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 

 Miss Moore ('15) to be often used as green food, and 

 Birge (' 1 1) 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. 



Digestive tract 

 Abdominal processes 



Brood- chamber 

 Heart 



Abdominol claws^ 



Post- abdomen 



Fig. 234. Daphne (after Dodds). 



Water- fleas — x\s 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 valuabh animal in water culture 

 on account of its abihty to turn the great grow-ths of 

 colonial diatoms and algae into excellent food for fishes. 

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

 tions that make for its growth. Plancton studies of 



