First stage.—The egg. Its outer covering is a very delicate membrane. The yolk is yel lowish-red, clouded, and marked with two rows of small spots, globule-like, distinct at one end, running together at the other. The eggs in the central parts of the ovary are consi derably further advanced than those in the cir cumference. Through the transparent covering of the egg the general form of the embryo can be seen.
Second stage.—In this stage the young Cir riped resembles the fry of Cyclops or Daphnia in its external characters. It is provided with two long antennm and three pairs of feet (arms?) placed along its ventral surface.* Each foot of the first pair is single, and is furnished with bristles at its free extremity. Each of the other pairs is divided into two members, also tipped with bristles. The posterior part of the body is tapering, compressed, and slightly bifurcated at its extremity, where it is beset with bristles. No eyes could be seen in this stage, but Pro fessor Burmeister nevertheless conjectures that they really do exist. The appeamnce of two rows of small globules on the surface of the body continues to present itself, but here they are more numerous, although not larger. The middle part of the body is clear and transparent.
Third stage.—Materials for the description of this stage were obtained by Burmeister from the examination of only one individual, which was found attached to the frond of a fucus hard by the bases of some adult indivi duals. The shell, in this the first stage of its growth, is of leathery consistence, and formed of one piece, placed dorsally. A fleshy protu berance serves as the peduncle. The organs by which the young animal fixes itself are evi dently the long antennce situated near the mouth. Behind these are placed the very large eyes. Burmeister satisfied himself of the ex istence of a single transparent cornea, and saw behind it a round black spot, but no lens. The two eyes are very closely approximated by their bases. Both the eyes and the brownish con tents of the alimentary canal can be distin guished through the translucent shell. In the structure of the posterior part of the body there is no great change from the former stage. Each arm of the first pair is single, and consists of three articulations, of which the basilar is the greatest: the smallest and terminal one bears four long stiff bristles. The arms of the follow ing pair are not single, but each is divided into two small articulated processes. The little globules of the two former stages are not dis cernible in this.
Fourth stage.—This stage was observed by Professor Burmeister in the Lepas anati lera from the coasts of Chili. All the indi viduals examined were about three-fourths of' a line in length. Soon after the animal fixes itself the old integuments are thrown off. The eyes and the antenn are entirely cast off along with these. After this process had been completed, the space within the man tle was found to be filled with a granular pulta ceous mass, at first occupying the greater part of the cavity of the shell, and covering all the young animal. This appeared to M. Burmeis
ter to be the same that is found in the pedicle of the older animals, and to rese:nble closely the matter contained within the cavities of the shells of Coronul and other Balanids. It is by a sack-formed process of the mantle filled with this yellowish matter that the peduncle is first formed. At the time of the animal's fixing itself the shell has no calcareous points, but in the course of this stage it becomes firm and gradually more and more solid. There are now six pairs of feet, each of three articulations, and terminated by bristles. A small tail of two articulations also appears, the rudiments of which, however, can be detected in the former stage. In the fifth stage the process of deve lopment is completed.
It must be admitted that the evidence in favour of Mr. Thompson's opinions on this subject is by no means conclusive. There is still wanting a series of minute and careful ob servations on the first appearance and motions of the embryo immediately after its exclusion from the egg ; and nothing, but the results of such a series can settle the question as to whe ther there be a real metamorphosis or not.
Mr. Gray's observations have led him to conclude that no great changes of structure, such as Mr. Thompson's views presuppose, actually take place ; although, in examining the mature egg of Balanus Cranchii, he found the appearance of the embryo nearly the same as is described by Burmeister as being that of the Lepads in the second stage of development. The egg of this Balanid Mr. Gray ascertained to be one-fiftieth of an inch in length. He de scribes the inclosed animal as being of an ovate form, tapering at one extremity, truncated and ciliated at the other ; bearing a general resem blance to the adult animal, but furnished with only three pairs of ciliated arms ; the base of each arm being two-jointed. He found only one lengthened process attached to the lower pair of arms; but, connected with the two upper pairs, tvvo fusiform thick, articulated and ciliated processes, similar to those of the anterior part of the perfect animal, but less elongated. He saw no shelly covering.* We have not yet had proper opportunities of devoting attention to this interesting subject so far as observations on the living animals are concerned ; but we have no doubt of its very soon meeting with a clear and satisfactory elu cidation ; meanwhile we may remark that the structure of the embryo within the mature egg (about which there can be no doubt) is such as strongly to indicate its adaptation to free loco motion ; and that, after a review of all the ob servations that have been published on the subject, we are inclined to conclude in favour of Mr. Thompson's opinion that, in the early stages of its development, the young Cirriped really enjoys locomotive powers, and then un derp.;oes such changes of structure as are re quired to fit it for its altered circumstances in adult age..