A Monograph on the Sub-class Cirripedia (Volume 1 of 2) / The Lepadidae; Or, Pedunculated Cirripedes

by Charles Darwin

INTRODUCTION.

I should have been enabled to have made this Volume more complete, had I deferred its publication until I had finished my examination of all the other known Cirripedes; but my work would thus have been rendered inconveniently large. Until this examination is completed, it will be more prudent not to discuss, in detail, the position of the Lepadidæ amongst the Cirripedia, or of these latter in the great class of Crustacea, to which they now, by almost universal consent, have been assigned. I may, however, remark that I believe the Cirripedia do not approach, by a single character, any animal beyond the confines of the Crustacea: where such an approach has been imagined, it has been founded on erroneous observations; for instance, the closed tube within the stomach, described by M. Martin St. Ange (to whose excellent paper I am greatly indebted), as indicating an affinity to the Annelides, is, I am convinced, nothing but a strong epithelial lining, which I have often seen ejected with the excrement. Again, a most distinguished author has stated that the Cirripedia differ from the Crustacea:—1st. In having “a calcareous shell and true mantle;” but there is no essential difference, as shown by Burmeister, in the shells in these two classes; and Cirripedes certainly have no more claim to a mantle than have the bivalve entomostraca. 2d. “In the sexes joined in one individual;” but this, as we shall see, is not constant, nor of very much weight, even if constant. 3d. “In the body not being ringed;” but if the outer integument of the thorax of any Cirripede be well cleaned, it will be seen, (as was long ago shown by Martin St. Ange), to be most distinctly articulated. 4th. “In having salivary glands;” but these glands are, in truth, the ovaria. 5th. “In the liver being formed on the molluscous type;” I do not think this is the case, but I do not quite understand the point in question. 6th. “In not having a head or organs of sense;” this is singularly erroneous: Professor Leidy has shown the existence of eyes in the mature Cirripede; the antennæ, though preserved, certainly become functionless soon after the last metamorphosis; but there exist other organs of sense, which I believe serve for smelling and hearing: and lastly, so far from there being no head, the whole of the Cirripede externally visible, consists exclusively of the three anterior segments of the head.

The sub-class, Cirripedia, can be divided into three Orders; the first of which, mainly characterised by having six pair of thoracic cirri, includes all common Cirripedes: these latter may be divided into three families,—the Lepadidæ, or pedunculated Cirripedes, the subject of the present memoir; the Verrucidæ containing the single genus Verruca or Clisia; and, lastly, the Balanidæ, which consist of two very distinct sub-families, the Balaninæ and Chthamalinæ. Of the other two Orders above alluded to, one will, I believe, contain the remarkable burrowing genus Alcippe, lately described by Mr. Hancock, and a second burrowing genus, or rather family, obtained by me on the coast of South America. The third Order is highly singular, and differs as much from all other Cirripedes as does a Lernæa from other crustaceans; it has a suctorial mouth, but is destitute of an anus; it has not any limbs, and is as plainly articulated as the larva of a fly; it is entirely naked, without valves, carapace, or capitulum, and is attached to the Cirripede, in the sack of which it is parasitic, by two distinct threads, terminating in the usual larval, prehensile antennæ. I intend to call this Cirripede, Proteolepas. I mention it here for the sake of calling attention to any parasite at all answering to this description.

NOMENCLATURE OF THE VALVES.

[ Figure I.
CAPITULUM. ]

Figure II.
SCUTUM of LEPAS.

Figure III.
TERGUM of LEPAS.

Although the present volume is strictly systematic, I will, under the general description of the Lepadidæ, give a very brief abstract of some of the most interesting points in their internal anatomy, and in the metamorphoses of the whole class, which I hope hereafter to treat, with the necessary illustrations, in detail. I enter on the subject of the metamorphoses the more readily, as by this means alone can the homologies of the different parts be clearly understood.

On the Names given to the different parts of Cirripedes.

I have unwillingly found it indispensable to give names to several valves, and to some few of the softer parts of Cirripedes. The accompanying figure of an imaginary Scalpellum includes every valve; the two most important valves of Lepas are also given, in which the direction of the lines of growth and general shape differ from those of Scalpellum as much as they do in any genus. The names which I have imposed will, I hope, be thus acquired without much difficulty.

Whoever will refer to the published descriptions of recent and fossil Cirripedia, will find the utmost confusion in the existing nomenclature: thus, the valve named in the woodcut the Scutum, has been designated by various well-known naturalists as the “ventral,” the “anterior,” the “inferior,” the “ante-lateral,” and the “latero-inferior” valve; the first two of these titles have, moreover, been applied to the rostrum or rostral valve of sessile Cirripedes. The Tergum has been called the “dorsal,” the “posterior,” the “superior,” the “central,” the “terminal,” the “postero-lateral,” and the “latero-superior” valve. The Carina has received the first two of these identical epithets, viz. the “dorsal” and the “posterior;” and likewise has been called the “keel-valve.” The confusion, however, becomes far worse, when any individual valve is described, for the very same margin which is anterior or inferior in the eyes of one author, is the posterior or superior in those of another; it has often happened to me that I have been quite unable even to conjecture to which margin or part of a valve an author was referring. Moreover, the length of these double titles is inconvenient. Hence, as I have to describe all the recent and fossil species, I trust I may be thought justified in giving short names to each of the more important valves, these being common to the pedunculated and sessile Cirripedes.

The part supported by the peduncle, and which is generally, though not always, protected by valves, I have designated the Capitulum.

The title of Peduncle, which is either naked or squamiferous, requires no explanation; the scales on it, and the lower valves of the capitulum, are arranged in whorls, which, in the Latin specific descriptions, I have called by the botanical term of verticillus.

I have applied the term Scutum to the most important and persistent of the valves, and which can generally be recognised by the hollow giving attachment to the adductor scutorum muscle, from the resemblance which the two valves taken together bear to a shield, and from their office of protecting the front side of the body. From the protection afforded by the two Terga to the dorso-lateral surface of the animal, these valves have been thus called. The term Carina[2] is a mere translation of the name already used by some authors, of Keel-Valve.

[2] In the Carina of Fossil Species of Scalpellum, I have found it necessary to distinguish different parts, viz., A, the tectum, of which half is seen; B, the parietes; and C, the intra-parietes.

The Rostrum has been so called from its relative position to the carina or keel. There is often a Sub-carina and a Sub-rostrum.

The remaining valves, when present, have been called Latera; there is always one large upper one inserted between the lower halves of the scuta and terga, and this I have named the Upper Latus or Latera; the other latera in Pollicipes are numerous, and require no special names; in Scalpellum, where there are at most only three pair beneath the Upper Latera, it is convenient to speak of them (vide [Woodcut, I,]) as the Carinal, Infra-median, and Rostral Latera.

As each valve often requires (especially amongst the fossil species) a distinct description, I have found it indispensable to give names to each margin. These have mostly been taken from the name of the adjoining valve, (see [fig. I.]) In Lepas, Pollicipes, &c., the margin of the scutum adjoining the tergum and upper latus, is not divided ([fig. II]) into two distinct lines, as it is in Scalpellum, and is therefore called the Tergo-lateral margin. In Scalpellum ([fig. I]) these two margins are separately named Tergal and Lateral. The angle formed by the meeting of the basal and lateral or tergo-lateral margins, I call the Baso-lateral angle; that formed by the basal and occludent margins, I call, from its closeness to the Rostrum, the Rostral angle. In Pollicipes the carinal margin of the tergum can be divided into an upper and lower carinal margin; of this there is only a trace ([fig. I]) in Scalpellum.

That margin in the scuta and terga which opens and shuts for the exsertion and retraction of the cirri, I have called the Occludent margin. In the terga of Lepas ([fig. III]) and some other genera, the occludent margin is highly protuberant and arched, or even formed of two distinct sides.

Occasionally, I have referred to what I have called the primordial valves: these are not calcified; they are formed at the first exuviation, when the larval integuments are shed: in mature Cirripedes they are always seated, when not worn away, on the umbones of the valves.

The membrane connecting the valves, and forming the peduncle, and sometimes in a harder condition replacing the valves, I have often found it convenient to designate by its proper chemical name of Chitine, instead of by horny, or other such equivalents. When this membrane at any articulation sends in 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.

The animal’s body is included within the capitulum, within what I call the sack (see [Pl. IV], [figs. 2] and [8´ a], and [Pl. IX], [fig. 4]). The body consists of the thorax supporting the cirri, and of an especial enlargement, or downward prolongation of the thorax, which includes the stomach, and which I have called the prosoma. ([Pl. IX], [fig. 4 n]). The cirri are composed of two arms or rami, supported on a common segment or support, which I call the pedicel. The caudal appendages are two little projections, either uni-or multi-articulate ([Pl. IV], [fig. 8´ a]), on each side of the anus, and just above the long proboscis-like penis. On the thorax and prosoma, or on the pedicels of the cirri, there are in several genera, long, thin, tapering filaments, which have generally been supposed to serve as branchiæ; these I call simply filaments, or filamentary appendages ([Pl. IX], [fig. 4 g-l]). The mouth ([fig. 4 b]) is prominent, and consists of palpi soldered to the labrum; mandibles, maxillæ, and outer maxillæ, these latter serve as an under lip; to these several organs I sometimes apply the title used by Entomologists, of “trophi.” Beneath the outer maxillæ, there are either two simple orifices or tubular projections; these, I believe, serve as organs of smell, and have hence called them the olfactory orifices. Within the sack, there are often two sheets of ova ([Pl. IV], [fig. 2 b]), these I call (after Steenstrup, and other authors) the ovigerous Lamellæ; they are united to two little folds of skin ([Pl. IV], [fig. 2 f]), which I call the ovigerous Fræna.

From the peculiar curved position which the animal’s body occupies within the capitulum, I have found it far more convenient (not to mention the confusion of nomenclature already existing) to apply the term Rostral instead of ventral, and Carinal instead of dorsal, to almost all the external and internal parts of the animal. Cirripedes have generally been figured with their surfaces of attachment downwards, hence I speak of the lower or Basal margins and angles, and of those pointing in an opposite direction as the Upper; strictly speaking, as we shall presently see, the exact centre of the usually broad and flat surface of attachment is the anterior end of the animal, and the upper tips of the Terga, the posterior end of that part of the animal which is externally visible; but in some cases, for instance in Coronula, where the base is deeply concave, and where the width of the shell far exceeds the depth, it seemed almost ridiculous to call this, the anterior extremity; as likewise does it in Balanus to call the united tips of the Terga, lying deeply within the shell, the most posterior point of the animal, as seen externally.

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 full confidence,—the specific determination being in each case made by myself.

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