276 ELEMENTARY ZOOLOGY 



scarlet, blue, creamy white, are developed in the breeding 

 season, the then vigorous males being the most highly 

 colored. Many of the feeble minnows even become very 

 brilliant in the nuptial season of May and June. Color in 

 fishes is formed by minute oil-sacs on the scales, and it 

 often changes quickly with changes in the nervous condi- 

 tion of the individuals. 



Development and life-history. The breeding habits 

 of fishes are extremely varied. Most fishes do not pair, 

 but in some cases pairing takes place as among higher 

 animals. Ordinarily fishes lay their eggs on the bottom in 

 shallow water, either in brooks, lakes, or in the sea. The 

 eggs of fishes are commonly called spawn, and egg-laying 

 is referred to as spawning. The spawn of some fishes is 

 esteemed a special food delicacy. Spring is the usual time 

 of spawning, though some fishes spawn in summer and 

 some even in winter; generally they move from their usual 

 haunts for the purpose. The eggs of the different species 

 vary much in size, ranging from an inch and a half in 

 diameter (barn-door skate) down to the tiniest dots, like 

 those of the herring. The number of eggs laid also varies 

 greatly. The trout lays from 500 to i ,000, the salmon 

 about 10,000, the herring 30,000 to 40,000, and some 

 species of river fish 500,000, while certain flounders, 

 sturgeons, and others each lay several millions of eggs. 

 The adults rarely pay any attention to the eggs, which are 

 hatched directly by the heat of the sun or by heat absorbed 

 from the water. The length of incubation varies much. 

 When the young fish leaves the egg-shell it carries, in the 

 case of most species, a part of the yolk still hanging to 

 its body. Its eyes are very large, and its fins are repre- 

 sented by thin strips of membrane. It usually undergoes 

 no great changes in development from the first, resembling 

 the adult except in size. But some of the ocean fishes 



