LAMPREYS AND FISHES 



Fig. 269. — Diagram of a fish: i, mandible; 2, pre- 

 maxillary; 3, nostrils; 4, opercle or gill cover; 5, 

 pectoral fin; 6, dorsal fin; 7, ventral or pelvic fin; 8, 

 anal fin; 9, caudal fin; lo, lateral line of sense organs; 

 II, scales. 



The fins of fishes are thin membranes supported by stiff 

 spines or by finely segmented rods, the fin rays, which are 

 often branched (Fig. 269). Fins are very constant in num- 

 ber and position. Just back of the head are the paired pector- 

 al fins, and further backward, low down on the sides of the 

 body, are the paired pelvic or ventral fins ; others — the dorsal, 

 anal, and caudal fins — are single. 



The opercle forms part of a fish's cheek and covers the four 

 blood-filled gills, that are often visible when it is lifted in 

 breathing. The jawbones, mandible and movable premaxil- 

 lar^'-, usually bear sharp teeth, and besides these, fishes have 

 several patches of short teeth on the roof of the mouth. 



Breeding habits. — As the spawning season approaches, 

 fishes move toward the shores of ponds andlakes or upstream 

 in the creeks. Some of them, such as perch and sunfish, only 

 swim in among nearby weeds or protecting stones. The 

 journeys vary from such short ones to the famous migrations 

 of river salmon, extending over hundreds of miles ; but they all 

 end at spawning grounds or nesting sites. During this time 

 color differences between male and female appear or become 

 more marked. Fins of the male darters redden, the colors 

 of male sunfish (PL XVIII) take on the brilliance of a hum- 



325 . 



