Food Relations of Fresh- Wafer Fishes. 509 



Some mediate correlations are inverse, others coincident, 

 the related structures varying oppositely or in the same direction. 

 An interesting inverse correlation is exhibited by the gill- 

 rakers and the pharyngeals in the suckers; as the former 

 lengthen and multiply, the latter become weaker and bear 

 smaller and more numerous teeth. The cause of this corre- 

 lation is seen in the food, the species with heavy pharyngeals, 

 few and large pharyngeal teeth, and few and short gill-rak- 

 ers being mollusk feeders, and the other group depending 

 largely on insects and crustaceans and using mollusks sparely, 

 and then only the small and thin-shelled sorts. A similar in- 

 verse relation is seen between the large mouths and the weak 

 pharyngeals of many piscivorous fishes; between the weak 

 pharyngeals and the muscular stomach of the gizzard shad; and 

 between the long gill-rakers and the rudimentary pharyngeals 

 of the shovel fish. Such correlations are often evidence of a 

 specialization and corresponding limitation of the feeding habit, 

 — the increased efficiency of one structure corresponding to the 

 increased importance to the fish of the related kind of food, and 

 the defective development of the correlated structure indicating 

 an abandonment of the food for whose appropriation it was 

 especially fitted. On the other hand, the absence of these in- 

 verse correlations marks an omnivorous habit, — as in the cat- 

 fishes, whose jaws, teeth, gill-rakers, and pharyngeals are all 

 moderately developed, while the food is correspoudingly indis- 

 criminate. 



