40 THE Y(WAGE OF H.M.S. CHALLENGER. 



which is composed of two halves separated by means of dorsal and ventral mesenteries. 

 These five spaces are completely separated from one another, and their arrangement is 

 in exact accordance with that of the corresponding sections of the body-cavity in 

 Balanoglossus. The similarity of Cephalodiscus to the same animal is rendered still 

 more striking by the existence in the former of a diverticulum {nch.) of the front part 

 of the alimentary canal, extending a short distance forwards into the region of the 

 proboscis, and homologous with the notochord described by Bateson. 



The course of the alimentary canal is sufficiently explained by means of this figure.^ 

 The mouth (m.) is seen to be overhung by the large proboscis (p.), otherwise known as 



ri6- 2. — Longitudinal section of an adult Cephalodiscus, supposed to be taken sufficiently on one side of the middle line to 

 .lUow of the representation of one of the ovaries, and of one of the proboscis-pores, p., proboscis ; stk., stalk ; m., mouth ; 

 ph., pharynx or branchial region of gut ; ces., oesophagus ; st., stomach ; int., intestine ; a., anus ; nch., notochord (really 

 visible only in a median section) ; cm., ovary ; ord., pigmented oviduct ; ft.c.i, b.c.-, b.c.^, divisions of body-cavity as in 

 fig. 1 ; p.p., one of tlie proboscis-pores ; op., operculum ; n.s., central nervous system. 



the epistome or buccal shield. The dorsal and ventral mesenteries are not visible, since 

 the section is not exactly median. The third body-cavity (6.c.') is very large, and contains 

 the alimentary canal and ovaries. The anterior body-cavity {b.c}) continues to form the 

 lumen of the proboscis, whilst the relations of the collar-cavities are not quite those of 

 the preceding figure. Dorsally the two halves of the body-cavity of the collar have 

 extended forwards into the proboscis region, whilst ventraUy they are to be found 

 (apparently fused ?) solely in the post-oral lamella or operculum. The cavities of the 

 proboscis and of the collar (and more particularly of the latter) are to some extent 



' A fold in the intestinal region of the alimentary canal, of the existence of which I have been informed by 

 Professor M'Intosh, has not been inserted iu the diagram. 



