686 FRESH-WATER BIOLOGY 



With them are frequently associated members of the Daphnia 

 pulex group. These last are also found in temporary pools of clear 

 and weedy water, and, less frequently, in lakes. Daphnia magna 

 in Europe is found in waters which are slightly brackish and very 

 possibly does not disdain slightly alkaline waters in this country. 



The Hmnetic region of the inland lakes has a cladoceran popula- 

 tion, large in number of individuals but not rich in species. Chy- 

 dorus sphaericus is almost the only Chydorid which is ever abun- 

 dant here, though any species may be present as an accidental 

 visitor. The regularly limnetic species belong chiefly to the genera 

 Bosmina, Diaphanosoma, Daphnia, and Holopedium. These forms 

 are transparent — an obviously protective character. Chydorus 

 is an exception and the size of this species is so small that trans- 

 parency may be unnecessary. Apart from transparency and a 

 general lightness of build, the limnetic forms have generally no 

 pecuhar characters. Holopedium forms a marked exception to 

 this statement, as its globular gelatinous case is wholly unique in 

 the group and indeed in the Crustacea. 



Certain forms are intermediate in character between the limnetic 

 and the Httoral forms. Such is Ophryoxus gracilis (Fig. iioo), 

 which paddles about in the open waters between weeds, and such 

 also is Sida crystallina (Fig. 1051). Both of these forms are trans- 

 parent, but they are never present in large numbers in the open 

 water, nor are they likely to be found far out from the weedy 

 margin. 



In southern waters, where are found masses of floating plants 

 such as the water hyacinth, the distinction between the littoral 

 and limnetic species quite disappears. 



The Gymnomera differ widely in structure and habits from 'the 

 Calyptomera. The section includes two species in our fresh 

 waters: Polyphemus pediculus (Fig. 1169), and Leptodora kindtii 

 (Fig. 1 1 70). In both forms the shell is reduced to an egg case and 

 the feet are free, Jointed, and provided with stout spines and hairs. 

 In Leptodora the body is long and jointed, while in Polyphemus it 

 is very short. Both animals are predacious, feeding on protozoa, 

 rotifers, and minute Crustacea. Polyphemus lives chiefly in marshes 

 and in the weedy margins of ponds and lakes, but may also be 



