STRUCTURE OF CEPHALODISCUS 



25 



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- 

 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 

 diverticulum, the notoehord, 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,^ 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 {si), and this by the intes- 

 tine (inf), which opens by the anus 

 (a) near the front end of the body. 



The trunk contains paired third body -cavities (6.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 



Fig. 11. — Longitudinal section 

 througli CepJudodiscus dode- 

 calophus, passing through the 

 two sides of the hody ; a, 

 tentacular arm; 6.c^ collar- 

 cavity ; h.c '', trunk - cavity ; 

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

 intestine ; n.s, central nervous 

 system ; 0, 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. 



