COELOM OF CEPHALODISCUS NIGRESCENS. 33 



while the stalks of buds are not, in other words, why the stolon is so much more 

 acutely sensitive than the stalks of the buds to those irritant influences which cause 

 contraction, remains yet to be answered. 



Coelom. 



There are, as in C. dodecalophus, five divisions of the coelom ; one in the 

 buccal shield (proboscis cavity), opening among the bases of the plumes by two 

 small pores (proboscis pores), and having a pericardial cavity lying within it ; a 

 pair of cavities in the anterior part of the body (collar cavities) continued into the 

 post-oral lamella and into the plumes ; and a pair of large trunk cavities. 



The collar cavities are a little smaller than the prol)oscis cavity. In their 

 middle portions, just dorsal to the pedicle of the buccal shield, they come close 

 together, and are separated by a small, thin mesenteric fold ; they extend into 

 the short pedicle of the buccal shield, and are separated from the proboscis 

 cavity by a thin wall of a similar nature. The two collar cavities come 

 together also l)ehiud the mouth, where they are separated by a small mesenteric 

 septum only. Each collar cavity opens on to the exterior by a narrow tubular 

 passage, which is lined, by ciliated epithelium, and curves round and opens 

 dorso-anteriorly to the gill slit. Posterior to this " collar-pore " is a ridge of 

 specialised epithelium, consisting of tall columnar cells, presumably of a sensory 

 nature. The collar canal marks the posterior limit of the collar cavity. 



The separation of the two large trunk cavities (abdominal cavities) is 

 probably in all cases incomplete owing to the imperfection of the mesenteric 

 fold. In the anterior part of the trunk coelom there is a fold which extends 

 from the pharynx to the gastric caecum, and from the sides of this fold there 

 pass out short folds to the anterior portions of the two gonads. Behind the 

 mouth, but anterior to the point at which the stolon leaves the body, is 

 another fold, extending from the pharynx to the ventral body wall. A similar 

 fold is found between the intestine and the dorsal body wall, but there appears 

 to be none between the stomach and the intestine, which seem invariably to be 

 in close contact. The trunk cavity is not definitely continued into the stolon, 

 for this is largely choked up with coelomic corpuscles, but a continuation of 

 the mesenteric fold can be recognised in the basal part of the stolon, extending 

 from the one blood-vessel to the other. 



General Internal Structure. 



In the present section of the paper a general account is given of the internal 

 organs, based mainly upon the appearances which they present in sections of the body 

 taken through structures of particular interest. Six such sections are drawn in 

 text-figures 10-15. These figures are composite figures, each constructed from sections 



