l68 E. A. ANDREWS. 



are all fused into one mass, forming a groove, with the middle 

 plates as the bottom and the side plates as the elevated edges 

 to which the legs join. But between the fourth legs the middle 

 plate is recognizable as a distinct part of the sternal skeleton 

 and we will call it the annular plate as in the female crayfish 

 of the genus Cambarus it is specialized as the so-called annulus 

 ventralis, that contains the sperm-receptacle. In the next 

 somite, which bears the fifth legs, the middle plate is separate 

 and often spine-like while the side plates stand up on edge as 

 diagonal wings. 



While the receptacle in Cambarus is contained within a single 

 middle plate, in the lobster it is formed by three plates, one 

 middle plate and two side plates, and thus though it has the 

 same position and the same physiological value it has not the 

 same morphological value. This will be more evident after a 

 description of the receptacle in the lobster, where it was first 

 recognized by Bumpus in 1897 (J. M., Vol. V). This organ is 

 made by two side wings or plates of the fourth leg-bearing somite 

 together with a wedge-like middle piece. These three plates 

 surround a middle space which is several mm. long, somewhat 

 less in width and more in depth. Bumpus states that it maybe 

 filled, in part, by a mass of sperm and largely by a wax-like 

 substance. 



In the lobster the legs are inserted so close to the middle line 

 that the side plates come into contact at the middle line between 

 the fourth legs, but passing diagonally backwards flare apart 

 so that the middle or wedge-plate pushes in between them. 

 The receptacle is just above the wedge-plate which underlies 

 it and shuts it off from the exterior, as it runs forward like a 

 slanting shelf from its broad posterior base to its small, free 

 tip. The receptacular space would thus be entirely shut off 

 and inaccessible were it not for the fact that the two side plates 

 in front of the tip of the wedge may easily be forced apart by 

 inserting between them the hard, spatula-like tips of the male 

 stylets, or first abdominal appendages, which are very well 

 fitted for this purpose. Once inserted these organs are not 

 readily removed since the special ridge each bears, catches on 

 the face of the lateral plate. The lateral plates are normally 



