II LIFE-CYCLE OF CLADOCEKA 49 



egg is invariably a partlienogenetic female : the characters of the 

 succeeding generations differ in different cases. 



In a few forms, of which Moina is the best known, the 

 parthenogenetic female, produced from a winter-egg, may give 

 rise to males, to sexual females, and to parthenogenetic females, 

 so that the cycle of forms which intervene between one winter- 

 egg and the next is short. A sexual female produces one or 

 two winter.- eggs, and if these are fertilised they are enclosed 

 in an ephippium and cast off; if, however, the eggs when ripe 

 are not fertilised, they atrophy, and the female produces partheno- 

 genetic eggs, being thenceforward incapable of forming sexual 

 " winter " eggs. An accidental absence of males may thus lead 

 to the occurrence of parthenogenesis in the whole of the second 

 generation. The regular production of sexual individuals in the 

 second generation from the winter-egg appears to depend on a 

 variety of circumstances not yet understood. Mr. G. H. 

 Grosvenor tells me that Moina from the neighbourhood of 

 Oxford may give rise to several successive generations of 

 parthenogenetic individuals, when grown in small aquaria. \ 



( j^^ In the greater number of Daphniidae, the parthenogenetic 

 female, produced from a winter - egg, gives rise only to 

 parthenogenetic forms, and it is not until after half a dozen 

 parthenogenetic generations have been produced that a few sexual 

 forms appear, mixed with the others. Such sexual forms are fairly 

 common in April or May in this country ; they produce 

 " winter " eggs and then die, the generations which succeed them 

 through the summer being entirely parthenogenetic. In late 

 autumn sexual individuals are again produced, giving rise to a 

 plentiful crop of winter-eggs, but many parthenogenetic females 

 ' are still found, and some of these appear to live and to re- 



\ produce through the winter. x 



In Sida, in the Polyphemidae and Leptodoridae, and in most 



of the Lynceidae, sexual individuals are produced only once in 



every year, while in a few forms which inhabit great lakes the 



sexual condition occurs so rarely that it is still unknown. 



Weismann ^ has pointed out that the sexual forms, with their 

 property of producing eggs which can endure desiccation, recur 

 most frequently in species such as Moina, which inhabit small 

 pools liable to be dried up at frequent intervals, while the 



^ Zeitschr. iviss. Zool. xxvii., xxxiii., 1876, 1879. 

 VOL. IV E 



