1 78 AMPHIOXUS 



surface of the pharynx it rises up into folds. Two or more of 

 these folds unite on each side to form the suspensory folds of 

 che 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 helow 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. 



E. The Coelom or Body-cavity. 



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

 sections. 



The coeloiri, 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 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, readily recognisable as a narrow space im- 

 mediately 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 coelomic 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 coelomic 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 



