38 W. G. RIDEWOOD. 
passing along the mesial wall of the collar coelom, are seen in transverse section 
(mu. 1b.), 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. ¢.) ; 
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 
g.d. 
\ / 
‘N \ / 
\ \ / 
\ 
Text-FiaurE 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 coelom; g.d. = gastric 
diverticulum; go. = gonad; i. = intestine; m. = mouth; p.c. = cavity of the buccal shield, proboscis cavity ; 
vh. = dorsal wall of pharynx; pl. = pleurochord; po.l. = lateral flap of the postoral lamella; mu.1b, mu.3, mu.4, 
mu.5 = muscle (see text). The tract marked mu.4 is the problematical body of Harmer. 1.1.2, n.t.3 = nerve 
tracts; ¢.c. = trunk coelom. 
between the right and left collar cavities is situated immediately posterior to the 
part marked se, ¢. 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 
