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.