40 



THE VOYAGE OF H.M.S. CHALLENGER. 



Tided with an anterior, undivided body-cavity, the collar and the trunk each containing 

 a body-cavity 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 



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

 allow of the representation of one of the ovaries, and of one of the prohoscis-pores. j)., proboscis ; stk., stalli ; »«., mouth ; 

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

 visible only in a median section) ; m., ovary ; <md., pigmented oviduct ; h.c.\ b.c,^, b.c.^, divisions of body-cavity as in 

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



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. 



