THE PERCH 69 



called operdes. How many are there? Along the lower 

 and posterior edge of the operculum is a membrane sup- 

 ported by seven parallel rays. This is the branchiostegal 

 membrane and rays. 



Make a drawing of a side view of the perch, showing all 

 the foregoing points possible. 



Raise one of the gill covers and note the position of the 

 gills. How many are there? Open the mouth widely and 

 depress the tongue to determine the connection between the 

 gill cavity and the mouth. Remove most of the gill cover. 

 Note that each gill consists of a bony arch with comblike 

 filaments on the posterior edge and teethlike organs, the 

 gill rakers, on the anterior edge. The slits between the 

 gills are the gill clefts. How many gill clefts are there? 



Draw a complete gill, showing arch, filaments, and rakers. 



Again open the mouth wide and note its size. Note the 

 flat, short tongue. Find the teeth on the upper and lower 

 jaws. What shape and size are they ? Which way do they 

 point ? What purpose would they seem to serve, judging 

 from their size and the way they point? 



C. INTERNAL ORGANS. Make a shallow incision from 

 just in front of the anal opening along the ventral wall of 

 the abdomen through the tip of the lower jaw, being care- 

 ful not to cut into the internal organs. Now make a deep 

 cut with the scalpel along the lateral line on the left side 

 from behind forwards and then remove one whole side of the 

 abdomen being very careful not to injure the air bladder. 



Note the spinal column along the dorsal side of the ab- 

 dominal cavity. The liver, intestines, and other organs lie 

 in the lower part of the abdominal cavity, while the thin 

 air bladder lies in the upper part and its ventral wall 

 forms a wall across the body cavity. 



