SUMMER PASTURAGE 



191 



formed. In summer, when algal growth is excessive, the 

 Daphnids gluttonise on a mass of pasturage far beyond 

 their requirements ; the excess of nourishment, powerless 

 to increase the size of the individual beyond its fixed 

 limits, is converted into summer eggs, which develop 

 parthenogenetically within a brood chamber inside the 

 shell. The number of individuals is thus enormously 

 increased, but the supply of food persistently remains in 

 excess of the demand and multiplication continues 

 unchecked.* Now the feast is made ready for the fish ; 

 perch, bleak, ruff, sticklebacks devour greedily the 

 Daphnids and hosts of other minute crustaceans. Then, 



FIG. 62. The Winter Egg, or Ephippium, ot the Water 

 Fleas. 1. Alona quadrangular is. (x 25.) 2. 

 Scapholebris mucronata. (x20.) From Lamport, 

 " Das Leben der Binnengewasser." 



as summer passes, the algal growth quickly falls off, and 

 the Daphnids form their winter eggs, which are fertilised, 

 well provided with yolk, and enclosed in a protective 

 covering, the " ephippium " (Fig. 62), death follows and 

 the Daphnids disappear, leaving the ephippial eggs at the 

 bottom of the water, where they lie till spring comes 

 round, when they give rise to fully-formed young 

 Daphnids. At least one exception is, however, afforded 

 to the last statement ; the large and beautiful Daphnid, 

 Leptodora hyalma, which enjoys a pelagic existence in 



* Thus the formation of summer eggs both in Rotifers and 

 Cladocera would seem to be an adaptation to the rapid appearance of 

 a large supply of food. The annual cycle of excessive multiplication 

 followed by a high death-rate may help to account for the exceptional 

 preponderance of freshwater genera in these groups (see p. 170). 



