92 The Food of the Smaller Fresh- Water Fishes. 



these facts we find upon analysis to be evidently due to Plienaco- 

 bius, by which jrenus nearly all the Chironomus larvae were taken; 

 and tliis, as already shown, is explained not by any structural 

 feature, but by its peculiar habitat; and when we note that 

 aquatic larvjv together amount in Group III to twenty-five per 

 cent., and in Group IV to twenty-seven, we see that the signifi- 

 cance of the diff"erence mentioned disappears. A similar explana- 

 tion is found of the difference in the ratios of Entomostraca, — 

 that of the first group amounting to twenty per cent, and that of 

 the second only to four. An examination of the tables shows that 

 this predominance in the group first mentioned is nearly all tracea- 

 ble to Hemitremia, a very small fish with rather elongate gill-rakers. 

 The importance of these gill structures is still more clearly 

 indicated, as already noticed, by the difference between Notemi- 

 gonus and Chrosomus of the second group, and clearly far out- 

 weighs the structure of the teeth as an indication of the food 

 habits of the fish. 



The o-eneral conclusions reached may be thus briefly stated: 

 An extraordinarily elongate intestine indicates the limophagous 

 habit, rather than an especial preference for vegetable food. The 

 length or number of the gill-rakers has much to do with the 

 abundance of Entomostraca and other minute animal forms in the 

 food of the fish, while the presence or absence of the terminal 

 hook or the masticatory surface to the pharyngeal teeth is not thus 

 far shown to have any sensible influence upon the general average 

 of the food. Finally, a species may depart widely in food char- 

 acters from those more nearly allied to 'it in structure, if its 

 favorite haunts are peculiar. 



