THE GYMNOTID EELS OF TROPICAL AMERICA. 175 



inferior. The snout of S. mulleri is also straight and is only slightl}^ out of line 

 with the long axis of the body. It is, however, directed downward to some extent 

 and the mouth is more or less inferior. All three species are thus adapted to feed 

 upon the bottom fauna with the minimum of effort. It seems probable that they 

 feed by sucking up quantities of mud and food after a suitable feeding-place has 

 been found. Forms like these taken from the stomachs are usually found in large 

 numbers, close together. 



No stomachs of the long-snouted Gymnorhamphichthys hypostomus Ellis were 

 examined. 



General Considerations. 



Two factors control the nature of the food taken by any fish, namely, (1) the 

 structure, and (2) the size of the fish. In the Gymnotidce the only structure that 

 needs consideration is the mouth. The other structures which might influence the 

 selection of food, such as shape, nature of fins, and the like, are all held in common 

 b}^ the several species of this small famil3^ Considering size and mouth there are 

 four types of Gymnotidce, (1) large, large-mouthed specimens (adult only); (2) small, 

 large-mouthed (yoimg of large-mouthed); (3) large, small-mouthed eels; and 

 (4) small, smaU-mouthed eels. The last three are ecologically the same, since 

 they all have small mouths. Class two, however, differs in that the members of 

 this group ultimately by growth attain to the first class. G. carapo, E. eledricus, 

 S. macrurus, S. albifrons and S. brasiliensis are the large-mouthed species of this 

 family. 



They are the only species examined which had eaten either fish or freshwater 

 shrimps, or both. These two items, which are the most bulky food taken from 

 Gymnotid stomachs, were found only in the largest specimens of the species be- 

 longing to the first class. 



A comparison of the various tables shows that the young of all species partake 

 .of much the same food. They feed upon entomostraca, the larvae of insects and 

 small annelids. The second, third, and fourth classes are therefore alike as regards 

 the food taken. There remains, however, the ability of the second class to change. 

 Forbes has found that the food of all small North American fishes is much the 

 same, being for the most part entomostraca and the larvae of insects. For the 

 predaceous fishes those which as adult feed largely upon other fishes he has also 

 shown a regular cycle of foods from the young to the adult. Beginning with 

 entomostraca and insect larvae they pass to Annelida and adult insects, small 

 Crustacea, large Crustacea, and finally to other fishes. Precisely this progression 

 is shown in the first two tables (pp. 168 and 170). On the other hand, the non- 

 predaceous members of this family, which are limited by smaU mouths, pass only 

 from entomostraca to insect larvae. 



Schlesinger divides the species of this family into three groups. The first 

 group contains E. eledricus and G. carapo. He imagines that the food of these 

 two species must be the same because of their general resemblance. This was 



