ANATOMY OF CEPHALODISCUS NIGRESCENS. 



39 



seen passing backward obliquely to complete their course along the septum between 

 the right and left collar cavities (text-fig. 10, mu. \c.). 



The gill-slit slopes from the pharyngeal cavity ventro-laterally and in the 

 transverse plane. The collar canal slopes from the collar cavity dorso-laterally, 

 and slightly backward. The external opening of the collar canal is more dorsal 

 than that of the gill-slit, and a little more anterior. The gill-slit is such a 

 short distance behind the collar canal that only occasionally, as is shown on the 

 right side of the text-fig. 14, is the canal wall missed entirely; more usually 



muS. 



ceo 



Text-Pigcbe 14. — Section of a polypida of Cephalodiscus nigrescens taken transversely to the length of the body, and a 

 little farther back than that shown in text-fig. 13. It passes through the giU-slits, and immediately behind the mouth. 

 b.s. = thickened ventral wall of the buccal shield ; c.ca. = part of the posterior wall of the collar canal cut 

 tangentially ; c.co. = collar coelom ; g.d. = gastric diverticulum ; go. = gonad ; g.s. = gill-slit ; i. = intestine ; 

 ph. = dorsal wall of pharynx ; jjI. = pleurochord ; po.l. = postero-lateral flap of the post-oral lamella bent over 

 towards the mouth ; mu.5, mu.Q = muscle (see next) ; n.t.3 = longitudinal nerve tract ; set. = subepidermal 

 tissue behind the mouth, cut tangentially ; t.c. — trunk coelom. 



the tangential section of the external part of the canal wall is included 

 (text-fig. 14, c. ca.). 



As in the previous figure, the trunk cavity is conspicuous, and is occupied, at 

 this particular level of section, by the two gonads, the intestine and the anterior 



