66 VERTEBRATE ZOOLOGY 



The mouth and pharynx form a single large cavity, which ex- 

 tends back to the opening of the oesophagus, the pharynx being 

 that part of it which contains the respiratory openings ; that is, 

 the glottis and gill clefts. Note the large lips; also the two 

 rows of teeth in the upper jaw, and the single row in the lower jaw 

 which fits between them. In the floor of the mouth is the short 

 tongue, supported by the large cartilaginous bars which form the 

 hyoid apparatus. These bars can be felt through the mucous 

 membrane of the mouth. Just back of this apparatus and joined 

 with it are the gill arches. Note the gill clefts and probe them. 

 Between the gills in the median line is a small longitudinal slit, 

 the glottis,— the opening into the lungs. It is bounded on the sides 

 by the delicate arytenoid cartilages, which form the primitive 

 larynx. On the roof of the mouth the internal nares will be found 

 between the lateral rows of teeth ; they are a pair of small openings 

 which form the communication between the pharynx and the nasal 

 capsules. Probe them. 



Exercise 4. Draw a sketch of the mouth and pharynx, showing both 

 . floor and roof. 



The Internal Organs. Open the body cavity in the following 

 way: Place the animal on its back in the dissecting pan or on a 

 dissecting board, with the head away from you, and fasten it there 

 firmly by means of a large pin at each end and one through each 

 leg. Then with a sharp scalpel and scissors make an incision 

 through the body wall, a short distance to one side of the median 

 line (in order to avoid cutting the large abdominal vein, which is 

 midventral in position), from the hinder end of the head to the 

 anus, taking care not to cut too deeply, especially at the forward 

 end, where the heart and its great vessels lie near the ventral sur- 

 face. The pectoral girdle and also the pelvic girdle will be cut 

 through by this incision. Pull the two flaps of the body wall apart, 

 but without cutting or disturbing any organs within, and examine 

 the viscera as they lie in the body cavity. 



The body cavity is divided into two very unequal divisions, the 

 larger and posterior of which is the abdominal cavity ; this is lined 

 by the peritoneum, the glistening membrane on its inner surface, 



