FISHES 125 



attained this condition by growth from side to side, instead of 

 dorso-ventrally. 



93. Some differences in the fins of fishes. We have seen that 

 the goldfish has one dorsal fin, the perch two, and that both fishes 

 have a single anal fin. A glance at Figure 108 will show that the cod- 

 fish has three dorsal fins and two anal fins. Dorsal and anal fins 

 vary not only in number, but in extent. In some fishes they are 

 very short, as in the mackerel (Fig. 95), while in the flounder (Fig. 

 93) these fins extend nearly the whole length of the dorsal and 

 ventral surfaces. 



Most common fishes possess both pectoral and pelvic fins, but 

 in the eels (Fig. 96) the pelvic fins are entirely wanting and the pec- 



FIG. 96. Eel. (Jordan and Evermann. Courtesy of Doubleday, 

 Page & Co.) 



toral fins are very small. The paired fins vary in position as well. 

 In the perch, for example (Fig. 90) the pelvic fins are immediately 

 below the pectorals, while in the cod (Fig. 108) they are anterior 

 to the pectoral fins, and in the salmon (Fig. 107) they are even farther 

 back on the body than in the goldfish. 



94. Adaptations for swimming. Laboratory study. 



1. Carefully watch for a time a goldfish when it is swimming 



around in a large battery jar or aquarium. 



a. Which of the three regions of the body is principally 



.used in pushing the animal forward ? 



b. Describe the 'movements of this body region. 



2. WTiich of the paired fins are used in swimming? De- 



