Diagram of an Ascidian, showing the position of the viscera.

Fig. 244. Diagram of an Ascidian, showing the position of the viscera: a, the oral orifice; b, tentacula guarding it; c, d, nervous ganglia; e, respiratory sac; f, longitudinal vessel; h, i, k, I, m, stomach and intestine; o, ovary; p, termination of oviduct; q, common excretory orifice.

* Cuvier, Memoire sur les Ascidies, p. 14.

(1269). The reproductive system in these humble forms of Mollusca presents the utmost simplicity of parts, being composed of an ovarian nidus, in which the germs of their progeny are elaborated, and a duct, through which their expulsion is accomplished.

(1270). The researches of Milne-Edwards*, however, relative to this part of their economy, tend to show that the structure of these creatures is more complex than was previously supposed; and in Amaroucium Argus, one of the compound Ascidians, that indefatigable observer also succeeded in recognizing the presence of a male apparatus. This consists of a largely-developed testicular gland, that occupies almost all the lower part of the post-abdomen, and communicates with the common excretory cavity by means of a long filiform canal that was regarded by Savigny as the oviduct. This gland is made up of a multitude of whitish vesicles, which, at first sight, have much the appearance of rudimentary eggs, but which, in reality, are found to swarm with spermatozoa, thus revealing the true nature of the organ. A similar arrangement has since been detected in numerous other genera; so that its existence throughout the entire class is now no longer doubtful. The ovarium is situated in close juxtaposition with the testes. In all the Polyclinian group it is lodged in the post-abdomen, and posteriorly is hardly distinguishable from the male apparatus, but towards the upper part becomes easily recognizable in consequence of the size and colour of the ova contained within it.

The eggs, of which a few only are developed at a time, as they issue from the ovigerous organ pass immediately into the cloaca, or even become lodged in the lateral part of the thoracic chamber, between the proper tunic of this cavity and the branchial sac, where they remain for some time exposed to the influence of the surrounding aerated medium.

(1271). Whilst the ova remain enclosed in the upper part of the post-abdomen, they increase considerably in size and assume a spherical form; the yelk acquires a deep-yellow colour; and the vesicle of Pur-kinje, which was distinctly visible during the commencement of their development, disappears, and is replaced by a cloudy spot, which appears to be the blastoderm, or proligerous layer, from which proceeds the embryo of the young Ascidian.

(1272). During incubation the egg acquires more of an oval form, the vitelline mass seems to contract, and its surface, growing denser, appears to become organized into a membrane which is distinct from the more deeply-seated substance of the yelk, and gradually the whole becomes moulded into something like the shape of a tadpole (fig. 245, a), the anterior extremity of which is surmounted with a sort of tentacular apparatus; and on the bursting of the egg, this embryo, by means of its long tail, swims about in the water with considerable vivacity; after the lapse of a few hours, however, the little creature, in size not yet larger than the head of the smallest pin, fixes itself to some foreign object by means of one of the little suckers situated on the anterior extremity of its body, and permanently loses all capability of locomotion.

* "Observations sur les Ascidies composees," Mem. de l'Acad. torn, xviii.

(1273). Having thus fixed itself for life, the larval Ascidian soon begins to change its form (fig. 245, b.) The anterior extremity of its body becomes expanded, the tentacular appendages disappear, the central portion of the tail becomes retracted into the central mass, and, lastly, the tail itself, which was at first such an important locomotive agent, gradually withers away, until no traces remain of such an organ having existed (fig. 245, c, d).

Larva and progressive stages of metamorphosis in an Ascidian Mollusk.

Fig. 245. Larva and progressive stages of metamorphosis in an Ascidian Mollusk.

(1274). From the above description of the development of the young Ascidians, it appears that during the first part of their existence they are solitary and isolated animals, although at a later period they are found united into numerous colonies, either connected together by means of a creeping common stem, or associated into a compact mass by a tegumentary tissue wherein the entire colony is arranged after a certain order, or regular pattern, which is constant in each species; and the manner in which this is effected thus presents itself as a problem possessing considerable interest.

(1275). Savigny, while prosecuting his dissections of the Botrylli, had remarked, situated upon the borders of the stellate groups formed by the association of individuals belonging to that genus, a multitude of membranous tubes, slightly dilated at their extremities, to which he gave the name of the marginal tubes, at the same time pointing out their existence in other families, but without entering into any details concerning the relations existing between them and the associated Ascidians contained in the tegumentary mass. Milne-Edwards, however, ascertained, by the examination of transparent groups of these creatures whilst in a living state, that each of these canals is at first a little tubercle, developed from the surface of the abdominal portion of the inner tunic of an adult Aseidian. This tubercle becomes elongated by growth into a tube, the extremity of which is closed, but free, while its internal cavity communicates freely by the opposite end with the abdominal cavity of the Aseidian from which it originally sprouted; so that the blood circulating in the latter easily penetrates into the caecal appendage, wherein an active circulation is kept up.