1224 



TUNICATA. 



nal tunic or the thorax, which had been 

 smaller than the abdominal or posterior por- 

 tion, was much increased, and far more dia- 

 phanous, and the part occupied by the whitish 

 anterior spot was somewhat elevated in the 

 form of a nipple, marking the future position 

 of the mouth. The obscure circle that sur- 

 rounded the base of the buccal region was now 

 replaced by a very narrow yellow band ; and 

 on the inferior part of this thoracic portion 

 of the body there were observed two yellow 

 lines, vertically dividing it into three nearly 

 equal lobes. The abdominal portion of the 

 internal tunic was, on the contrary, much 

 straitened ; the pericardial spot was more dis- 

 tinct; and another less distinctly limited spot, 

 situated more in front, presented apparently 

 the first indication of the stomach. Towards 

 the middle of the second day, the middle 

 lobe of the thorax was much enlarged, and in 

 certain positions of the animal appeared to be 

 formed by a new interior cylindrical sac, which 

 in front united with the anterior wall of the 

 thorax at the point occupied by the yellow 

 ring before described, whilst laterally it was 

 separated from the internal tunic by the spaces 

 corresponding to the lateral lobes already men- 

 tioned. One of these lobes became very much 

 narrowed, and seemed destined to form the 

 great vascular sinus: subsequently, traversing 

 the anterior surface of the thorax, the other 

 lateral lobe appeared to correspond to the 

 future cloaca ; and the middle lobe was evi- 

 dently the branchial sac, from the base of 

 which arises the digestive tube. 



A few hours after, the anterior nipple-like 

 prominence was more salient, and seemed to 

 be contractile. The situation of the stomach 

 and the course of intestine were also distin- 

 guishable in the abdomen (fig. 784. F). The 

 yellow substance had now in a great degree 

 disappeared, but it was still present in a pretty 

 considerable quantity in the alimentary tube, 

 and appeared to pervade all the interior parts 

 of the young animal. 



Towards the end of the second day, the bran- 

 chial orifice was easily distinguished at the 

 summit of the thorax, and its margin began to 

 be somewhat crenulated. This orifice, how- 

 ever, was present only in the internal tunic, the 

 tegumentary substance being continued over 

 it without interruption. The nerve -ganglion 

 appeared in the form of a minute tubercle. 

 The yellow line encircling the summit of the 

 thorax appeared as the superior margin of the 

 branchial sac. All the thoracic portion of the 

 body contracted itself from time to time. 

 Lastly, the anal orifice began to be visible. 



On the third day, the heart was seen to 

 pulsate, and pellets of faecal matter were visi- 

 ble in the intestine. The following day, the 

 mouth opened externally, and the water passed 

 through it to the branchial cavity. About the 

 same time, the integument was perforated also 

 by the anal orifice, from which faecal matter 

 was discharged, provided without doubt by 

 the digestion of nutritive matter furnished by 

 the vitelline mass (fig. 784. G). 



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 ofaDidemnian than 

 of a Polycliuian, 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 Polypifera, 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 be 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 Amaroucium 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 Amcebee 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 



