38 



W. G. RIDEWOOU. 



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

 (mil. lb.), and the fibres of the collar canal muscle {mil. 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 taugentially {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 



-pl 



~-n,b.3. 



c.ca. 



c.co. 



Tkxt-Figuee 13. — Section of a polypide of Cephalodiscus nigrcscens taken transversely to the length of the body, and 

 passing through the coUar canals and the mouth aperture. 



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

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

 vh. = dorsal wall of pharynx ; pi. = pleurochord ; po.l. = lateral flap of the postoral lamella ; mti.lb, mu.3, tnu.i, 

 mu.5 = muscle (see text). The tract marked mu.i is the problematical body of Harmer. n.i.2, 7i.t.3 = nerve 

 tracts ; t.c. = trunk coelom. 



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



