THE WATER FLEAS (CLADOCERA) 683 



of water at the surface of a lake may be deserted by the Clado- 

 cera. A little deeper may be found young forms, and still deeper, 

 perhaps one or two meters below the surface, the adult animals. 

 The temperature of the water also has much influence on the reac- 

 tion to Hght. In cold water Cladocera are attracted by a light 

 which will repel them at higher temperatures. The limnetic forms 

 of Daphnia pulex ordinarily remain during the daytime in the cool 

 water immediately beneath the thermocHne, though they may rise 

 into the warm water during the night. In the winter, when the lake 

 is skimmed with ice, the same animals may be seen in the bright 

 sunshine immediately below the ice. Practically all of the Cladocera 

 react negatively to the blue rays of the spectrum, are nearly im- 

 infiuenced by the rays at the red end, and find the yellow rays the 

 most attractive. 



The ocellus is rarely absent (Diaphanosoma, Daphnia retrocurva, 

 longiremis) ; sometimes rudimentary (many forms of Daphnia); 

 sometimes larger than the eye {Leydigia, Dadaya) ; and rarely the 

 sole organ sensitive to light (Monospilus). It is not known in what 

 respects its function differs from that of the eye. 



This imperfect sketch shows how complex the structure of the 

 Cladocera is, — wonderfully complex, when their small size is con- 

 sidered. The smallest of them are hardly more than one one- 

 hundredth of an inch in length. Yet these have ten complicated 

 legs, besides the numerous other structures named and many which 

 have not been mentioned. Probably no other animals of so small 

 size have so complex a structure, yet they must suffer the disgrace 

 of being eaten by Stentor and so being among the few Metazoa 

 which are swallowed whole by one-celled animals. 



The reproduction of the Cladocera is noteworthy. During the 

 open season the females produce eggs which develop without being 

 fertilized. These may number only two, the usual number in the 

 Chydoridae, or, in the larger Daphnidae, there may be more than 

 twenty. These eggs are deposited in the cavity bounded by the 

 dorsal part of the valves and the upper side of the body — the 

 brood case. Here they develop and hatch in a form quite like 

 that of the parent and are well grown before they are set free. 

 Hence there are no free-living larval forms of Cladocera, such as 



