228 FISHES OF ILLINOIS 



It evidently avoids the lower Illinoisan glaciation. It is more 

 abundant north than south, the frequency ratios of our 121 collec- 

 tions of it being approximately as 1, 2, and 3, for southern, central, 

 and northern Illinois respectively. 



It occurs in a great variety of waters from Lake Michigan and the 

 northeastern glacial lakes of Illinois to the borders of the main 

 stream of the Illinois River and the muddy lakes of the Illinois 

 bottoms, commonest, however, in the quieter and clearer parts of 

 the waters which it inhabits. We have found it somewhat most 

 abundant in the smaller rivers (coefficient 1.67), and next in the 

 glacial lakes (1.13) and in lowland lakes and sloughs (1.01). It 

 is not infrequent, however, in our collections from creeks and the 

 larger rivers ( . 76 and . 77) . 



Outside the state, it is present in all the Great Lakes, and ranges 

 thence southward to Florida and southwestward to Missouri, 

 Arkansas, and Texas. 



It seems to live wholly on the animal plankton, apparently 

 catching its minute prey one by one, as a pike captures fish. Its 

 mouth, though small, is well equipped with teeth, and its gill-rakers 

 are unusually well developed, being numerous, slender, armed with 

 minute denticles, and longer than the gill-filaments. Corresponding 

 to its predaceous habit, its intestine is uncommonly short, the whole 

 alimentary canal being considerably shorter than the body without 

 the head. The food of twenty-five specimens, obtained from widely 

 scattered localities, was wholly animal, and consisted mostly of 

 minute larvae of gnats (Chirononins) and many species of Entomos- 

 traca, both copepods and Cladocera. Land insects and spiders, 

 washed or fallen into the water, were also frequent in its food, in- 

 cluding forms as small as plant-lice, chalcids, springtails, and thrips. 

 One specimen had taken a very small unrecognizable minnow. 



Family APHREDODERIDiE 



(the pirate-perches) 



Body oblong, elevated at base of dorsal, compressed behind; caudal 

 peduncle thick; scales strongly ctenoid; sides of head scaly; lateral line 

 imperfect; skeleton osseous ; anterior vertebrae simple; ventrals thoracic, 

 with a small spine and more than 5 soft rays; dorsal fin single, with 3 or 

 4 small spines; anal with two slender spines; caudal rounded; no meso- 

 coracoid; gill-membranesslightly joined to isthmus anteriorly; branchios- 

 tegals 6; pseudobranchiae obsolete; gill-rakers tubercle-like, dentate; 

 preopercle and preorbital with free edges sharply serrate; opercle with a 



