The larva has now lost all power of loco motion, and quickly undergoes further changes of form. The anterior extremity of the trunk is widened, and the prolongations of the in ternal tunic quickly disappear. The central portion of the tail becomes at the same time empty, its contents being returned to the cen tral yolk-mass of the body. The sac or internal tunic enclosing the yolk becomes much con tracted and spherical ; lastly, the yellow matter, which was unequally divided, seems again to be rearranged. The tail, which during the early period of the existence of the larva per formed so important a part, being the only instrument of locomotion, is now reduced to its gelatinous or tegumentary portion ; and this, after becoming more and more trans parent, withers, and finally is detached, or falls away in shreds at a more advanced period of the growth of the animal. The trunk, on the other hand, is the seat of important and active changes. The tegumentary portion of the body is much widened, taking an oval out line, and is visibly augmented in bulk. The interior tunic continues at first to lessen, and becomes quite spherical, and many large patches of a lighter yellow than the rest are apparent, one of which occupies the anterior, and two others the posterior portions of the tunic (fig. 784. D).
M. Milne-Edwards further remarks, that the modifications already noticed ordinarily occupy the space of from ten to twelve hours ; and if the larva' are again examined towards the end of the first day of their sedentary existence, further changes in the interior tunic may be observed. In a speci men carefully watched by him, the follow ing changes were noted. Instead of being spherical, the large yellow sac became oval, and its anterior part much thinned. It soon afterwards again became elongated, and a circular contraction divided it into two por tions (fig. 784. a). The anterior portion, smaller and lighter coloured than the pos terior, was rounded in front, and presented at that part a large annular patch of a deep yel low, vaguely circumscribing a central paler portion. The posterior part was swollen and of a deeper yellow than the anterior, and quite behind there was observable a minute patch of a very clear yellow. This latter spot sub sequently became the heart, and the annular spot on the other extremity of the body was developed into the thorax of the animal. The following day all these parts grew still more distinct. The anterior portion of the inter 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 fxcal 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 fmcal matter was discharged, provided without doubt by the digestion of nutritive matter furnished by the vitelline mass (fig. 784. G).