1 STRUCTURE OF CZPAALODISCUS 
iS) 
wal 
pairs of plume-like arms, which bear an immense number of 
pinnately-arranged tentacles. The arms, which may end in a 
swollen bulb,’ have ventral grooves along which food doubtless 
travels to the mouth by ciliary currents. The anterior edge of the 
ventral half of the collar is drawn out into a narrow flap or oper- 
eulum (Fig. 11, op), in front of which 
is the mouth, and behind it the gill- 
slits (g) and collar-pores (¢). The 
central nervous system (7.s) 1s a thick 
mass of nerve-tissue in the dorsal 
epidermis of the collar; it is not sunk 
beneath the skin as in Balanoglossus. 
The details of the nervous and vascular 
systems, and the development of the 
buds, have been described by Master- 
man. In the dorsal region of the collar 
the alimentary canal has a_ slender 
diverticulum, the notochord, which 
passes into the base of the proboscis; 
it is believed by Masterman to have a 
function similar to that of the neural 
gland (ef. p. 52) of Tunicates. 
The next part of the alimentary 
Fic. 11.—Longitudinal section 
canal, the pharynx,’ has a single pair 
anal, pharynx,” has ¢ gle pe 
of simple gill-slits opening to the 
exterior immediately behind the collar- 
pores. The short oesophagus (Fig. 
through Cephalodiscus dode- 
calophus, passing through the 
two sides of the body; a, 
tentacular arm; 0.c*, collar- 
cavity; b.c®, trunk - cavity ; 
c, collar-pore ; g, gill-slit ; 7, 
intestine ; 72.s, central nervous 
system ; 0, oesophagus ; op, 
operculum; y, pharynx; s, 
stomach. 
10, oes) is followed by the wide 
stomach (st), and this by the intes- 
tine (int), which opens by the anus 
(a) near the front end of the body. 
The trunk contains paired third body-cavities (.c*), the 
septum between which and the collar-cavities is slightly behind 
the line of origin of the operculum. Two ovaries (ov) are 
situated between the pharynx and the last part of the intestine, 
each opening to the exterior dorsally between the central nervous 
1 Cf. Cole, J. Linn. Soc. xxvii. 1899-1900, p. 256. 
* Two dorsal portions of this region, which are regarded by Masterman as 
lateral notochords, appear to me to represent the dorsal part of the pharynx of 
Ptychodera. 
