38 



W. G. KIDEWOOD. 



passing along the mesial wall of the collar eoelom, are seen in transverse section 

 (mu. lb.), and the fibres of the collar canal muscle (mu. 4), the ' problematical body' 

 of Harmer, are cut obliquely, but more longitudinally than transversely. A special 

 section of the paper is devoted to a consideration of this body (p. 41). 



The section drawn in text-fig. 14 is taken only slightly behind that represented 

 in text-fig. 13. It cuts the hind edge of the mouth almost tangentially (se. t.) ; 

 but if the polypide dies with its mouth widely open, a transverse section taken 

 through the gill-slits will pass also through the mouth opening. The small septum 



mu 



mu 



mu 



c.co. 



po 



m. 



pc. 



TsxT-PiGUBE 13. Section of a polypide of Cephalodiscus nigrescens taken transversely to the length of the body, and 

 passing through the collar canals and the mouth aperture. 



b.s. = thickened ventral wall of the buccal shield ; c.ca. = collar canal ; c.co. = collar eoelom ; g.d. = gastric 

 diverticulum ; go. = gonad ; i. = intestine ; m. = mouth ; p.c. cavity of the buccal shield, proboscis cavity ; 

 T>h. = dorsal wall of pharynx; pi. = pleurochord; po.l. = lateral flap of the postoral lamella; mu.U, mu.B, muA, 

 mu.5 = muscle (see text). The tract marked muA is the problematical body of Harmer. n.t.Z, n.t.3 = nerve 

 tracts ; t.c. = trunk eoelom. 



between the right and left collar cavities is situated immediately posterior to the 

 part marked se. t. By the side of se. t. are seen some of the median ventral muscle 

 fibres (text-fig. 10, mu. 2) passing forward obliquely into the anterior horns of the 

 trunk eoelom, and some of the fibres of the oral muscle in the collar cavity are 



