CHAPTER XxXI 
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 sexualforms. 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 alive (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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