75 



gtp, ninety per cent, of the food of the fourth being Chironomus liuva-, Jind 

 the remainder, larvjio of Neuroptera, — Ilexagenia and Corydalis. 



On the other hand, two small collections of the same species made at 

 Peoria. Oct. !>, 1878— four of /. urus and Hve of 7. qiprindlus—exMhit simi- 

 lar food, composed chiefly of Entoniostraca. Cliirononuis larva', dislillcry 

 waste (meal, etc.), and aiiuatic vegetation. The Mrw.s group alone had eaten 

 Entoniostraca, these being replaced in the other by a larger quantity of 

 meal. 



The facts above recited are evidence that fishes are not mere animated 

 eating-machines, taking indiscriminately and indifferently whatever their 

 structures tit them to capture, to strain from the waters, or to separate 

 from the mud. but that psychological preferences as well as physical capa- 

 bilities have something to do with their choice of food. 



The Structures of Alimentation. 



A brief review of the principal facts respecting the structures of ali- 

 mentation in fishes will be necessary to exhibit clearly the relation of habit 

 and organization in this particular. 



These structures may be conveniently divided into those of search, of pre- 

 hension, of mastication, and of digestion. ]Means of defence and escape may 

 also properly be mentioned, as belonging to the obverse side of the food 

 relation. 



Structural peculiarities relating to the methods and situation of the 

 search for food are illustrated by the barbels of the cattishes and the stur- 

 geons, the shovel of Polyodon, the square head of the stone roller, the flat 

 heads of the top minnows, and the pointed snouts of the darters, — which 

 fit them for prying about between and under stones in running water. 

 Similarly related, are the bare breasts of many darters and the large pec- 

 toral flns of the stone roller and Phenacobius. 



The structures of food prehension are the lips, the jaws, the teeth, and 

 the gill-rakers, with which should be considered, perhaps, the gill slit or 

 branchial opening. The sucking lips of the Catostomatida?, organs of touch 

 as well as of prehension, are of course related to the mud-searching habit of 

 these fishes, the protractile jaws aiding in their use. The stout wide jaws 

 of the catfishes, with their wide bands of minute, pointed teeth, are prob- 

 ably to be understood as an apparatus for seizing, holding, and pulling 

 about relatively large objects, whether hard or soft, and are perhaps most 

 useful in feeding upon mollusks. The very large but weak jaw of the 

 shovel fish is explained by the minute character of its food, which offers 

 no resistance, but necessitates the passage of large quantities of water 

 through the mouth; while the long and slender jaws of the long nosed 

 gar (Lepidosteus) armed witli several rows of acute raptatorial teeth, are 

 the best apparatus in our waters for the destruction of a relatively small 

 but active living prey. 



The teeth of our fresh-water fishes are always pointed and acute, there 

 being no examples of pavement teeth or cutting incisors among them, such 

 as are found in several marine forms, nor are tliere any instances of either 

 jaw being toothed and the other not. The evanescent teeth of the young 

 of several species which become toothless when mature, are sometimes to 

 be understood as rudiments, as in the shovel fish, and sometimes as related 

 to the early food, as in the white-fish and the gizzard shad. 



The gill-rakers of fishes vary widely in number, length, and usefulness, 

 but are as important and significant as any other part of the feeding ap- 

 paratus. As they oppose the only obstacle to the escape through the gill 

 slit, of objects which enter the mouth with the water of respiration, they 

 set the minimum of size for objects of the fishes food, the only exception 

 to this rule being afforded by the few fishes which swallow mud with lit- 

 tle or no discrimination. 



