672 



EVOLUTION OF ANIMALS 



Part V 



Fig. 33.10. A scale from a seven pound female rainbow trout (Salvelinus 

 gairdnerii) taken at spawning time. May 20, in Paul Creek, British Columbia. 

 The age is indicated by winter rings (7-7) showing slowed growth in such 

 periods of low food supply. The ring at 4 is a typical example of a spawning mark, 

 when feeding stops and life is strenuous. This fish probably did not spawn in its 

 fifth (5) year, but did so again in its sixth (6). The dark part of the scale without 

 rings is embedded in the flesh. (After Mottley. Courtesy, Needham: Trout Streams. 

 Ithaca, N. Y., Comstock Publishing Co., 1940.) 



products. The silvery sheen on the undersides of fishes also is due to guanin. 

 Some fishes show remarkable changes of color when against different back- 

 grounds. Sunfishes and others are brilliantly colored during the breeding 

 season. 



Skeleton. Fishes are the early models of vertebrates. The main parts of 

 the skeleton are the skull, vertebral column, the pectoral and pelvic girdles, and 

 the pair of pectoral fins with the pelvic fins behind them. There are other fins 

 but these are the most important to the skeleton (Fig. 33.8). The pairs are far 

 apart in most primitive bony fishes such as trout, and closer together in the 

 more specialized yellow perch. 



The great feature of the skeleton is the strength and flexibility of the chain 

 of vertebrae that form the backbone (Fig. 33.11). Its weakness is with the 

 paired fins and their girdles that are not attached to the backbone, but are only 

 embedded in the flesh. This arrangement is adequate for the fishes that do 



