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 proboscis 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 behind 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 
