FIELD BOOK OF PONDS AND STREAMS 



Water-fleas, Cladocera. — Water-fleas are minute crusta- 

 ceans which swarm through the water in amazing numbers 

 (Fig. 124). Their short bodies are completely covered by a 

 transparent carapace. By far the largest of them, the beau- 

 tiful and fierce carnivore, Leptodora, is less than three-quarters 



Fig. 124. — Common cladocerans: l, Daphnia; 2, 

 Bosmina; 3, Simocephalus; 4, Sida; 5, Latona; 6, 

 Scapholeheris. 



of an inch long; the smallest measures scarcely one one-hun- 

 dredth of an inch. Several even average-sized cladocerans 

 can be held in one trap of the bladderwort Utricularia (p. 102). 

 Yet every one of them has ten legs and highly organized sys- 

 tems for digestion and reproduction and the like. They are 

 beautifully transparent under a microscope, with every detail 

 of their structure showing distinctly without any tools or prep- 

 aration except a glass slide and a drop of water, and they have 

 long been favorite animals for study. In 1669 the Dutch 

 naturalist, Swammerdam, described the common water-flea 

 (Fig. 124) as Piilex aquaticus arhorescens — the water-flea with 

 the branching arms — and "water-flea" it has remained ever 

 since. 



Cladocerans and copepods (p. 166) are the staple food of 

 young fishes. Their importance is obviously not due to their 



164 



