36 VERTEBRATE ZOOLOGY 



pectoral fins ; and a posterior pair, the ventral fins. The former are 

 nearly vertical in position and are situated on the side of the 

 trunk just behind the opercle. They are supported by a bony 

 arch within the body wall just back of the gills, which is called 

 the pectoral girdle. The ventral fins are a short distance behind 

 them and are nearly horizontal in position. In the more primi- 

 tive fishes the ventral fins are situated just in front of the anus. 

 Note that in all the fins the rays with split tips are segmented. 



Exercise 1. Draw a view 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. Make then a deep cut downward and inward 

 along the lateral line on the left side of the body until the scalpel 

 strikes the spinal column or the basal portion of the ribs ; cut al- 

 ways against the outer edges of the scales, from behind forward. 

 When the scalpel will go no farther, pass it ventrally along the 

 outer surface of the ribs and remove the muscles of the side of 

 the body. 



Determine now definitely the position of the spinal column, 

 since it marks the dorsal boundary of the abdominal cavity, and 

 carefully cut away the entire left body wall, removing thus the 

 ribs (with any muscles which may still be attached to them), the 

 left pectoral fin and pectoral girdle, and the left ventral fin. It 

 is best in doing this to work from the midventral incision upward, 

 as in this way the internal organs are brought gradually into view 

 as the work proceeds. The liver, intestine, and reproductive or- 

 gans, which are 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; 



