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.
Unquestionably, without a rigid examination, these four forms would have been ranked in different families, if not orders, of the Articulated kingdom.
Fig. 236. Ibla Cumingii, showing the supplemental male: a, b, c, d, e,f, g, i, body of the female Ibla; h, supplemental male. (After Darwin).
(1182). The observations of Mr. Thompson* relative to the progress of the ova after their escape from the pedicle throw much additional light upon this portion of our subject. "In the whole tribe of Cirripeds," says this industrious naturalist, "the ova, after their expulsion from the ovarium, appear to be conveyed by the ovipositor into the cellular texture of the pedicle, just beneath the body of the animal, which they fill to the distance of about an inch. When first placed in this position they seem to be amorphous, and inseparable from the pulpy substance in which they are imbedded; but as they approach to maturity they become of an oval shape, pointed at both ends, and are easily detached. Sir Everard Home has given a very good representation of them at this stage of their progress, in his 'Lectures on Comparative Anatomy,' from the elegant pencil of Mr. Bauer.
(1183). "During the stay of the ova in the pedicle, they render this part more opake and of a bluish tint, - the ova themselves, and the cellular texture in which they are surrounded, being of a pale or azure-blue colour. It is difficult to conceive in what manner the ova are extricated from the situation above indicated; but it is certainly not by the means suggested by Sir E. Home in the above-mentioned lecture, viz. by piercing outwards through the membranes of the pedicle; for the ova are subsequently found forming a pair of leaf-like expansions, placed between either side of the body of the animal and the lining membrane of the shells. These leaves have each a separate attachment at the sides of the animal to the septum which divides the cavity occupied by the animal from that of the pedicle: they are at first comparatively small, having a rounded outline, and possess the same bluish colour which the ova had in the pedicle; but as the ova advance in progress, these leaves extend in every dimension, and lap over each other on the back, passing through various lighter shades of colour into pale pink, and finally, when ready to hatch, become nearly white.
* Phil. Trans, for 1835, p. 356.
These leaves appear to be composed of a layer of ova, irregularly placed, and imbedded in a kind of parenchymatous texture, out of which they readily fall, when about to hatch, on its substance being torn asunder; indeed, it appears at length to become so tender as to fall entirely away, so that, after the period of gestation is passed, no vestige of these leafy conceptacles is to be found".
(1184). In the second form of Cirrhopoda (Balani), the animals, instead of being appended to foreign substances by elastic and flexible pedicles, are sessile, - the shelly investment of the body being in immediate contact with the rock or other submarine body to which the Barnacle adheres. The soft tube of Pentelasmis is, in this case, represented by a strong testaceous cone composed of various pieces accurately joined together, and generally closed inferiorly by a calcareous plate; while the representatives of the valves of the pedunculated species form a singular operculum, which is moved by special muscles, and accurately shuts the entrance of the shell when the animal retires into its abode. In their general structure, however, the Balaniform Cirrho-pods accord with the description above-given; and, from the similarity of their habits and economy, a more elaborate account of the peculiarities which they exhibit would be superfluous in this place.
(1185). One of the most remarkable circumstances connected with the history of the Cirrhopoda is the recently-discovered fact of their undergoing a distinct metamorphosis; so that, in the earliest periods of their existence, instead of being rooted by means of a pedicle or otherwise, the newly-hatched young are endowed with locomotive organs calculated to enable them to swim freely about, and giving them rather the appearance of Entomostracous Crustacea than of animals of their own class. This singular fact was first announced by Mr. J. V. Thompson, of Cork*; and its correctness has since been admitted by various anatomists who have devoted their attention to this subject. Mr. Thompson's first observations were made upon minute animals, which, although at first actually taken for Crustaceans, turned out to be the young fry of Balanvs pusillus; and the following is that gentleman's account of their appearance and subsequent change. The young Cirrhopod is a small translucent animal, 1/10.th of an inch long, of a somewhat elliptic form, but very slightly compressed laterally, and of a brownish tint.
When in a state of repose, it resembles a very minute mussel, and lies upon one of its sides at the bottom of the vessel of sea-water in which it is placed; at this time all the members of the animal are withdrawn within the shell, which appears to be composed of two valves, united by a hinge along the upper part of the back, and capable of opening from one end to the other along the front, to give occasional exit to the limbs. The limbs are of two descriptions, viz.: anteriorly, a large and very strong pair provided with a cup-like sucker and hooks, serving solely to attach the animal to rocks, stones, etc.; and posteriorly, six pairs of natatory members, so articulated as to act in concert, and to give a very forcible stroke to the water, causing the animal, when swimming, to advance by a succession of bounds, after the same manner as the Water-flea (Daphnia) and other Monoculi, but particularly Cyclops, whose swimming-feet are extremely analogous. The tail, which is usually bent up under the belly, is short, composed of two joints, and terminates in four setae, forming an instrument of progression. The animal, moreover, is furnished with large pedunculated eyes.