ONTOGENY OF THE ANNULUS VENTRALIS. I 2/ 



descending artery from the heart, between the fourth and fifth 

 ventral thoracic ganglia. 



Young crayfish in the above third stage are about 78 mm. 

 long. In the fourth stage they are usually 1 1 mm. long. By 

 this time the males and females differ noticeably in the lengths of 

 the first pleopods. In the female, Fig. 9, they are still very small 

 but larger than in the previous stage, this figure being enlarged 



FIG. 10. 



but thirteen, and Fig. 5, fifty diameters. In the male, Fig. 10, 

 on the same scale, the pleopods are long, slender, but simple 

 cylinders pointing toward one another. These two figures also 

 show the increasing longitudinal diameter of the anmulus in the 

 female and its simple form in the male, as well as the reproductive 

 openings upon the fifth legs in the male and the third legs in the 

 female. 



In these females 1 1 mm. long the median groove of the annulus 

 is much more evident than at first, but is still a simple groove as 

 shown in Fig. 1 1 enlarged 200 diameters. The walls of the folded 

 in and thickening shell that bound the deep and narrow groove 

 are indicated by the broken line. In this preparation the epider- 

 mis had shrunken far away from the shell and is indicated in 

 optical section to show its invaginated state where it came under 

 the cuticular groove. The groove seems to grow forward from 

 the posterior face of the annulus and becomes more narrow and 

 closed in anteriorly. At the anterior tip the groove was partly 

 overhung by a cross fold, or tended to burrow under the surface 

 as a short citl de sac. 



In different females, however, the groove had different lengths 

 and more or less of this covering-over of the front end. In a 

 specimen 10 mm. long the groove was longer and more narrow 

 than the one above figured. In others of 1 1 mm. the anterior 



