[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.