ANATOMY OF CEPHALODISCUS NIGRESCENS. 



37 



branchial nerves. The shield shows nerve tracts in both its ventral and dorsal walls ; 

 that of the former is a continuous sheet, that of the latter is paired. 



The section passes tangentially through the anterior part of the " visceral mass " 

 of the body, and includes sections of the gonad ducts and of the terminal portion of 

 the intestine. Had the section been taken a little farther back the spacious trunk 

 cavity would have come into view. 



Text-fig. 1 3 shows a section taken through the mouth and the collar canals. Such 

 a section passes behind the stalk of the buccal shield, and shows a very thick dorsal 

 wall to the pharynx, with lateral parts, the pleurochords (pi), of a paler tint and with 

 a strong groove on their inner surface. The lateral flaps of the post-oral lamella 

 (po. I.) are shown as bent over towards the mouth, a relation which they frequently 



.b.l. 



c.n.m.- _ 



~--mu.1 a. 



no. 



TEXT-FIGDBB 12. Section of a polypide of Cephalodiscus nigrescens taken transversely to the length of the body, and a 

 little farther back than that shown in text-fig. 11. It passes through the front part of the notochord and the hinder 

 part of the heart. 



a. = anus ; b.l. = base of the lophophoral system ; b.s. = thickened ventral wall of the buccal shield ; 

 b.w. = anterior body- wall, cut tangentially ; c.n.m, = central nerve mass ; d.b.s. dorsal blood-sinus ; d.g. = duct 

 of the gonad ; h. = heart, lying within the pericardium ; mu.la. = muscle fibres passing along the side of the 

 septum between the right and left collar cavities ; no. = notochord, in transverse section ; n.t.2 = nerve tract ; 

 p.c. = cavity of the buccal shield ; proboscis cavity. 



possess, but in some slides the flaps are outwardly directed, so that their free edges 

 are near, or succeed in covering, the openings of the collar canals. 



The collar cavity is here fairly spacious (c. co.) and is seen to extend into 

 the post-oral lamella, the two walls of which are connected here and there by 

 fine coelomic trabeculae with lateral, spherical nuclei. The epithelium of the dorsal 

 wall of the collar canal is taller than that of the ventral wall, and the internal 

 opening of the canal is smaller than the external. The fibres of the oral muscle, 



