ABDOMINAL CAVITY 217 



the meta-pterygium supports twelve or more mucli 

 narrower rays. 



To the outer ends of the fin-rays two or more 

 rows of polygonal plates of cartilage are attached, 

 forming the peripheral portion of the skeleton. 



Horny fibres are present in large numbers, as 

 in the pelvic fins. 



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 

 ^j/perture. 



Open the abdominal cavity by a median ventral incision, 

 ■extending from the pelvic girdle behind to the pectoral girdle 

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

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

 ing-hoard. Gut through the pelvic girdle in the median plane, 

 and prolong the cut backwards to the level of the cloacal aper- 

 ture, but keeping a little to its right side. Wash or sponge 

 cut the abdominal cavity. 



The coelom 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 sUghtly beyond, the cloacal aper- 

 ture, where it communicates with the exterior through the 

 abdominal pores (p. 199). 



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. 



