The Food of Fresh-Water Fishes. 467 



dragon-fly larvae (Libellulidae and Agrion), and Caenis larvae, 

 with a few Corisas, aquatic beetles (Coptotomus), and chance 

 terrestrial insects, were the remaining items of this class. 



The crustaceans were all Entomostraca, with the exception 

 of the amphipod Allorchestes dentata, noted in two specimens. 

 Five of the specimens had eaten Entomostraca, one of them 

 ninety per cent., and another eighty, the remaining ratios 

 being thirty-five, thirty, and twenty. Water mites (Hydrach- 

 nida) were noticed in a single specimen, leeches also in one, 

 and Plumatella in another. The smaller Crustacea were so 

 numerous that no attempt was made to exhaust the possible 

 determinations ; but in some cursory examination of this ma- 

 terial the following forms were observed: Daphnia pulex^ 

 Bosmina, Chydorus, Eurycercus, Leptodora, Cypris, Cyclops, 

 and Canthocamptus. 



To the comparative anatomist, Polyodon is peculiarly not- 

 able as among the oldest of fishes, distinguished, when com- 

 pared with higher species, by the persistence of juvenile charac- 

 ters ; and similarly we find that the most remarkable feature 

 of its food is one which it shares with young fishes in general. 

 This is, however, a simulated correlation, the food habit not 

 being due to a persistence of youthful structures of alimen- 

 tation, but to a remarkable specialization of the apparatus of 

 food prehension. It must consequently be correlated with a 

 superabundant supply of minute animal life when and where 

 these structures originated, or, at least, when they took their 

 present form ; and taken together with the great size of this 

 fish and its out-worn dental furniture, seemingly indicates a rad- 

 ical change in the feeding habits of the species, and a capacity 

 for adaptation to new circumstances which possibly accounts 

 for its long survival. 



