400 TUNICATA. 



tubes (/>) on either side of the intestine, and, on the right side of the 

 body, the pericardial tube (Fig. 192, pc), are continued direct from 

 the Oyathozooid into the chain of Ascidiozooids. The central nervous 

 system, on the contrary, arises independently in the Ascidiozooids 

 (Salensky). From this point the development of the Cyathozi 

 and that of the Ascidiozooids will be treated separately. 



D. Further Development of the Oyathozooid. 



The structure of the < 'yathozooid is fairly simple. One of the poles 

 of the body is marked by the presence of an ectodermal invagination, 

 the atrial invagination (Fig. 193 B, 194, cl) which occupies the most 

 anterior end of the germ-disc (Fig. 189, »•/), and the origin of which 

 has already been discussed (p. 394). This invagination originally 

 communicates with the peribranchial tubes (Figs. IS!), 192 B, 193 

 A, ]>). Very soon, however, that part of these tubes which lies in the 

 Oyathozooid degenerates and completely disappears (Fig. 193 B). 

 The atrial invagination, on the contrary, in which a narrow thick- 

 walled portion can be distinguished later from a thin-walled portion, 

 the actual cloaca, becomes connected with the alimentary canal of 

 the Oyathozooid, the lamellae that separate the two cavities being 

 perforated (Fig. 194). The alimentary canal (Fig. 194, d) of the 

 Oyathozooid is a simple thin-walled sac, nan-owed in the shape of 

 a funnel posteriorly, which adopts a somewhat curved position in 

 accordance with the curving of the stolon in later stages. Its 

 posterior, narrowed end passes over into the enteric rudiment <>i 

 the Ascidiozooids. There is no sign of endostyle-folds in the intestine 

 of the Oyathozooid, that part of the organ in which, in the germ-disc, 

 the rudiments of these folds appeared (Fig. 189 en) being used up 

 in the formation of the Ascidiozooids (Fig. 193, en). 



The rudiment of the nervous system of the Oyathozooid, which is 

 derived from an ectodermal invagination lying close behind the atrial 

 rudiment near the anterior margin of the germ-disc (Fig. 189. u) is 

 originally a somewhat long and completely closed vesicle which, when 

 the alimentary canal changes its position, in consequence of the 

 curvature of the stolon, also shifts from its original position. The 

 posterior end of the neural vesicle now enters into open communica- 

 tion with the enteric cavity (Figs. 194 B, g, and 197). This is the 

 rudiment of the ciliated pit ( //). The anterior part of the vesicle 

 now becomes divided by a furrow from that part which is used for 

 the formation of the ciliated nit : it swells and changes into the rudi- 



