176 AMPHIOXUS 



behind the atrial pore it is confined to the right side of the 

 body. 



E. The Ccelom or Body-cavity. 



Like the atrial cavity this can only be traced by means of 

 sections. 



The coelom, or body-cavity, is perfectly distinct from the 

 atrial cavity, though its boundaries are not easy to follow, 

 and its relations vary considerably in different regions of the 

 body. It contains during life a coagulable fluid. 



1. Behind the atrial pore the ccelom is a cavity of some 



width, surrounding the intestine and separating it 

 from the body-wall. 



2. In front of the atrial pore it becomes greatly reduced 



owing to the increased size of the atrial cavity : it 

 is, however, readily recognisable as a narrow space 

 immediately surrounding the intestine and the liver. 



3. In the region of the pharynx the coelom becomes much 



subdivided and more difficult to trace. Its chief 

 divisions are as follows : 



a. The dorsal ccelomic canals are a pair of irregular 



spaces lying at the sides of the dorsal part of the 

 pharynx, between the atrial folds and the sus- 

 pensory folds of the pharynx. 



b. The branchial ccelomic canals are a series of tubular 



cavities in the primary gill- arches, lying within 

 the folds into which the atrial epithelium is 

 thrown on the outer sides of these arches. These 

 cavities are very small in the ventral portions of 

 the arches, but widen out towards their dorsal 

 ends, and open, as shown in figs. 32 and 33, 

 into the large dorsal ccelomic canals above the 

 suspensory folds. 



c. The metapleural canals are a pair of wide spaces 



running along the lateral or metapleural folds. 



d. A series of ccelomic spaces surround the reproductive 



organs. 



