6ii 



Case-worms (Phiy^-anoidM') were sonunvhat rarely found, rising to Itt'leen 

 per cent, in tlie rock l)ass and Iwclvi; per cent, in the minnows of the 

 Ilybopsis yn)up, but otherwise averaging from one to six per cent, in less 

 than half of the species. 



rilE CHTTSTACEAN ELEMENT. 



Of the four principal classes of the animal food of fishes; viz., fishes, 

 mollusks, insects, and Crustacea, the latter stand third in importance 

 according to my observations, mollusks alone being inferior to them. That 

 insect larva' should be more abundant in the food of fresh-water fishes 

 than are crustaceans, is a somewhat unexpected fact, but while the former 

 nuide about twenty-five per cent, of the food of our entire collection, the 

 crustaceans amounted to only fourteen per cent. These divide conveniently 

 into crayfishes, the medium-sized sessile-eyed crusteceans (Isopoda and 

 Amphipoda), and Entomostraca. The so-called fresh-water shrimps (Pala^- 

 mon and PaUemonetes) appeared so rarely in the food that they need 

 scarcely be taken into the account. 



Crayfishes made about a sixth of the food of the burbot ; about a tenth 

 that of the common perch, a fourth that of half a dozen gars, not far 

 from a third that of the black bass,* the dog-fish, and our four rock bass. 

 Young crayfishes appeared quite frequently in some of the larger minnows 

 (Semotilus and Hybopsis), and also in catflshes, especially the pond and 

 river bull-heads, averaging nearly fifteen per cent, of the entire food of 

 the two most al)undant species. 



The small, sessile-eyed crusteceans eaten by fishes were nearly all of four 

 species; viz., Allorchestes dentata, — excessively abundant in the northern part 

 of the State, — a species of Gammarus not uncommon in running streams, 

 and two representatives of the isopod genera Asellus and Mancasellus. To 

 fishes at large, this group is of little importance ; but the perch of northern 

 Illinois rinds about one third of its food among them, and the common 

 suufishes (Lepomis) eat a considerable ratio (eleven per cent.). The miller's 

 thumb of southern Illinois seems also to search for them among the stones. 



The little Allorchestes mentioned above I found in a single white bass. 

 in eleven of the common perch, in one of the largest darters, in five young 

 black bass, in seventeen sunfishes of various species, in the rock bass, the 

 pirate perch, a single grass pickerel and six top minnows, in only two of 

 the true minnow family, in two only of the sucker tribe, in seventeen cat- 

 flshes, — mostly young or of the smallest species, — in a single dog-fish, and 

 in a single spoon-bill'. The common Asellus, or water wood louse, was 

 less generally eaten : by only two of the miller's thumb, a single sheeps- 

 head, a white l)ass, four perch, two young black bass, eight sunfishes 

 (Lepomis), two pirate perch, a grass pickerel, three small catfishes, and a 

 dog-fish. 



The minute crustaceans commonly grouped as Entomostraca are a much 

 more important element. Among full-grown fishes, I find them especially 

 important in the shovel fish, — where they made one third the food of the 

 specimens studied, — in the common lake herring', in the brook silversides 

 (forty per cent.), in the stickleback (thirty per cent.), in the darter family 

 (eleven per cent.), and in the mud minnows (ten per cent.). The perch 

 had taken scarcely a trace of them. Among the sunfishes at large tliey 

 were present in only insignificant ratio: but two genera (Pomoxys and 

 Centrarchus), distinguished by long and numerous rakers on the anterior 

 gill, had derived al)out one-tenth of their food from these minute crusta- 

 ceans. In the early spring especially, when the backwaters of the streams 

 are filled with Entomostraca, the stomachs of these fishes are often dis- 

 tended with the commonest forms of Cladocera. 



* Our specimens— especially of the small-mouthed black bass— were too few in number 

 to make this average reliable. 

 1 Polyodon. 

 - Corepron.is artec^i. 



