242 MICROSCOPY FOR BEGINNERS. 



a sunshiny day, they are occasionally seen in immense 

 numbers, sinking when a cloud shades them, and rising 

 again to the sunlight. Others are to be taken only in 

 deep water, while still others can be obtained only at 

 night. Very many, however, are collected in every 

 gathering of aquatic plants. They abound at all sea- 

 sons of the year, even in midwinter. Their movements 

 are rapid and characteristic. An Entomostracan can be 

 readily recognized as such by the unaided sight, on ac- 

 count of the peculiar leaping, or short, jerking motions 

 with which it travels through the water. 



They are not only interesting little creatures to the 

 microscopist, but they are extremely useful as well. 

 They play a very important part in the food supply of 

 fishes, forming the chief article of diet of some of our 

 best fresh- water fishes. And they are almost as impor- 

 tant as scavengers. Their favorite food is dead and 

 decaying Algae and animal niatter, which, if allowed to 

 remain in the great abundance in which it exists, our 

 ponds ancl slow streams would before long become pu- 

 trid and unbearable. But these numerous little creat- 

 ures, by eating this refuse matter, transform it into an 

 innocent and innocuous material, and confer a benefit 

 both on themselves and us. Mr. C. L. Herrick, writing 

 on this subject, says, " Their importance depends large- 

 ly on their minute size and unparalleled numbers. The 

 majority of non-carnivorous crustaceans are so consti- 

 tuted that their diet is nearly confined to such floating 

 particles of matter as are present in the water in a state 



