CEPHALODISCUS. 



107 



mouth- opening which lies between it and the base of the proboscis 

 stalk. The arms are prolongations of the dorso-lateral parts of 

 the collar commencing at the point where this lip-like projection 

 passes into the dorsal part of the collar. They are tentaculi- 

 ferous and vary in number from four to six pairs (one pair in 

 the males of C. sibogae). In 

 C. dodecalophus they are 

 slightly swollen at their ex- 

 tremities. 



Each arm contains a pro- 

 longation of the collar body- 

 cavity (Fig. 79) and is grooved 

 on its ventral surface, the 

 grooves uniting in pairs and 

 converging to the corners of 

 the mouth, near which they 

 terminate. 



The ectoderm of the swellings at 

 the end of the arms of C. dodecalo- 

 phus contains a number of ovoid 

 globules of a clear substance which 

 have been variously interpreted as 

 organs akin to the rhabdites of 

 Turbellaria, and as the refractive 

 structures of a rudimentary visual 

 organ. Similar refringent vesicles 

 are found along the whole course 

 of the arms of the males of C. 

 sibogae. 



FIG. 79. Transverse section through Cephalo- 

 discus dodecalophus (after Harmer). 1 central 

 nervous system of the collar region ; 2 oper- 

 culum turned backwards ; 3 pharynx ; 4 

 oesophagus ; 5 stomach ; 6 intestine ; 7 

 trunk-coelom ; 8 gill-slit ; 9 collar-pore ; 10 

 collar-coelom ; 11 arm. 



Behind the collar region is 

 the trunk, the ventral portion 

 of which is prolonged into the 



huge hump before referred to. The single pair of gonads 

 (Fig. 78, J5) are placed in the anterior part of the trunk, 

 just behind the collar, and open dorso-laterally one on each 

 side. The anus is at the hind end of the dorsal surface 

 (Fig. 78, r6). The proboscis presents a broad disc-like surface 

 towards the front (Fig. 77) and is marked by a crescentic band 

 of pigment in its ventral portion (j). Its anterior wall is covered 

 with a glandular ectoderm, which appears to play some part in 

 the formation of the tube, as in RJidbdopleura. The mouth is on 

 the ventral side at the junction of the proboscis and collar (Fig. 78) 



