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.