The Food of Fresh- Water Fishes. 467 



dragon-fly larvio (Libellulidji! and Agrion), and C;unis larvoG, 

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

 terrestrial insects, were the remaining items o£ this class. 



The crustaceans were all Entomostraca, with the exception 

 of the aniphipod AUorehestes dentcda^ 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- 

 uida) 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., 

 Bosraina, 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. 



