466 



TUNICATA. 



The blind end of the rectum comes into contact with the wall of the 

 atrium and fuses with it, perforation finally taking place at this 

 point (Fig. 237, ed). 



In the meantime, the rudiment of the central nervous system has 

 also appeared. It can be seen in the form of a rather long tube 

 (Figs. 236, 237, n) which is continued posteriorly as a solid strand. 

 The latter evidently corresponds to the visceral cell-strand observed by 



FiG. 237. — Dorsal view of a bud of Distaplia stylifem at a lat • .stage of development 

 (after Kowalevskv). cl, cloaca ; dr, digestive gland ; e, atrial aperture ; ed, 

 rectum ; g, genital organs ; i, branchial aperture ; n, nervous system ; «', visceral 

 nerve-strand ; oe, oesophagus ; p, peribranchial sacs ; st, branched stolon. 



van Beneden and Julin (" cordon ganglionnaire visceral "). Authors 

 differ as to the origin of the neural tube. According to Kowalevskv, 

 it is of entodermal origin and is derived from a furrow which forms 

 in the wall of the branchial intestine. Hjort (No. 59) accounts in 

 the same way for the nervous system in the bud of Botryllus* 



* [Although Hjort derives the dorsal tube and nerve-ganglion from the 

 inner vesicle of the bud which also gives rise to the pharynx and atrium, he 

 does not regard these structures as entodermal, for in Botryllus the inner 

 vesicle grows out from the peribranchial sac of the Ascidiozooidand this cavity 

 is primarily ectodermal, hence all these organs are ectodermal.— Ed.] 



