THE ANATOMY OF PHORONIS AUSTRALIS. 137 
a modified kind of enteroccele, and is everywhere lined by a 
flattened epithelium, with rather prominent nuclei. The 
celomic fluid contains corpuscles of two sorts (fig. 20), prob- 
ably amoeboid in life. They are not very numerous, and 
perhaps as their only difference is in size, we are scarcely 
justified in distinguishing them. Some of the corpuscles at any 
rate are smaller, and others larger than the blood-corpuscles. 
Both are very finely granular, and contain a small nucleus. 
Some of the corpuscles, however, are more coarsely granular 
than the rest. All are circular in outline in preserved speci- 
mens. They are easily distinguished from the corpuscles 
contained in the blood-vessels by the difference in staining. 
The celom is divided into two very unequal cavities by a 
transverse septum, which passes somewhat obliquely across the 
distal region of the body at the level of the lophophoral ridge. 
The septum is somewhat dome-shaped, with the concavity 
downwards. The oral edge is slightly lower down than the 
anal edge. The anus and rectum lie completely outside of, 
whilst the cesophagus perforates, the septum (fig. 86). The 
smaller division of the ccelom, the supraseptal cavity, is con- 
tinued into the tentacles and epistome. The larger infra- 
septal cavity is the general body-cavity, and is further sub- 
divided by the mesenteries already mentioned. 
The chambers are all in communication proximally, as the 
mesenteries cease before the apex of the ventral surface is 
reached. The relations of these mesenteries has already been 
accurately described by Caldwell. 
The histological structure of the septum is of some interest ; 
it appears to be a sort of cartilage (fig. 22). It consists of 
a nearly homogeneous, dense matrix, sometimes fibrous, in 
which are embedded small spindle-shaped cells with rounded 
nuclei. Here and there are larger and smaller spaces, lined 
by cells, which appear to place the supraseptal cavity in com- 
munication with the infraseptal cavity. 
The basement tissue and the cesophageal mesentery are of 
the same nature as the septum, but the cells are rare. The 
two lateral mesenteries are so thin that I have been unable 
