346 



CRUSTACEA. 



(383.) The female generative organs of Crustacea very accurately 

 resemble those of the male ; and in the unimpregnated condition it 

 is not always easy, from a superficial survey of the internal viscera, 

 to determine the sex. In Astacus Fig. 163. 



Jluviatilis, the ovaria (Jig. 163, a) 

 occupy a position analogous to that 

 of the male testis, and a simple 

 canal derived from each side (6, c) 

 conducts the eggs to the external 

 apertures found upon the first joint 

 of the third pair of legs. 



In crabs an important addition 

 is made to the female generative 

 system : -prior to the termination 

 of each oviduct it is found to com- 

 municate with a wide sacculus, the 

 function of which is apparently ana- 

 logous to that of the spermatheca 

 of insects ( 328), inasmuch as it seems to form a receptacle for the 

 fecundating secretion of the male, in which the seminal fluid re- 

 mains ready to impregnate the ova as they successively pass its 

 orifice during their expulsion from the body. 



It is not precisely known in what manner copulation is effected 

 by these animals ; neither, indeed, is it positively ascertained in 

 many species whether the ova are impregnated prior to their 

 expulsion or afterwards, although the latter supposition seems by 

 far the most probable. 



(384.) The eggs are almost invariably carried about by the female 

 until they are hatched, and in order to effect this various means 

 are provided. In the Decapoda they are fastened by a stringy 

 secretion to the false feet under the abdomen, and a female crab 

 may generally be readily distinguished from a male of the same 

 species by the greater proportionate size of this part of their body. 

 In Asellus, a small Crustacean very common in stagnant water, 

 the male may be observed during the breeding season to carry the 

 female about with him for many days ; after which her eggs are 

 found impregnated, and enclosed in a membranous sac placed under 

 the thorax, from which when the young are hatched they escape 

 through a longitudinal fissure provided for the purpose. In many 

 genera, broad laminse, or scaly plates, are found upon the under 

 surface of the body, beneath which the eggs are lodged. 



