CHAPTER XXI 



INFLUENCE OF EXTERNAL CONDITIONS ON THE LIFE- 

 CYCLE OF THE LOWER CRUSTACEANS 



In the lower subdivisions of the group of Crustacea there 

 are a number of forms in which parthenogenetic and sexual 

 modes of reproduction alternate with each other. Experiments 

 have shown in some cases, at least, that the occurrence of these 

 two modes of reproduction is dependent on external conditions. 

 Here, again, we meet with the same fact, referred to in the case 

 of the aphids, that the external conditions determine, in the 

 main, whether the reproduction is to be parthenogenetic or sex- 

 ual, not whether individuals of one or the other sex appear. In 

 other words, the effect is not to produce males or females, but 

 a different mode of reproduction, involving only females in the 

 case of parthenogenetic, and both males and females in the case 

 of sexual formes. It might be claimed that the change only brings 

 about the appearance of males when the sexual generation ap- 

 pears, while females develop under both conditions. This view 

 of the matter does not seem to hold in those cases where the 

 sexual females are quite different forms from the parthenogenetic 

 females, as in the aphids, but in the case of the crustaceans to 

 be described this claim might be maintained. It is also true 

 that the proportion of sexual females and males may be to some 

 extent affected by the character of the external conditions, but 

 this may be a different side of the question, and will be discussed 

 later. 



Artemia salina reproduces both by summer eggs that develop 

 in the embryo sac, the young being born ahve (viviparous) or 

 by means of resting eggs that are deposited (oviparous) ; but the 

 time of year at which the one or the other method is followed 



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