CYPRINID^ THE MINNOWS AND THE CARP 95 



The minnow family, much the largest and most complex of the 

 fish families of the state, has become variously differentiated in 

 respect to habits, ecological relations, and some of its more impor- 

 tant structures, in a way to adjust the group with considerable ex- 

 actness to the various features of its environment. In respect to 

 territorial distribution, we may distinguish among the minnows a 

 group distributed mainly through the Mississippi drainage, another 

 mainly through the Ohio drainage, and a third which is generally 

 distributed throughout the state. We may also distinguish a 

 group of species which does not enter or remain in the persistently 

 turbid waters of the southern Illinois region covered by the fine- 

 grained drift of the lower Illinoisan glaciation ; another group which 

 is common in the lowland lakes, and a much larger group which is 

 rarely found in lakes of any kind ; a group of minnows which prefer 

 large ri\'ers, and another which is most abundant in the smaller 

 streams; one more than normally common over a mud bottom, and 

 another evidently most at home over a bottom of rock and sand; 

 one which prefers a swift current, a:nd another which seeks quiet 

 waters. 



The various species of the family show also considerable dift'er- 

 ences of preference in respect to the kinds of food which they choose 

 from the general supply offered to them. They are mainly carniv- 

 orous, on the whole, in this country, although we have found fishes 

 and mollusks only rarely in the food of our native species. Insects 

 and crustaceans, including Entomostraca, are their principal de- 

 pendence, except for a few w^hich eat largely of vegetation and a few 

 others which feed almost wholly on the highly organic mud of the 

 bottoms of our ponds and streams. The special structures of ali- 

 mentation correspond in their variations, in the several divisions of 

 the family, to these difiierences of their food. 



Fishes so small as most of our minnows, are, as a rule, in no need 

 of a specially developed set of gill-rakers,- since the gill-arches them- 

 selves are so small and the spaces between them so narrow that 

 any object large enough to be useful for food is little likely to be 

 carried out through the gills with the respiratory current. In two 

 of our species, however (Abrafnis crysoleucas and Notropis hetero- 

 cfo«) , the gill-rakers are considerably developed, and in these species 

 Entomostraca appear more largely in the food than in any other 

 minnows. Even Protozoa and unicellular algas have been found 

 common in the stomachs of N. heterodon. 



The intestine varies greatly in length, being longest in the mud- 

 eating minnows and shortest in those dependent wholly or mainly 



