular valves are
capable of varied slight movements, within the limit allowed by the
width of the flexible opercular membrane. By the action of the lateral
scutal depressores, the orifice leading into the sack is opened, the
movement being generally aided by the protrusion of the cirri. By the
sudden contraction of the rostral scutal depressores, the blows which
are sometimes given by the beaked terga at the opposite end of the
operculum, are probably effected. By the contraction of all three pairs
of muscles, the opercular valves are held down with quite surprising
force. The valves can be raised only by the action of the animal's body
against the basis.
In Coronula these muscles are more spread out, and do not extend
down to the basis; their lower portions, as is likewise the case in
Tubicinella, do not exhibit transverse striæ, and hence tend to pass
either into the involuntary class, or into ligament. This condition of
the muscles, in the above two genera, accords with the little-developed
state of their opercular valves. In Xenobalanus, there is no longer
any evidence of the muscles being collected into five or six bundles,
for they are thinly and almost uniformly spread out, and show in no
part transverse striæ. I may add that in much elongated specimens
of _Balanus balanoides_, these muscles become in their lower part
ligamentous, and destitute of striæ.
_Branchiæ._
In the Balaninæ, a pair of Branchiæ is always present: they lie on
each side, in a somewhat curved position, in the angle between the
sides of the shell and the basis. In Pl. 25, fig. 1, they are exactly
covered, on the further side, by the body of the animal. They are
attached near each other at the carinal end of the sack in a vertical
line, and likewise on each side in a transverse line, extending from
close beneath the spur of the tergum towards the point of attachment
of the body to the scutum. In Balanus, as in the figure (Pl. 25, fig.
3) of _B. tintinnabulum_, each branchia consists of a medial fold of
skin, a little curved conformably with the sack, and slightly tapering
towards its rostral and free extremity; but this fold is almost hidden
by the vertical sub-folds or membranous ridges, themselves plicated and
sub-plicated, which project on both sides: these vertical folds are
free at their tips: at their lower attached ends, they are thickest.
On the side nearest the wall of the shell, the whole branchia has a
bilobed appearance, owing to a very deep indentation caused by the
projection of the scutal lateral depressor muscle; the sub-folds on
this side are also more plicated. The branchia essentially is an inward
plicated fold of the membranes of the sack; for its outer, very thin
tunic is continuous with and moulted with that lining the sack; and
within it we have two layers of delicate, pulpy, transparent corium,
united together (as is best seen in Coronula) by ligamentous fibres,
branched at their two ends, all exactly as in the corium surrounding
the sack. There are here no distinct vessels, any more than in
other parts of the body, but a fluid could easily circulate in the
interspaces of the corium. From the large size of this organ, and its
simplicity of internal structure, being adapted exclusively to expose
a great surface of skin to the water, I do not doubt that it has been
correctly considered as a respiratory organ. By the voluntary movements
of the opercular valves (_i. e._ part of the carapace) the water is
constantly being pumped in and out of the sack; the movement, indeed,
may be almost compared to the heaving of a man's chest. Moreover,
the branchiæ on each side are attached so closely to the spur of the
tergum, that each time the latter is moved, the whole branchia must, I
think, be agitated, and the folds opened, as by the action of a lever.
In our two commonest, tidal, sessile Cirripedes, viz. _Balanus
balanoides_ and _Chthamalus stellatus_, I have observed that, when left
uncovered by water, they kept the orifice of their operculums a little
open, with a bubble of air within their sacks, so that the orifice
was in fact closed by a thin septum of water, with air beneath; when
disturbed, they closed their operculums with force, and expelled the
bubble of air with a clicking noise, which has been noticed by Dr.
Coldstream,[27] and has been thought to be made by the movement of the
operculum itself. _Bal. crenatus_, a deep-water species, when out of
water, keeps its operculum closed.
[27] 'Encyclopædia of Anatomy and Physiology;' article Cirrhopoda.
In Coronula, Platylepas, Tubicinella, and Xenobalanus, each
branchia[28] consists of two unequal folds, both plicated on both
sides: in the two latter genera, they extend far down the deep and
elongated sacks, and hence the area of surface altogether gained is
extremely great. In most of the species of Chthamalus, the branchia
consist of a small fillet barely plicated: in the allied _Chamæsipho
columna_, they are rudimentary, forming a smooth little pouch only
1/100th of an inch in length: in _Chthamalus scabrosus_ they are quite
aborted, being perhaps represented by a slight hairy ridge; but in
_Chthamalus dentatus_, and therefore within the limits of the same
genus, the branchiæ (and this seems to me a singular fact) are large,
each being composed of two plicated folds, as in Coronula. Tapering
filaments situated near the bases of the cirri, such as those occurring
in several species of the Lepadidæ, are not found in any sessile
Cirripede; but I have observed nearly similar filaments, projecting
upwards and inwards at the base of the sack, in several species of
Balanus and in Coronula; those which I examined were simply occupied
by delicate corium, and no doubt must aid in exposing a greater surface
of corium to the circumambient water.
[28] Burmeister has given a good figure (Tab. 2, fig. 10) of the
branchiæ of Coronula, (but the two folds are shaded too unequally),
in his 'Beiträge zur Naturgeschichte der Rankenfüsser.'
In my former volume on the Lepadidæ, I have described the _ovigerous
fræna_ occurring on the two sides of the sack, to which the
_ovigerous lamellæ_ are attached by a peculiar glandular secretion:
in the Balanidæ there are no ovigerous fræna, but the branchiæ just
described are identical with the fræna in essential structure and
in position; differing only in being placed a little nearer to the
carinal end of the sack, and in being generally (but not always)
larger and more plicated: seeing this, and that in _Alcippe lampas_,
and in some species of Pollicipes,--the genus which comes nearest to
the Balanidæ,--the ovigerous fræna are large and are destitute of
glands, and have therefore lost their normal function of supporting
the ovigerous lamellæ, I can hardly doubt that the _branchiæ_ in the
Balanidæ are the _ovigerous fræna_ of the Lepadidæ in a modified
condition; a transformation of function not greater than that of the
swimming bladder of a fish into the lungs of the higher Vertebrata.[29]
[29] There is, I conceive, no foundation for the belief of some
authors that the branchiæ of the Balanidæ are in any way connected
with the ovaria.
_Thorax and Body._
_Parts of the body included within the shell or carapace._--These
parts (Pl. 25, fig. 1) consist of the prominent mouth, and of the
thorax (_c′_), with its largely developed portion, called the prosoma
(_c_), and with its appendages. The abdomen is quite rudimentary,
being represented merely by a small portion of membrane surrounding
the anus, and sometimes inserted like a wedge between the inwardly
inflected posterior thoracic segments; in only two genera (Catophragmus
and Pachylasma), its nature is rendered somewhat plainer by supporting
caudal appendages. The probosciformed penis lies folded under the
thorax; and I believe (from what is seen in the anomalous genus
Proteolepas), that it normally arises from the ventral surface of
the terminal point of the rudimentary abdomen.[30] The thorax is
laterally compressed, the ventral surface being very narrow, with
the bases of the cirri placed closely together. It consists, in
appearance, of two very different portions; one a soft, more or less
rounded bag, which I have called the prosoma; and the other, which
supports the five posterior pairs of cirri, is narrower, invested
with stiffer membrane, and is more or less distinctly composed of
five segments. These segments (Pl. 26, fig. 8) on their dorsal and
dorso-lateral surfaces, are generally driven like wedges one into the
other, with their points directed anteriorly: on the ventral surface
the articulations are transverse. The prolongation (_e_) of the thin
membrane (_a_) surrounding the anus (_b_), that is, the rudiment of
the abdomen, which sometimes carries caudal appendages, almost divides
(in appearance, whether really I know not) the hindermost thoracic
segment along the medio-dorsal line, into two parts. I have given the
above drawing of these segments, but with the dorsal surface much
flattened, in _Coronula diadema_; in most species of Balanus, however,
the wedges formed by one segment being driven into another, are much
sharper; on the other hand, in Xenobalanus they are nearly straight and
transverse. The three posterior segments are always the most distinct;
the two next segments are also distinct laterally, but along the dorsal
surface they become, excepting in Xenobalanus and some few other
cases, completely confluent. The greater distinctness of the posterior
segments is conformable to what takes place in the higher Crustacea.
The articulations between the segments are folded inwards, and are
formed of thin membrane, which in some cases, as in _Coronula diadema_,
forms a marked contrast with the much thicker, stiffer, and yellowish
membrane of the segments themselves; in _Balanus tintinnabulum_,
however, the whole membrane of the five thoracic segments is very
thin, excepting small wedge-shaped portions along the medio-dorsal
line. The infolded articulations between the segments supporting the
three anterior pairs of cirri (at least in the Balaninæ), are much
wider than those between the three posterior segments; the former
segments, with their cirri, being consequently capable of being moved
further apart from each other. Could there have been any doubt as to
the distinctness and reality of the five thoracic segments, it would
have been set aside by the arrangement of the muscles attached to
them, as will presently be described. I need only add, that in many
genera there are shield-like swellings at the exterior bases of the
pedicels of the posterior cirri, which I for some time thought were
the epimeral elements of the thoracic segments; but I now believe them
to be parts of the pedicels of the cirri. The basi-exterior margin,
moreover, of the pedicel of the third pair of cirri, in many species of
the Balaninæ (Pl. 25, fig. 1), is produced as a plate, thickly fringed
with fine hairs, half across the dorsal surface of the thorax; serving,
apparently, as a brush to clean the sack, or to prevent the ingress of
any intruding substance.
[30] Von Siebold and Stannius, in their 'Anatomie Comparée,' tom.
i, p. 473, and p. 440, (foot-note), consider the articulated
probosciformed penis as an elongated abdomen; a view which, at the
commencement of my examination, I was tempted to admit; but the
position of the caudal appendages on the dorsal basis of the penis,
suffice, I think, to show that this view is not correct; for these
caudal appendages evidently correspond with those borne on the
very extremity of the abdomen in the pupa. Nor, indeed, does the
position of the anus accord well with such a view.
The soft, rounded, bag-like portion of the body, which I have called
the prosoma, is usually separated by a notch from the five posterior
thoracic segments; at its upper end it may be said to carry the mouth
and first pair of cirri. The prosoma includes the main part of the
stomach and the broad ends of the vesiculæ seminales. It is always
clothed by very thin membrane, which in _Chthamalus dentatus_, is
hairy. In Tubicinella and Xenobalanus, the prosoma is much elongated,
being produced far down the deep sack. That the prosoma is mainly
formed by a great development of that segment (homologically the second
thoracic segment) which carries the first pair of cirri, is certain,
and I should not have hesitated to have said that it was exclusively
so formed, had not the first thoracic segment in the anomalous genus
Cryptophialus been developed as a distinct and free segment, not
attached to the carapace; showing that possibly in other Cirripedes,
the dorsal half of this first thoracic segment may be concerned in the
formation of the free prosoma.
_Muscular System._
_Attachment of the Body to the Shell._--The prosoma which carries the
posterior thoracic segments, and in appearance the mouth, is the only
part of the body which is attached to the general covering (Pl. 25,
fig. 1), namely, to the opercular valves. Except through the continuity
of the lining membranes of the sack, the body lies free within the
walls of the shell. The area of attachment (shown by a sinuous broken
line round _a_ and _b_) extends from about the middle of the two scuta
down to their basal margins. As these valves lie obliquely across the
orifice of the shell, the animal's body comes to be suspended almost
in the middle of the sack. The two scuta, as we have seen, have the
power of opening and shutting a little; and are brought together by
the adductor scutorum muscle (_a_), which is generally very powerful.
The body is attached to these valves, round and beneath the adductor,
so as to hide it until one of the valves be removed. The attachment
is chiefly effected by three pairs of widely expanded, superficial
muscles, two pairs of which are spread over the flanks of the prosoma,
and the third pair over its rounded (properly dorsal) surface, which
lies close to the rostral compartment (A, fig. 1) of the shell. I
should have stated, that my chief examination of the attachment of
the body to the scutal valves, has been made on _Coronula balænaris_,
and less closely on _Balanus tintinnabulum_. Within these three pairs
of superficial muscles, there are (besides the adductor) no less than
five other pairs; of these one long pair is attached at one end to
the basal margin of the labrum (_e_), and at the other end, to the
under side, near to the basal margin of the scuta: two other, shorter,
parallel pairs of muscles are attached at one end to the interspace of
membrane between the basal edge of the labrum and the adductor scutorum
muscle, and at the other end, to the under side of the scuta, above the
attachment of the first pair: the fourth and shortest pair curls close
under the adductor, and is there attached at both ends beneath it. The
action of these four pairs of muscles must be to draw back, from the
orifice of the shell, the mouth, and that interspace of body between
the basal margin of the labrum and the adductor muscle. This movement I
saw in living specimens. The last and fifth pair of muscles is small,
but of considerable length; it is a diverging pair, attached at the
converging end, above and exteriorly to the adductor muscle; and at
the diverging end, low down on the under side of the scuta; I am very
doubtful regarding the function of this pair. Altogether we have seen
that round and within the fleshy pedicel, by which the body is attached
to the scuta, there are no less than eight pairs of muscles. The
central space between these muscles is hollow, and here many lacunal
channels seem to converge. These muscles receive nerves from the
supra-œsophageal ganglions. The interspace above alluded to, between
the basal edge of the labrum and the adductor scutorum muscle, occupies
a very different position according as the animal's body is protruded
as far as it can be, or is retracted. It is homologically part of
the third cephalic segment; and consequently the mouth ought to have
stood posteriorly (_i. e._ above, in the position figured in Pl. 25,
fig. 1) to this interspace; yet, in fact, when the animal is retracted
within its shell, the mouth usually lies almost directly beneath this
interspace and the adductor scutorum muscle.
Besides these muscles of attachment, the prosoma is furnished with
several other muscles. There are superficial muscles running up
towards the basal margin of the sides of the mouth; and other deeper
muscles, to which, I presume, the movements of the mouth, as a whole,
are due. The muscles moving the gnathites do not, as far as I could
make out, extend beneath the basal edge of the mouth. There are, also,
powerful muscles giving movement to the basal segments of the pedicel
of the first pair of cirri. Again, there are superficial muscles
running to the next succeeding thoracic segment; the anterior ends of
which are separated by a clear interspace from the lower ends of the
above-described superficial muscles, by which the prosoma is attached
to the scuta. On each flank, moreover, but more deeply imbedded, are
the long flexor and extensor muscles, presently to be described,
running to the five posterior thoracic segments. The last muscle which
I need here mention, is a deep-seated diverging pair, attached near
the upper end of the stomach, on its ventral surface, and diverging
from this point to the sides of the prosoma high up beneath the mouth.
The probable action of this pair, as well as of the three superficial
pairs of muscles by which the body is attached to the scuta, is to
draw up the whole prosoma towards or from the orifice; and likewise to
contract it firmly, so as to serve as a fulcrum for the movements of
the five posterior thoracic segments, together with the cirri, which
they carry.
The muscles of these five thoracic segments are numerous and powerful;
they are also complicated, chiefly owing to the segments on their
dorsal and dorso-lateral surfaces being driven, like wedges, one into
the other. As far as I could make out, there are on each side three,
superficial, dorso-lateral and lateral muscles (generally, if not
always, destitute of striæ), which do not cross the articulations,
but extend merely from articulation to articulation; and of which
the function can be only to contract each separate segment, and
consequently to open out the intermediate infolded articulations;
the effect of this would be to separate slightly the cirri from each
other,--more especially those borne on the two or three anterior
segments, between which the infolded articulations are deeper or
broader. There are other more deeply imbedded, powerful, long,
dorso-lateral extensor, and ventri-lateral flexor muscles, attached
at one end within the flanks of the prosoma, and at the other end to
the successive segments of the thorax. The action of the former is to
straighten and stretch out the thorax; of the latter, or ventri-lateral
muscles, to retract it. In tracing these muscles, a fascia could be
seen to become attached to a segment, and then this same fascia would
run on to the next succeeding segment: the effect of this must be, that
each segment can be retracted and protracted either from the prosoma
as a fulcrum, or from the antecedent segment as a fulcrum: we have,
also, seen that each segment can, by the agency of the superficial,
non-striated muscles, contract itself. Hence these thoracic segments
are capable of diverse movements, as was very evident when the shell
of a living specimen was opened. By one movement in common, the
whole five posterior segments could be drawn back, so as to become
even partly imbedded in the prosoma: lateral, twisting or wriggling
movements were also quite distinct: the three posterior segments
seemed to be capable of less independent movements than the anterior
segments; and I observed that the more powerful flexor and extensor
muscles did not run into these three posterior segments. The cirri, of
course, partake of the movements of the thorax; and in watching, in an
uninjured specimen, the alternate, protruding, gracefully sweeping and
retracting movements of the posterior pairs of cirri, it was evident
that the thorax was the chief agent in their movement. Besides the
muscles now mentioned, there are some immediately to be noticed, which
extend from within the thoracic segments to within the pedicels of the
cirri.
_Movements and Muscles of the Cirri._
Although the cirri have not been described, it will be most convenient
here to treat shortly of their muscles. Each cirrus consists of a
pedicel, having a long basal and a short upper segment, supporting two
multiarticulate rami. The lower segment of the pedicel can be drawn
forward by an adductor muscle, attached low down within this segment,
and crossing at right angles (at least in the case of the anterior
cirri) the corresponding muscle of the opposed cirrus, on the central,
ventral surface of the thorax. This segment can also be drawn back
by a muscle springing from the dorso-lateral surface of the thorax,
and running only a little way within the segment: I am far from sure
that the lower segment does not possess other muscles. The short upper
segment of the pedicel can be moved backwards and forwards, as I saw
in living specimens, independently of the lower segment; this movement
being best seen in the anterior cirri, which are much more often moved
independently of each other than are the posterior cirri. The rami
are capable, I believe, of being moved backwards and forwards _as a
whole_, by the movement of the few lower segments, which are generally
more or less confluent. They can, also, be curled up and uncurled by
the combined movement of each separate segment. The uncurling seems
to separate the two rami a little laterally. Each ramus, at least in
the two or three anterior pairs, can be moved to a certain extent,
independently of the other ramus of the same cirrus; and the few
terminal segments, either of both rami or of one ramus, are often a
little moved and curled (and this is especially the case with the long
anterior ramus of the first pair), without the lower segments or the
pedicel being moved.
The flexor and extensor muscles, which, as I believe, move the upper
segment of the pedicel (_a_ and _b_, Pl. 29, fig. 1), are attached at
their upper ends to its basal margin, and are thus enabled to draw it
a little way down within the lower segment, and so move it. The short
flexor muscle (_c_), which is attached at its lower end within the
upper segment of the pedicel, and the longer extensor (_d_), also,
attached within this same lower segment, serve, I believe, to move the
lower, partially confluent segments of each ramus as a whole. In the
case of these muscles, and of those last mentioned, I am surprised that
the extensors (_b_) and (_d_) are not attached nearer to the exterior
and dorsal surface. Other muscles (_e_, _f_) attached at their lower
ends within the upper segment of the pedicel, run up each of the two
rami to their tips, with some of the fasciæ terminating within each
segment: of these muscles, the outer one (_f_, _f_) appears to be
the extensor, and the inner one (_e_, _e_) the flexor. But besides
these, there are other short flexor muscles (_g_, _g_) which run on
the anterior face,[31] from segment to segment, serving to pull the
front edge of one segment within the edge of the next lower segment.
These muscles differ much in plainness in the several genera: they are
very distinct in Coronula. In some specimens of this genus, a few of
the articulations between the basal segments of the rami having been
obliterated, the short muscles (_g_, _g_) running from articulation to
articulation were absent, and their presence and nature in the upper
segments thus rendered the plainer. The muscular system in the several
pairs of cirri seems to be the same, with the exception of the first
pair, in which the muscle answering, as I suppose, to (_a_), namely,
the flexor of the upper segment of the pedicel, is much spread out at
its lower end, and is there attached to the exterior surface of the
lower segment.
[31] For a considerable time I thought that there were muscles
going to the spines, especially to those which arise from the upper
dorsal edge of each segment; but I have since ascertained that
these are the cases within which new spines, with their lower ends
doubled like the fingers of a glove hastily pulled off, are in
process of formation.
The backward and forward movements of the segments, both in the rami
and in the pedicels of the cirri, are apparently effected, as already
noticed, by the outer or inner (as the movement may be) basal edge of
one segment being drawn a little way down within the next succeeding
lower segment. If, at the same time, both the inner and outer margins
of all the segments were drawn one within the other, the whole limb
would necessarily be shortened; and I distinctly saw a shortening
action, with very slight movement in any other direction, in the
first and second pairs of cirri; and I think it almost certain that
this movement might be performed by the other cirri. If I correctly
understand a statement of Milne Edwards,[32] this is an important fact,
as he asserts that only the higher Crustaceans possess the power of
shortening their limbs.
[32] 'Annales des Sciences Naturelles,' tom. xviii, 1852, p. 121.
When a Cirripede is alive, the action of the cirri is really beautiful:
from the position of the thoracic segments, the posterior cirri (three
pairs in the Balaninæ and four pairs in the Chthamalinæ) form a sort of
semicircle facing the mouth: the anterior cirri stand further apart,
and are opposed in pairs to each other, with the first pair pointing
beyond the mouth. Together the cirri form a hollow cone, not circular
but elongated, with the mouth situated at the lower anterior end. The
posterior cirri are protruded, by the movement of the whole thorax,
curled up, close along the carinal end of the orifice; as they are
protruded, they diverge, both by the movement of their pedicels, and,
as I believe, by the separation of the thoracic segments. As the two
rami of each separate cirrus are uncurled, they also diverge a little;
as do the double rows of spines on the segments in each ramus, by their
elasticity. By the movement of the thorax, the cirri are then swept
towards the rostrum; and, lastly, they are brought perpendicularly down
towards the mouth with a rapid movement, which would be beautifully
adapted to catch any object floating or swimming in the water; hence
I have called the action of the cirri, captorial. When the shell of
a Balanus is broken open, the second and third pairs of cirri are
repeatedly clasped over the mouth with a convulsive movement, in a
manner indicating, I think, that their chief function is to seize
and carry to the mouth any object entangled by the sweeping movement
of the three posterior pairs. The first pair is also well adapted
to aid in this seizing action; but I suspect that the long anterior
ramus likewise acts as an organ of touch, warning the animal of
danger. The mouth being itself moveable as a whole,--the outer maxillæ
being capable of a backward and forward sweeping action, and being
furnished with orifices apparently olfactory,--the inner maxillæ having
more diversified movements,--the toothed mandibles overhanging the
œsophagus,--and the œsophagus itself possessing a powerful swallowing
movement, are all admirably adapted to secure any prey, when once
entangled by the cirri.
_Mouth._
The mouth, in the sub-family Chthamalinæ, cannot be distinguished
from that of the Lepadidæ, which has been pretty fully described
in my former volume. In the Balaninæ, however, the labrum differs
considerably in not being swollen; that is, in its outer and inner
fold of membrane being close together, and in having a central notch:
the palpi are also larger, and the lower teeth on the mandibles, are
laterally (Pl. 26, fig. 5) double, as will be more particularly stated
under these two sub-families. I have given a drawing (Pl. 26, fig.
1) of the mouth, seen from above, of _Balanus perforatus_, with the
right-hand palpus (_d′_) and outer maxilla (_a′_) cut off, in order
that the labrum (_e_), mandibles (_c_), and inner maxilla (_b_) might
be better shown; the cut-off bases (_x_, _x_) of the first cirrus on
each side are also shown. In fig. 2 we have the deep supra-œsophageal
cavity in _Bal. improvisus_ torn open and laid flat, with the inner
surfaces of the labrum (_c_) and outer maxillæ (_a_) exhibited, the
palpi, mandibles, and inner maxillæ having been removed. Figs. 3 and
4 will presently be referred to; they are parts of the mouth, with
the muscles, &c. removed, of Coronula. The mouth differs extremely
little in the different genera and species of the Balanidæ, much less
than amongst the Lepadidæ. In the Balaninæ, the crest of the labrum
is sometimes hairy, instead of having, as is usual, from two to six
teeth on each side of the central notch: in _Balanus improvisus_ (Pl.
26, fig. 2) and _eburneus_, and in _Chelonobia_, the crest on each
side of the central notch (_e′_) is furnished with a row of finely
graduated teeth. A sub-triangular portion of the inner fold of membrane
of the labrum, which overhangs the œsophagus, is always thickened and
yellowish; it is also often punctured in patterns (Pl. 26, fig. 2,
_f_), which, I believe, give attachment to little muscles that serve
to open the upper end of the œsophagus. Opposite to this thickened,
sub-triangular portion of membrane, the thin membrane forming the
supra-œsophageal cavity (or the cavity surrounded by the gnathites)
is strengthened by a pair of curved ribs (_h_, fig. 2) of thickened
yellowish membrane, running down from the inner bases (_a′′_) of the
bilobed outer maxillæ to the opening of the œsophagus (_g_): a broad
branch from each of these ribs supports the sides of the orifice of
the œsophagus; and this branch almost joins on to a slightly thickened
rim or bar (_f′_), which branches off from the upper part of the
sub-triangular (_f_) inner fold of the labrum. This structure, in _Bal.
improvisus_, is represented in Pl. 26, fig. 2, as well as it could be,
considering that the deep supra-œsophageal cavity has to be torn open;
and then laid flat.
The _Palpi_ (Pl. 26, fig. 6) differ little, except in size, in the
different genera, being squarish, more or less elongated, or even
approaching to club-shaped: in most of the Balaninæ they are larger
even than the mandibles, of which they normally form a part. Their
upper margins, especially towards their free extremities, are always
thickly clothed with spines; and there is generally a single row,
either short (_r_) or long, of spines of greater length, which arise
from a little above, and stand almost in a parallel line to, the basal
margin. On the internal surface, there is sometimes a row (_t_) of
very short little spines, which overhang the crest of the labrum. The
_Mandibles_ (Pl. 26, fig. 5) have from three to five teeth; the lower
point or angle is generally pectinated. In Coronula and its close
allies, there are some small teeth intermediate between the four or
five main teeth; and in these genera, though members of the sub-family
Balaninæ, the lower teeth exhibit only rudiments of being laterally
double.[33] The _Maxillæ_ sometimes have a notch under the upper large
pair of spines, and in _Octomeris brunnea_ there is a double notch: in
many species of Balanus, the inferior corner stands up like a step (Pl.
26, fig. 7, _a_): in many other genera and species, the whole edge is
straight. In all, or almost all cases, the row of spines on the middle
portion is double. The _Outer Maxillæ_ are always bilobed on their
inner faces, and are clothed with bristles. On all the gnathites, the
bristles are often doubly serrated.
[33] M. Martin St. Ange describes, in his 'Mémoire sur
l'Organisation des Cirripèdes,' pp. 15 and 32, "_une petite
langue_" in the mouth of Lepas; but I may venture to assert that
such does not exist; it is merely the point of union between the
outer maxillæ. M. St. Ange, in his comparison of the mouth of
Lepas with that of Phyllosoma, compares the mandible of the latter
with the palpus of Lepas; the first maxilla of Phyllosoma with the
mandible of Lepas; and so on with the other gnathites.
_Muscles and functions of the Gnathites, and their confluence._--The
outer maxillæ appear at first like a deeply-lobed lower lip, for they
reach over almost to the labrum (Pl. 26, fig. 1), and thus partially
cover the other organs; they are separately capable of a strong and
rapid, to and fro movement, by which no doubt they sweep any prey,
entangled by the cirri, towards the other gnathites. Each outer maxilla
is furnished with a pair of muscles, apparently a flexor and extensor;
there is also a little muscle between the two maxillæ, I presume for
the purpose of bringing them together. The outer and inner maxillæ
generally stand close together, and in several genera a little way
apart, from the mandibles; but there is no trace of any labrum or true
lower lip, bounding the mandibles and orifice of the œsophagus. The
outer and inner maxillæ and mandibles are not opposed in pairs to each
other, but against the thickened inner fold of the labrum; almost in
the same manner as the posterior pairs of cirri are not opposed one to
the other, but to the mouth.
I have described pretty accurately the muscles of the mandibles in
my former volume, and there given a drawing (Pl. 10, fig. 1) of them.
There are four muscles: first, the depressor muscle, which is the
largest, and is attached, at its upper end, to ligamentous apodemes
under the free toothed portion of the jaw; and at its lower end,
spreading considerably out, is attached to a concavity close above the
basal margin of the labrum; to understand the action of these muscles,
it should be borne in mind that the mandible almost faces the labrum.
In some genera, as in Coronula,[34] the swelling near the basal margin
of the labrum (Pl. 26, fig. 3, _k_), caused by the internal concavity
for the above muscle, is conspicuous. The depressor muscle is opposed
by a small elevator, attached to the mandible close by the depressor;
thence it runs upwards, and is united at its upper end to the base of
the palpus, at the point where the latter adheres to the labrum: I have
ventured to call this muscle the elevator, from being apparently so
well fitted for this purpose; but I feel some little doubt, from having
observed an apparent slight movement in the palpi of living Balani;
and this is the only muscle entering those organs. The free part of
the mandible is articulated on a square, thickened piece of membrane,
forming part of the side of the mouth (Pl. 26, figs. 3, 4, _c^1_;
and Pl. 10, fig. 1, _a_, _b_, in my volume on the Lepadidæ); to this
square piece of membrane, two short muscles are attached, one above the
other, and which ought, in the Plate in my former volume, to have been
represented crossing the depressor muscle at nearly right angles; at
their further ends they are attached to about the middle of the labrum,
where, at least in Coronula (Pl. 26, fig. 3, _i_), a slight concavity
can be detected. The action of these two muscles must be to draw the
whole mandible against the labrum; and the depressor muscle might, at
the same time, draw the toothed edge downwards, and thus force any prey
into the œsophagus.
[34] This is figured by Burmeister in his 'Beiträge zur
Naturgeschichte der Rankenfüsser,' Tab. 2, fig. 6.
The inner maxillæ are likewise furnished with four muscles, very
nearly as figured in my former volume (Pl. 10, fig. 10); namely, two
muscles, one inside and the other outside the curious apodeme, which
in the Balanidæ (Pl. 26, fig. 7, _b′_) is as invariably present as in
the Lepadidæ: these two muscles are attached at their lower ends to
the outer membrane of the mouth, close to its basal articulation: the
outer one of these two muscles would, I presume, act as an elevator,
and the inner one as a depressor; the free part of the organ working on
the top of the apodeme, like an axe, on a hinge, on the top of a pole.
But there is also a larger depressor muscle, in an analogous position
with that (_i. e._ the first-mentioned muscle) of the mandibles; and a
fourth muscle, crossing the latter depressor at nearly right angles,
and attached (as far as I could make out) on the side of the orifice
of the œsophagus, close under the mandibles: the action of this latter
muscle would be to draw the whole organ towards t
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