THE PERCH 20I 



the pectoral fins ; and a posterior pair, the ventral, or pelvic, fins. 

 The former are nearly vertical in position and are situated on the 

 side of the trunk just behind the gill cover. They are supported 

 by a bony arch within the body wall just back of the gills, which 

 is called the pectoral girdle. The pelvic fins are a short distance 

 behind them and are nearly horizontal in position. Note that in 

 all the fins the rays with split tips are segmented. 



Exercise 1. Draw an outline of the right side of the animal ; do not 



draw the scales. Label the organs carefully. 

 Exercise 2. Draw the ventral view. 



The Internal Organs. It will be well first to cut off the sharp 

 tips of the dorsal fins to keep them from hurting the hands. The 

 internal organs will be exposed by removing the left side of the 

 body wall. After placing a probe in the anus to mark it, make a 

 straight incision through the body wall from just in front of the 

 anus to the mouth, cutting through the midventral point of the 

 lower jaw. Care must be taken not to cut the organs which lie in 

 the body cavity. Now lay the fish in a dissecting pan with the 

 head to your left and its ventral side toward you, lift up the cut 

 edge of the body wall with forceps, and with scissors cut away 

 the left body wall and remove it, taking great care not to injure 

 any of the internal organs. The ribs of the left side and the 

 muscles attached to them, the left pectoral fin, and the left pelvic 

 fin will thus have been removed. 



It is best in doing this to work from the midventral incision up- 

 ward, as in this way the internal organs are brought gradually 

 into view as the work proceeds. The liver, intestine, and repro- 

 ductive organs, the latter sometimes very large, will first be seen ; 

 then, dorsal to them, the air bladder. This organ adheres closely 

 to the body wall and special care must be taken not to injure it ; 

 its ventral wall forms a wall across the body cavity which is 

 tough and strong, its lateral walls becoming thinner dorsally. 



Place the animal in a pan of water and examine the organs, 

 without, however, disturbing them. Note the glistening perito- 

 neum, the membrane which lines the abdominal cavity ; it passes 

 along the ventral side of the air bladder. The mesenteries. 



