On the following days, the growth of the young animal was more rapid, its organs became more distinct, and soon afterwards the spiracles of the branchial sac, disposed in transverse rows, were visible, as well as the vibratile movements of the cilia, with which the stigmata are fringed. The number of these rows, however, was but four, the adult animal having ten.
The young Amaroucium was now provided with all its necessary organs except those of generation, of which no trace was yet visible, and the future situation of which was occupied by other organs, the heart being close up to the intestinal tube. The general form of the body resembled more that of a Didemnian than of a Polyclinian, for it had as yet no post abdomen, and the loop of the intestine was folded up against the inferior extremity of the thorax. Lastly, during the succeeding days, the abdomen very much lengthened itself (fig. 784. H), and at the end of the second week there was present, between the heart and the intestine, a granular mass, which by its ap pearance and position could readily be re cognised as the generative organs.
With regard to the development of the integument of this, at first solitary, but sub sequently compound, Ascidian, and which is evidently the analogue of the polypary of the Polyp fern, we have mentioned that at first it is a gelatinous layer, surrounding the yolk. An inner membrane, immediately in vesting the yolk, and regarded as the blasto derm, becomes the internal tunic of the animal. Whilst the larva goes through its early changes, there appears no connection at all between the inner tunic and the integu ment. Indeed, says M. Milne-Edwards, the larva may be seen sometimes to be entirely turned round in the cavity of the tegumentary envelope; and sometimes, when it abandons its original position, it forms a kind of hernia on the exterior of this envelope, by distending it at a weak spot (fig. 784. G). The learned professor does not, however, regard this en velope as being either a deposit produced by secretion, or as an organised body that had ceased to live since it had ceased to he attached to the interior parts of the animal ; because, as he observes, it continues to grow and gives un mistakable signs of vitality. Thus, not only
does its bulk rapidly increase, but it frequently gives rise, as in ilmaroucium Nordmanni, to lobe-like expansions, frequently changing their form, contracting and dilating very gently, and appearing to have some analogy to the proteiform expansions of the Ameba and other inferior animals. Of these changes Professor Milne-Edwards has given an inte resting series of figures.
It is only when the mouth and the anus open externally, that an attachment is estab lished between the integument and the inter nal tunic of the animal ; and then, as through out its future existence, it is around the two orifices only that organic continuity exists be tween the two parts, one only of which is in direct relation with the organs of animal life. It is consequently probable that the nutrition of the test is carried on by imbibition only ; and M. Milne-Edwards points out the fact of the independence of these two portions of the body of these Ascidians during the early periods of their life, as worthy the considera tion of physiologists ; and he adds, that proba bly this kind of vitality of the integument of the larval Ascidians has some analogy with what obtains in Sponges, and may, perhaps, throw some light on the peculiar existence of the basilar portion of the Serlularice and other Polyps, that continue to live for some time after the loss of the soft parts that are generally, but wrongly, regarded as constitut ing the entire animal.
From observations made by the same natu ralist on the development of the ova and larvm of other species of the Polyclinina, and of the Dirk-mina and Clavellince, it appears that very similar modifications take place ; the time occupied in the development being of course variable according to specific and external conditions. The larvm of Clavellina have the internal tunic strongly lobed in front, very tumid behind, and destitute of the pecu liar appendages observed in the Polyclinina. These appendages exist in the Didenznina, but are very short ; and at their base are seen a row of pyriform lobules, which might easily be taken for the germs of young individuals, but all of which really belong to one individual.