STRUCTURE OF CEPHALODISCUS 



Tl.S 



pairs of plume-like arms, which bear an immense number of 

 piunately-arranged tentacles. The arms, which may end in a 

 swollen bulb, 1 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- 

 culum (Fig. 11, op), in front of which 

 is the mouth, and behind it the gill- 

 slits (g} and collar - pores (c). The 

 central nervous system (n.s) is 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 s- 

 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 (cf. p. 52) of Tunicates. 



The next part of the alimentary 

 canal, the pharynx, 2 has a single pair 

 of simple gill -slits opening to the 

 exterior immediately behind the collar- 

 pores. The short oesophagus (Fig. 

 10, oes) is followed by the wide 

 stomach (st\ and this by the intes- 

 tine (inf), which opens by the anus 

 () near the front end of the body. 



The trunk contains paired third body -cavities (b.c 8 ), 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 



V 



FIG. 11. Longitudinal section 

 through Cephalodiscus dode- 

 calophus, passing through the 

 two sides of the body ; a, 

 tentacular arm ; b.c", collar- 

 cavity ; b.c 3 , trunk - cavity ; 

 c, collar-pore ; g, gill-slit ; i, 

 intestine ; n.s, central nervous 

 system ; o, oesophagus ; op, 

 operculum ; p, pharynx ; s, 

 stomach. 



1 Cf. Cole, J. Linn. Soc. xxvii. 1899-1900, p. 256. 



2 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. 



