240 THE DOG-FISH. 



III. THE ABDOMINAL CAVITY AND VISCERA. 



Lay the dog-fish on its back on the dissecting -board, and 

 pin it down through the fins. Determine the positions of the 

 pectoral and pelvic girdles by feeling them through the skin, 

 the pectoral girdle being a little in front of the pectoral fins, 

 and the pelvic girdle a short distance in front of the cloacal 

 aperture. 



Open the abdominal cavity by a median ventral incision, 

 extending from the pelvic girdle behind to the pectoral girdle 

 in front. Cut through the body-wall transversely, just behind 

 the pectoral girdle, and pin the two flaps down to the dissect- 

 ing -board. Cut through the pelvic girdle in the median plane, 

 and prolong the cut backwards to the level of the cloacal aperture, 

 keeping a little to its right side. Wash or sponge out the abdo- 

 minal cavity. 



The ccelom or body-cavity extends forwards into the head 

 in the embryo, but in the adult is reduced to the pericardial 

 cavity which lodges the heart, and the abdominal cavity in 

 which he the principal viscera. The abdominal cavity is a 

 wide space extending from the level of the pectoral girdle 

 backwards as far as, or slightly beyond, the cloacal aperture, 

 where it communicates with the exterior through the abdo- 

 minal pores (p. 221). 



Pass a seeker from the abdominal cavity through one of 

 the abdominal pores to the exterior. 



Identify the positions and relations of the following organs 

 before proceeding further with the dissection. 



1 . The liver is a very large solid organ of a brownish yellow 



colour, attached by a median suspensory ligament to 

 the anterior wall of the abdominal cavity. It consists 

 of two lobes of about equal size, which are united 

 in front, and extend back along the sides of the ab- 

 dominal cavity for the greater part of its length. 



2. The stomach is a wide U shaped tube lying between the 



