CRUSTACEA 329 



The gonads are simple, elongated sacs lying in the trunk and con- 

 tinuous with their ducts, which open in the male on the telson, in the 

 female dorsally behind the last limb. The eggs are yolky. They are of 

 two kinds, "summer" eggs which have relatively little yolk and 

 develop rapidly by parthenogenesis in the brood pouch of the mother, 

 and "winter" eggs with much yolk which need fertilization and 

 develop slowly. The winter eggs are fertilized in the brood pouch, but 

 then the cuticle of the carapace, which has thickened, is thrown off 

 as a case — the ephippiiim — in which they are contained. They go 

 through the early stages of segmentation within a short time, but 

 after this a period of quiescence sets in, during which they may be 

 dried or frozen without injury. Sexual reproduction takes place at 

 certain times only, normally twice a year. After the winter eggs 

 develop in spring, there are for some half-dozen generations no males, 

 and reproduction proceeds by parthenogenesis. Then, about May, 

 a generation appears in which males are present. In this sexual and 

 asexual reproduction go on side by side. The same thing occurs again 

 in autumn or at other times when, in unfavourable circumstances, 

 such as cold or starvation, males appear. It is interesting to note that, 

 since parthenogenesis is never suspended by all the females, there is 

 nothing to show that a sexual phase in the life cycle is necessary. 



Suborder GYMNOMERA 



Cladocera whose carapace does not cover the trunk and its limbs, 

 which are slender, jointed, and prehensile. 



Leptodora (Fig. 236), a pelagic inhabitant of certain fresh waters 

 in Britain and elsewhere, is the extreme member of this group. The 

 body is long and slender owing to elongation of the head and of the 

 " abdomen", in which the segmentation is distinct. The fore part of 

 the trunk bears six pairs of slender, jointed, uniramous limbs. The 

 carapace has fused with the somites of this region and projects behind 

 it as a brood pouch. The winter egg gives rise to a Nauplius, the only 

 instance of a larva in the Cladocera. 



Class OSTRACODA 



Free Crustacea, with or without compound eyes; with a bivalve 

 carapace and an adductor muscle ; a mandibular palp , usually biramous ; 

 and not more than two recognizable pairs of trunk limbs, these not 

 being phyllopodia. 



The small crustaceans which compose this class differ little in the 

 general form of the body but show very great variety in that of their 

 appendages. There are among them freshwater and marine, pelagic 



