XVII 



PROTOCHOEDATA 



625 



degenerates and disappears. The greater part of the spinal cord is lost 

 with the degeneration of the tail, but the small portion which remains 

 loses Its cavity; whilst irom the posterior end of the hypophysial 

 canal glandular pockets are formed, which give rise to the sub- 

 neural gland of the adult, and which have been compared to the 

 pituitary body of the higher Vertebrate. 



The intestine, which in Ciona is less developed in the free- 

 swimming larva than is the case in some other species of simple 



® n. 



Fig. 455. — Longitudinal sagittal sections through the brain-vesicles and hypophysial 

 tubes of just-fixed tadpoles of Giona intestinalis. (After Willey.) 



A, earlier stage ; note that the hypophysial tube opens behind into the nerve tube. B, later stage. 

 l>.v, degenerating larval brain-vesicle ; ga/ng.ad, adult ganglion ; liyp, hypophysial tube ; n.t, nerve tube. 



Ascidians, grows longer, and acquires an opening to the exterior 

 by fusing with the ectoderm near the mid-dorsal hne, This opening 

 is the anus. The proximal portion of the intestine swells out to 

 ftrin a globular sac which is termed the stomach. From the 

 stomach grows out a pouch, which, by rapid branching, gives rise 

 to a tubular gland called by Willey the pyloric gland. This gland, 

 whose branches enswathe the intestine (Fig. 453, C), almost certainly 

 corresponds to the liver of Amphioxus. 



A median groove, which involves the anus and both atrial 

 openings, is formed in the dorsal surface of the animal. The sides 



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