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. 1c.). 
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 
rr Danity 
FNM 
S = 
QT 
Tuxt-FiguRE 14.—Section of a polypids 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 gill-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; 7. = intestine ; 
ph. = dorsal wall of pharynx; pl. = pleurochord ; po.J. = postero-lateral flap of the post-oral lamella bent over 
towards the mouth; muw.5, mu.6 = muscle (see next); .t.38 = longitudinal nerve tract; se.t. = subepidermal 
tissue behind the mouth, cut tangentially; ¢.c. = trunk coelom. 
the tangential section of the external part of the canal wall is included 
(text-fig. 14, ¢. 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 
