342 



GTLMAN A. DUEW. 



mantle lobes^ are soon formed. About this time the shell 

 cuticle is secreted and some lime salts are deposited. 



Soon after casting has been completed, swellings, the 

 beginnings of the gills (fig. 39, g.)j are formed near the 

 posterior margin of each lobe of the mantle. The gills are 

 thus formed as appendages of the mantle. 



The mantle now has the adult structure and appearance, 

 except that at a later stage a portion of its inner epithelium, 

 and of the epithelium covering the suspensory membranes of 

 the gills, becomes converted into the hypobranchial glands. 

 These glands are present in both sexes, but just before the 

 breeding season they are much better developed in the 

 females than in the males, and there is considerable evidence 

 that they furnish most, if not all, of the material from which 

 the brood-sacs are formed. The margins of the mantle lobes 

 remain thickened and contain the glands that secrete the 

 cuticle of the shell. Some cells along the ventral and pos- 

 terior borders of the mantle lobes bear cilia. Pallial muscles 

 are attached to the shell-valves, and extend out to the margins 

 of the mantle. These serve to retract the margins of the 

 mantle when the shell is tightly closed. 



Foot. 



At a stage such as is represented by figs. 14 and 15, a 

 group of cells lie between the gut and the stomodasum. 

 These cells, together with the ectodermal side walls, are 

 concerned iu the formation of the foot. The side walls of 

 the foot are continuous with the general ectodermal covering 

 of the body beneath the test. The cells lying between the 

 gut and the stomodeeum are apparently mesodermal, and en- 

 close a small space (figs. 15 and 24). The shell-gland spreads 

 out, arches dorsally, and folds laterall}^ to form the mantle, 

 and a large space is left between it and the stomach and in- 

 testine (figs. 20, 24, and 26). In some transverse sections 

 the space between the stomodaeum and the intestine, and the 

 space dorsal to the intestine, are more or less connected. 

 This connection may be due to shrinkage caused by pre- 



