384 



INYERTBERATE MORPHOLOGT. 



The second nephridium (Fig. 172, B) develops in connec- 

 tion with the second maxillary segment, and opens usually 

 upon the appendage of that segment. It is especially devel- 

 oped in the Entomostraca, in which it may lie in the folds of 

 the body-wall which form the shell, and hence is usually 

 known as the shell-gland. It occurs also in the larval stages 

 of many Malacostraca, and may possibly persist in a degen- 

 erated condition in the adults of some forms. In structure it 



sa 



Fig. 173. — A, Diageam of Nephbiditim (Gbeen-gland) of Astacus (after 



Marchal); B, Shell-gland of Wulimnadia. 



s = terminal sac. sa = saccule. 



resembles closely the antennary gland, but does not present 

 the complexity frequently found in that gland. 



The majority of the Crustacea are bisexual, hermaphro- 

 ditism occurring only in forms which have a parasitic habit 

 and in some which are sessile in adult life (Cirrhipedia). The 

 ovaries or testes (Figs. 173, A and B) are paired organs lying 

 alongside of the intestine or slightly dorsal to that organ, 

 transverse connecting bars in some cases passing from the 

 organ of one side to that of the other. Each organ may be 

 regarded as a tube, sometimes simple, sometimes branched, 

 and lined on its interior by an epithelium which gives rise 

 to the germ-cells. Special germ-producing regions are fre- 

 quently developed, as, for instance, at the extremities of the 

 tubes or along one side (Isopoda), the cells in other regions 

 ceasing to give rise to ova or spermatozoa. The reproductive 

 elements pass to the exterior by special ducts, oviducts (od) 



