496 Illinois State LahoratoDj of Natural History. 



From the above it is clear that young fishes in general 

 depend at first on Entomostraca and certain small insect larvae 

 (chiefly those of two genera of gnats), beginning with the 

 smallest of these forms, or with those especially exposed to 

 their attack. One-celled plants and animals are also eaten 

 freely by the young of two of the largest families. 



Correlated with these facts, I find that two at least of the 

 genera, 'which are toothless when adult, have minute raptatorial 

 teeth in this early stagte; viz., Coregonus and Dorosoma. Other- 

 wise young fishes have no apparatus specially adapted to 

 the capture of their minute prey, but this is brought within 

 their reach merely by their own small size and the correspond- 

 ing minuteness of their structures of food prehension. Later, 

 as the larger species grow, this apparatus becomes too coarse 

 to retain objects so minute, but other food resources are made 

 available, usually through some adaptive modification of the 

 fishes themselves. 



In other words, one-celled organisms and Entomostraca are 

 the natural, and practically the only, food of an undifferenti- 

 ated small fish; and to be at liberty to grow, the fish must 

 either change its food (as is usually done) or must develop a 

 special apparatus (commonly a set of fine long gill-rakers) 

 for the separation of Entomostraca from the waters in which 

 they swim. 



Of the fishes which emerge from this earliest stage, through 

 increase in size with failure to develop alimentary structures 

 especially fitted to the appropriation of minute animal forms, 

 some become mud-eaters, like Campostoma and the gizzard 

 shad; a few apparently become vegetarians at once; but most 

 pass into or through an insectivorous stage. After this a few 

 become nearly omnivorous, like the bull-heads; others learn to 

 depend chiefly on molluscan food, — the sheepshead and che 

 red horse species, — but many become essentially carnivorous. 

 In fact, unless the gars are an exception, as they now seem to 

 be, (attacking young fishes almost as soon as they can swallow,) 

 all our specially carnivorous fishes make a progress of three 

 steps, marked, respectively by the predominance of Entomos- 

 traca, of insects, and of fishes, in their food; and the same is 

 true of those strictly fitted for a molluscan diet. 



