STRUCTURE OF ASCIDIANS. 361 



say that mucus is wafted forwards to form a sort of web 

 near the mouth, from which entangled food-particles are 

 wafted backwards, along the dorsal wall of the pharynx to 

 the beginning of the digestive gut. On the dorsal wall there 

 is a ciliated fold — or dorsal lamina — which is sometimes 

 broken up into tongue-like processes or languets. On the 

 other hand, Herdman notices that comparatively few of the 

 endostyle cells are glandular, that much of the mucus seems 

 to be secreted elsewhere, perhaps even from the sub-neural 

 gland, that the food-particles are in all likelihood swept 

 backwards both ventrally and dorsally. As to the morpho- 

 logical importance of the ventral groove or endostylfe, it 

 may be compared with the ventral portion of the respira- 

 tory pharynx in Balanoglossus, with the " hypobranchial 

 groove " to be afterwards described in Amphioxus, and even 

 with the thyroid gland of higher Vertebrates, for that in part 

 arises as a constriction from the ventral wall of the pharynx. 



At the base of the pharynx, lies the opening of the 

 digestive and intestinal part of the gut. This is continued 

 downwards, and on to the other side of the animal where 

 an S-shaped curve is often described by the intestine. With 

 this part of the gut, a glandular mass is often associated, 

 and the wall is folded inwards as it is in the earthworm. 

 Finally, the intestine ends in a cloacal chamber near the 

 exhalent aperture. 



Respiratory System. — Let us now trace the course of the 

 water which was drawn in at the inhalent aperture. Bereft 

 of some of its food-particles, it passes through the numberless 

 ciliated slits in the walls of the pharynx, washing the blood- 

 vessels exposed on these walls. It passes into a peri-branchial 

 chamber which ensheathes the pharynx, and this opens out by 

 the exhalent aperture. This peribranchial chamber is in 

 part at least lined by ectoderm, for it is formed from a modi- 

 fication of the two gill-slits which the larva possesses, and 

 these result from two invaginations of ectoderm which meet 

 two outgrowths of the gut. The slits on the pharynx can 

 hardly be compared with ordinary gill-slits, though they serve 

 the same purpose ; they are secondary structures which 

 replace the two primary gill-slits of the larva, and commun- 

 icate only indirectly with the exterior, namely, through 

 the peribranchial chamber. 



