ATEIAL CAVITY AND COELOM. 187 



each of the gill-arches, and invests the liver and the anterior 

 portion of the intestine. 



On the primary gill-arches this layer of atrial epithelium 

 is wider than on the secondary arches, and towards the dorsal 

 surface of the pharynx it rises up to form the suspensory folds 

 of the pharynx, which run across from the dorsal part of the 

 sides of the pharynx to the inner surface of the atrial folds 

 and separate the atrial cavity below from the dorsal coelomic 

 canals above. 



The atrial cavity commences opposite the anterior end of 

 the pharynx, and extends back almost as far as the anus ; 

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

 body. 



F. The Coelom or Body-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 differ considerably in different regions of the 

 body. It contains during life a coagulable fluid. 



1. Behind the atrial pore the coelom 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, easily recognisable as a narrow space im- 

 mediately surrounding the intestine and the liver, 



3. In the region of the pharynx the coelom is much sub- 

 divided and more difficult to trace. Its chief divi- 

 sions are as follows : — 



a. The dorsal coelomic canals are a pair of wide 



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 coelomic canals are a series of tubular 



cavities in the primary gill-arches, lying within the 



