124 E. A. ANDREWS. 



In using the annulus as a sperm receptacle the male passes the 

 sperm into the orifice and thence into the posterior part of the 

 tube. The anterior part of the organ, the orifice itself and the 

 following transverse part, which may be called the vestibule, is 

 filled not with sperm but with a cement that protects the sperm 

 from the water. Eventually the sperm issues out through the 

 more posterior part of the suture at the right time to meet the eggs. 

 When the young C. affinis hatches from the egg there is no 

 annulus present. The ventral surface (Fig. 3), multiplied fifty 

 diameters, shows no specialization of the wide level area between 

 the bases of the fourth legs. The sexes are not yet distinguish- 

 able and the first abdominal appendages are in all cases but very 

 faintly indicated by slight elevations. Here under a higher power 

 the epidermis was seen to be specialized as a group of nuclei, over 



which the cuticle was elevated 

 as a slight protuberance in the 

 region that is later to grow out 

 as the first abdominal appendage. 

 After this larva sheds its shell 

 and passes into a second stage 

 the sternal surface is larger (Fig. 

 4), and the first abdominal ap- 

 pendages are somewhat more 

 protuberant. There is, however, 

 ^ s _ x _ no annulus and no external signs 



of sexual differences. Between 

 FIG. 4. 



the bases of the fifth legs there 



is now a transverse elevation of the sternum, which will become 

 the prominent tubercle posterior to the annulus in the adult. 



After a second moult the larva in the third stage has a much 

 larger sternal area and for the first time sexual openings and the 

 beginning of the annulus. In the female (Fig. 5), the sternum 

 between the fourth legs is divided by a cross-line into an anterior 

 part articulating with the legs, and a broad posterior region that 

 is, however, markedly short. On the middle of this plate and at 

 its posterior edge is a slight depression or groove destined to 

 become the receptacle for sperm in the adult. In the male the 

 same differentiation of a posterior sternal area is found, but the 



