96 FISHES OP ILLINOIS 



The intestine varies greatly in length, being longest in the 

 mud-eating minnows and shortest in those dependent wholly 

 or mainly on animal food. In Campostoma, a typical mud- 

 eater, it is five to nine and a half times the length of the head 

 and body, and is wound spirally about the air-bladder, while in 

 the more strictly insectivorous genera it is only two thirds to 

 five sixths as long as the head and body taken together. The 

 mud-eating forms also differ from the others in the fact that the 

 pharyngeal teeth have a large grinding surface at the free end, 

 and are without the terminal hook-like processes with which 

 those species are provided which feed mainly on insects. 



Although the cyprinoids are mostly of small size, the 

 European carp and a few native species, some of which are abun- 

 dant on the Pacific slope in America, attain a considerable weight. 



There are some two hundred genera in the world and about 

 a thousand species. In Illinois there are fourteen genera and 

 thirty-six species known, seventeen of the latter belonging to 

 the single genus Notropis. All our native species are small and 

 commercially insignificant except as they are used for bait and 

 serve as a valuable food resource for other fishes. The top of 

 the head in spring males, and often also the fins and sides 

 particularly the sides of the caudal peduncle are covered with 

 small tubercles called pearl organs, and the fins and lower parts 

 of the body are, in the breeding season, often highly colored 

 with bright pigments, either red, satiny-white, yellow to orange, 

 or black. The young of the deeper-bodied species are much 

 more slender than the adults and have much larger eyes. They 

 may also show color markings not found in adults of the same 

 species, such as a caudal spot or a black lateral stripe. 



Taken as a group the minnows are, on the whole, fishes 

 especially of the creeks and smaller rivers, and they show, in 

 these situations, a decided preference for a more or less rapid 

 current and for a clean bottom rather than one of mud. There 

 are notable exceptions, as already said, but the general fact is 

 well shown by our data of frequency of occurrence in the various 

 ecological situations, drawn from the 24 Illinois species of which 

 we have collections numerous enough to make them available 

 for this study. Of these 24 species, 6 are more than usually 

 .abundant in the larger rivers, 20 are extraordinarily so in rivers 

 of the second class and 19 in creeks, 5 are more numerous than 

 the average in lowland lakes, and only 1 is unusually so in 

 upland lakes of glacial orgin. 



