136 - E. A. ANDREWS. 



and sexually mature, has an annulus very much like that of the 

 full-grown adults, but it is still very small, as is seen on com- 

 paring Fig. 25 with Fig. i, both enlarged 12 diameters. Still 

 the tuberosities do not yet overhang the transverse depression 

 enough to conceal the hood and the transverse fold, nor are the 



transverse depression and the pos- 

 terior rim as well developed as later. 

 The receptacle itself is much like 

 that in Fig. 24, but its orifice is more 

 perfect, though not yet as patent as 

 in the large specimens, Fig. i. 

 This young annulus still lacks the 



posterior enlargement of the sperm tube, indicated in Fig. I, and 

 the complexity of bending of the tube is less, nevertheless it func- 

 tions as a sperm receptacle. 



SUMMARY. 



The specialized sternal plate of the shell of Cainbarns, which 

 in the adult female bears the sperm-receptacle, is first differen- 

 tiated in the third larval stage, in C. affinis. 



The epidermis under this plate grows inward and outward in 

 special areas to form the receptacle. First there is formed an 

 open median groove that then bends to one side, sinks away from 

 the surface and becomes closed. In C. Clarkii also one stage 

 shows a like development. 



Next definite folds overgrow the anterior end of the groove as 

 it sinks from the surface. 



Later elevations and depressions complete the external sculp- 

 turing of the annulus. 



The anterior part of the groove, with the accompanying folds, 

 form the orifice and vestibule of the sperm receptacle, while the 

 posterior part of the groove forms that part of the receptacle in 

 which the sperm is stored. 



A functionally complete annulus is made within five months, 

 but subsequently it becomes more complex. Comparative study 

 of the adults of several species has shown that in Canibarus the 

 essential part of the annulus, as a sperm receptacle, is a curved 

 pocket ; the above facts indicate that in Canibanis in general this 

 pocket arises as an open epidermal groove. 



