Water- Fleas 



391 



herbage may not necessarily mean good crops; for 

 weeds may be much more conspicuous in a pasture than 

 the close-cropped grasses that yield the forage there. 

 Certain species of pondweeds have been shown by 

 Aliss JMoore ('15) to be often used as green food, and 

 Birge ('96) has given many notes on the food preferences 

 of herbivorous plancton Crustacea. 



The above mentioned staples invite much attention 

 but we shall have space for noticing but a few represen- 

 tatives of the groups to which they severally belong. 



Di^esfivT tract 



X 

 N 



Abdominal processes 



Abdominal setae 



Brc^d' chamber 

 Heart 



Anal spines. 



Pecfen - , 



Abdominal clav/s'' * 



Post- abdomen 



Fig. 234. Daphne (after Dodds). 



Water-fleas — As a typical representative of this great 

 group of herbivores, we may speak of Daphne (fig. 234). 

 Its manner of life and its enormous reproductive 

 capacity have already been briefly mentioned (pp. 1 86-7 

 and 306) . It is a very valuable animal in water culture 

 on account of its ability to turn the great grow^ths of 

 colonial diatOms and algae into excellent food for fishes. 

 Little is known, as yet, unfortunately, about the condi- 

 tions that make for its growth. Plancton studies of 



