THE ANNULUS OF A MEXICAN CRAYFISH. 



127 



(9) : it is a rather simple infolding of the epidermis, lined by the 

 exoskeleton and is a mere flat pocket opening to the exterior by 

 a narrow chink. 



The mouth of the sperm pocket is the sinuous line seen in Figs. 

 1, 2, 3 and 5, passing as an S-shaped line from the neighbor- 

 hood of the median groove to the right or the left as the case 

 may be, across the posterior face and then around onto the 

 lateral face of the annulus. It is not extended onto the anterior 

 face. For most of its extent the line is a closed suture, or mere 

 morphological mouth, forming a chink that is apparently closed 

 up, but toward the edge of the posterior face and on the 

 lateral face the chink is more patently an opening into the 

 interior. The lips of this suture are more or less swollen espe- 

 cially on the lateral face and toward the edge of the posterior face 

 and this is much more pronounced in some individuals than in 

 others (Figs. 2 and 3). 



The internal pocket into which the suture leads may be seen 

 through the exoskeleton when properly prepared and, as indi- 

 cated in the above figures by the dotted areas, it extends out 

 rather far on either side of the suture in a peculiar way. 



A good idea of the shape 

 of the internal pocket may be 

 got by looking at it from the 

 inside, as in Fig. 6, that is, by 

 removing the anterior walls of 

 the annulus and all the con- 

 nective tissue and epidermis ; 

 the exoskeleton that lines the 

 sperm pocket stands forth as 



a prominent ridge passing sinuously from side to side. 

 6 is such an anterior view of the exoskeleton of the sperm- 

 pocket of a right-handed annulus, similar to Fig. 2. The flat, 

 sinuous ridge there indicated is attached all along the pos- 

 terior side of the annulus and free anteriorly, toward the 

 observer ; the part to the right is the beginning of the pocket on 

 the lateral face of the annulus and stands forward as do the 

 middle and the left parts, forming three roughly parallel parts of 

 an S. 



Fig. 6. 



Fig. 



