1921.] N. Annandat.e & R. B. S. Sewell : Vivipara. 



257 



over, greater changes in the course of post-embryonic hfe and show 

 a greater range of structural difference in different genera. I shall 

 first describe them as they occur in the 3'oung of Lecylhoconcha 

 lecythis at birth, for they are greatly hypertrophied at the time in 

 that species, occupying practically the whole of the external layer 

 of the roof of the branchial chamber, Iving immediately or almost 



g.c. 



:-9c 



Fig. iS — Vertical section (slightly obliquei thruugli part of a calciferous 

 gland of the same individual as figured in fig. 17 (same magnification). 



g.c'., necks of gland-cells opening on the external surface of the mantle ; 

 g.c",, degenerating gland-cells. Lettering otherwise the same as in fig. 17. 



immediately beneath the external epithelium and extending down- 

 wards into the substance of the mantle for nearly half its thickness. 

 In a vertical section of the mantle passing through the peri- 

 pheral marginal process (pi. iii, figs, r, 2) the tissues can thus be 

 separated at sight under a low power of the microscope into an 

 external glandular area and an internal vascular layer. It is with 

 the former we are at present concerned. At the extreme margin, 

 in the substance of the process, the periostracal glands can be dis- 



