This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
After keeping several of the above for some days in sea-water, they threw off their exuviae, and, becoming firmly adherent to the bottom of the vessel, were changed into young Barnacles; and the peculiarly-formed shells, with their opercula, were soon distinctly formed, while the movements of the cirri, although as yet imperfect, were visible. As the shell becomes more complete, the eyes gradually disappear, the arms become perfectly ciliated, and an animal originally natatory and locomotive, and provided with a distinct organ of sight, becomes permanently and im-moveably fixed, and its optic apparatus obliterated.
* Zoological Researches, 4th Memoir, 1830.
(1186). Similar results were obtained by watching the development of the pedunculated type of Cirripeds* (Lepades), many of which were proved in their earliest form to resemble different kinds of Monoculi, and to be possessed of the capability of locomotion.
(1187). The manner in which larvae thus constituted are converted into the fixed and pedunculated Cirriped is, indeed, one of the most remarkable features connected with the history of the class. The larva in its last stage has much the appearance of one of the Stomapod Crustaceans; and, as is the case in several genera belonging to that order, the part of the head bearing the antennae and organs of sense in front of the mouth equals or even exceeds in size the posterior part of the body, consisting of the enclosed thorax and abdomen. On the borders of the carapax at the anterior end, on the sternal surface, there are two minute orifices, sometimes having a distinct border round them, within which are contained minute sacculi, regarded as acoustic organs; and, moreover, large compound eyes, each consisting of eight or ten lenses, are situated near the bases of the antennae.
(1188). But it is the antennae themselves that principally claim our notice, inasmuch as it is by means of these organs that the creature ultimately attaches itself when about to assume its complete or fixed condition. They consist of three segments; the first, or basal one, is much larger than the others 1, and apparently always has a single spine on its outer distal margin. The second segment consists either of a large, thin, circular sucking-disk, or is hoof-like; in all cases it is furnished with spines on the exterior hinder margin. The third and ultimate segment is small; it is articulated on the upper surface of the disk, and is directed rectangularly outward; it is sometimes notched, and even shows traces of being bifid, and bears about seven spines at the end, some of which are hooked, others simple. The antennae, at first, are well furnished with muscles, and serve for the purpose of walking, one limb being stretched out before the other; but their main function is to attach the larva, for its final metamorphosis into a Cirriped, by-means of the appended disk, which can adhere even to so smooth a surface as a glass tumbler*. The attachment is at first manifestly voluntary, but soon becomes involuntary and permanent, being effected by special and most remarkable means.
* Phil. Trans, for 1835, p. 355. 1 Darwin, loc. cit.
(1189). In each of the antennae there is situated a duct, derived from a large glandular body (the cement-gland): the termination of this duct is situated in the immediate vicinity of the adhesive disk, by the assistance of which the little animal is about to fix itself permanently to some foreign body and assume the Cirriped condition.
(1190). Several times Mr. Darwin succeeded in dissecting off the integuments of the lately-attached larva, and in displaying the enclosed Lepas entire, of which, in this condition, he gives the following account: - Whilst the young Lepas is closely packed within the larva, the capitulum (or shell-clad portion of the body), as known by the five valves, about equals in length the peduncle. The peduncle occupies the anterior half of the larva; and even at this early period the muscles of its inner tunic are quite distinct. The compound eyes, as we have already seen, are attached to the sternal surface of the larval carapax, and are consequently cast off with it: but the antennae, which are not moulted with the carapax, are left cemented to the surface of attachment; their muscles are converted into sinewy fibres, the corium after a short time is absorbed, and they are then preserved in a func-tionless condition. If, indeed, the peduncle even of an adult Cirriped be very carefully removed from the surface of attachment, quite close to the end, but not at the actual apex, the larval prehensile antennae can always be found, and the cement-ducts traced, running in a slightly sinuous course on each side within the peduncle, until they arrive at the glandular organs whence the cement is furnished.
Each gland contains a strongly coherent, pulpy, opake cellular mass, like that in the cement-ducts; and it is this peculiar substance that constitutes the bond of union between the Cirriped and the surface whereupon it becomes fixed.
Fig. 237. Cement-ducts of Scalpellum, magnified.
* Rev. R. L. King, Annual Report of the R. Inst, of Cornwall, 1848.