300 



ZOOLOGY. 



asexual females, the males not appearing until the autumn, 

 when the females lay the fertilized " winter'' eggs, which are 

 surrounded by a very tough shell. Dohrn observed the de- 

 velopment of the embryo in the summer eggs. At first the 

 embryo has but three pairs of appendages, representing the 

 antennae and one pair of jaws. It is thus comparable with 

 the Nauplius of the Copepodous Entomostraca, and thus the 



Fig. 242. Sida. , egg in brood-sac. 



Cladocera may be said to pass through a Nauplius stage in 

 the egg. 



Afterwards more limbs grow out, until finally the embryo 

 is provided with the full number of adult limbs, and hatches 

 in the form of the mature animal, undergoing no farther 

 change of form. 



The members of the suborder Phyllopoda are more highly 

 developed than any of the Crustacea mentioned, though, like 

 the Ostracodes and Cladocera, the body is usually partly 

 covered by a large carapace (the mandibular segment greatly 

 developed), which is sometimes bent down, and opens and 

 shuts by an adductor muscle, so that they resemble bivalve 



