168 THE LIFE OF CRUSTACEA 



of the year. In most species, particularly in those 

 which live in lakes, the sexual period occurs in the 

 late autumn, and the ephippial eggs lie dormant 

 during the winter, and hatch in the spring. In 

 species living in small ponds exposed to the risk of 

 overheating or of drying up during summer, there is 

 often a distinct sexual period in the spring, when 

 ephippial eggs are produced to tide over the un- 

 favourable conditions of the warmer months of the 

 year. Although no species is known to be exclusively 

 parthenogenetic, yet it appears that purely partheno- 

 genetic colonies of certain species may be found in 

 favourable localities, where they may reproduce from 

 year to year without males ever being found. 



Certain species of Cladocera belong to the plankton 

 of lakes and large ponds, and show modifications 

 which adapt them for a floating life. Some of these 

 belong to the genus Daphnia, and differ from the 

 species found in other situations by their glassy 

 transparency. As in the case of many marine 

 plankton Crustacea, this transparency is probably 

 due to the thinness of the shell and to the general 

 watery condition of the body, giving the necessary 

 buoyancy to enable the animal to remain constantly 

 afloat. The same effect is no doubt produced by the 

 long terminal spine of the carapace and by the great 

 helmet-shaped crest into which the upper part ot 

 the head is often produced. A form very character- 

 istic of the plankton of large lakes is Bythotrephes' 



