GASTROPODA. 



359 



The shell is secreted by the epidermis of the mantle; the^epi- 

 dermic (periostracum) and porcellanous layers by the edge, and 

 the nacreous layer by the whole surface. In cases in which the 

 nacreous layer is absent, the mantle surface still has a shell-forming 



FIG. 28Q.Pymla tula, male, removed from the shell. The mantle has been cut through along 

 the right side of the mantle-cavity and turned over to the left. The pallial organs are accord- 

 ingly reversed (after Souleyet from Lang). 1 proboscis ; 2 head ; 3 foot ; 4 penis ; 5 vas 

 deferens cut and continued in the roof of mantle-cavity at 15; 6 floor of the mantle-cavity ; 

 7 spindle muscle ; 8 intestine ; 9 heart in the opened pericardium ; 10 liver ; 11 testis ; 12 and 

 13 kidney ; Ik kidney opening ; 15 vas deferens ; 16 rectum ; 17 hypobranchial gland ; 18 anus ; 

 19 ctenidium ; 20 roof of mantle-cavity ; 21 osphradium ; 22 siphon. 



