174 ™^ AMERICAN NATURALIST. [Vol. XXXVIII. 



the long axis of the annukis and anteriorly it is overhung, some- 

 what, by the two hills, while posteriorly it is bounded by an 

 elevated lip or transverse ridge that forms the posterior edge of 

 the annulus and sometimes shows a faint crack in it as if made 

 of right and left halves welded together. 



Though the hills are right and left the median slope of one 

 extends across the median line of the body of the crayfish so 

 that the chink between the hills is always asymmetrically placed, 

 sometimes on the right and sometimes on the left. In forty-one 

 females, all but two of which were young, probably but four 

 months old, examined in October, only three cases were found 

 in which the chink was on the left side; in the thirty-eight 

 other females it was upon the right side. 



In four still younger females, 32-40 mm. long, the annulus 

 was found less well developed and with but slight transverse 

 groove. 



Sections of this organ show that the chink opens into a small 

 rounded pouch or sac which as seen from a dorsal view projects 

 upward as a small, curved ridge. Its walls are stiffly calcified 

 cuticle and no opening could be found excepting the external 

 chink above mentioned. This cavity of the annulus serves as 

 the seminal receptacle, ^but it is only a small, specialized pit in 

 the external cuticle or exo-skeleton. 



After sexual union the female shows a white plug of waxy 

 substance projecting from the chink and filHng up the transverse 

 groove at that point as indicated in Fig. 2. This plug is 

 necessarily excentric, generally upon the right side, but in some 

 females upon the left side. It is the surplus sperm envelope or 

 macaroni-like case that came down the pleopod groove and was 

 forced into the chink of the annulus. This plug may remain for 

 weeks, but it disappears some time after the eggs are laid. It 

 may thus be used as evidence of foregoing union for a much 

 greater time than the vaginal plug of some rodents can be. 

 E.xamining the contents of the cavity of the annulus when the 

 plug is in evidence we find it full of a similar compact paste-like 

 material that may be quite hard and has the form of a tubular 

 sheath around a granular mass that proves to be the peculiar 

 sperm cells of the crayfish. The sperm in this sperm receptacle 



