THE SPERM-RECEPTACLE IN CAMBARUS. l8l 



and in being decidedly concave on the posterior face. When 

 isolated it has the form indicated in Fig. n, with the mouth of 

 the sperm-pocket lengthwise along its high middle part. 



Posterior to this annulus the middle plate between the fifth 

 legs is developed as a very high spine, which is shown in its 

 true proportions and relations to other adjacent organs in Fig. 

 10. In this side view, with one half of the body cut away, the 

 tall, conical, or somewhat flat- 

 tened spine juts forward to over- 

 hang the annulus, which in turn 

 stands up above the general level 

 of the mid-region of the sternum. 

 The spine bears at its apex several 

 long and sparsely plumose setae 

 and a scant fringe of much shorter FlG - I0 - Side view of 



r , , , , f and post-annular spine, left half of 



setae is found along the edges of bodj removed> 2<v 



the wing plates of the fourth and 



fifth legs. In the background of the figure are the bases of the 



fourth and fifth legs. The side plates are here seen in their 



true elevation, while in Fig. 9 they are seen on edge. 



In crayfishes the last thoracic somite is movable and the hard 

 middle plate or spine of this somite may be readily shoved 

 against the hind edge of the annulus. In Cambarus affinis 

 there is reason to believe that the female so shoves the middle 

 plate against the annulus as to liberate the sperms when they 

 are needed to fertilize the eggs, as these glide out of the open- 

 ings on the third legs and thence back over the annulus. In 

 Procambarus cubensis the low simple spine, Figs, i, 2, 3, may 

 thus act against the low posterior face of this annulus and, on 

 the other hand, in Paracambarus -paradoxus the very tall spine 

 may be useful against the unusually high annulus, which is 

 concave on its posterior face, as if to receive the spine. While 

 the very tall spine may thus find its use in connection with a 

 very high annulus, as yet no use has actually been demon- 

 strated, and the great development of the spine in this Para- 

 cambarus may not have any value. The spine is a specializa- 

 tion of the middle plate of the last thoracic somite and as such 

 is to be regarded as homologous with the annulus, so that the 



