THE LOWBll VBUTEBKATA. 



173 



ocr*cro*r 



The pericardial cavity is ventral to the pharnyx (fig. 89), 

 and its outer membrane no longer hangs like a bag in the 

 coelom : it forms a 

 vertical partition 

 between pericardial 

 and peritoneal 

 cavities. This par- 

 tition is apt to be 

 mistaken for a f'* 



diaphragm like the c "-*- ' 



rabbit's. This it 

 is not, for the 

 cavity anterior to 

 it contains the 

 heart alone, and 

 is wholly ventral 

 to the oesophagus. 

 At its dorsal end 

 this partition is 

 imperfect, and a 

 pericardio-peri- 

 toneal passage 

 unites pericardium 

 to coelom. At its 

 posterior end the 

 coelom communi- 

 cates directly with 

 the exterior by a 

 pair of abdominal 

 pores, one on each 

 side of the cloaca. 

 Their function is 

 not very obvious : 



Fig. 90. ALIMENTARY CANAL OF DOGFISH. 



i mouth and pharynx have been cut open (as in 

 fig. 68). The curved rows of dots in front of the 

 mouth represent the pores of the sensory tubules. 



possibly by allowing some of the coelomic fluid to escape 

 when the external pressure on the body- wall increases (as 

 when the dogfish descends to some depth in the sea) they 

 may prevent the increased pressure being felt by the viscera. 



6. The Alimentary Canal is simpler than in the frog. It 

 is a simple tube with one fold, giving it an N shape (fig. 90). 



