THE SPERM-RECEPTACLE IN CAMBARUS. 1 69 



kept close together by the elasticity of the soft cushions formed 

 by the thin inner face of each plate. While the wedge-plate is 

 especially well calcified, the walls of the side plates, or wings, 

 remain membranous over most of their inner faces, so that much 

 of the lining of the receptacle, as well as the entrance to it, is 

 not brittle shell, but thin chitinous material. 



Looked at from the inside of the body the protruding plates 

 of the under surface of the body are not solid thickenings of the 

 shell, but hollow protuberances, and from this standpoint the 

 receptacle may be described as an external space embraced on 

 the sides by two pouches of the shell, the right and the left 

 wing, and by a middle floor, a long, pointed pouch, the wedge. 



It should be emphasized that the receptacle is merely an en- 

 closed region outside the body of the lobster, and that, as far as 

 known, it has no communication with any internal structure. 



To determine the homology of this organ it is important to 

 know if all its parts belong to one somite or not. At first sight 

 the wedge-plate seems to belong to the somite bearing the fifth 

 pair of legs, as it sticks forward from the sternal ridge joining 

 these legs, while the side wings belong to the somite bearing the 

 fourth legs. But there are some reasons for holding that the 

 wedge belongs to the somite bearing the fourth legs, though 

 Herrick (The American Lobster, 1895), from a study of several 

 stages in the formation of the receptacle in young lobsters, con- 

 cluded that the wedge-plate belonged to the somite of the fifth 

 legs. However, an examination of his Fig. 5, Plate 33, sug- 

 gests that the cross line between the fourth and the fifth legs is 

 homologous with the similar line, which we have shown to be 

 the boundary of the annulus in young crayfishes (Biol. Bull., 

 1906), and as the annulus seems to belong to the somite of the 

 fourth legs, it may be that Herrick was in error in assigning the 

 wedge to the somite of the fifth legs. Again, in the series of 

 middle and side plates, the middle plate stands a little posterior 

 to the line joining the side plates of its somite, as far as one can 

 judge from the adult condition, so that a recession of the middle 

 plate from its side plates is what is to be expected, while the 

 existence of a middle plate in advance of the side plates would 

 be exceptional. Thus there is a middle plate posterior to the 



