PHYLUM MOLLUSCA 



673 



end 



during regularly recurring periods of arrest of growth, probably 

 annual. 



The mouth of the shell is bordered on the side turned away 

 from the columella by a prominent rim or outer lip of the 

 peristome ; this is produced at the extremity farthest from the apex 

 of the shell into a spout-like process the siphonal process. The 

 prominent edge of the peristome is in relation to the dorsal 

 surface of the body of the 



:he opposite side 



jrominent edge, 



lided off to form 

 inner lip ; a 

 ridges on_this 

 jowards the apical 

 the "animal in 

 drawing itself out after it 

 has become retracted into 

 the interior of the shell. 

 The outer lip is in rela- 

 tion to the dorsal surface 

 of the body of the animal, 

 the inner lip in relation 

 to the ventral surface ; the 

 siphonal process is for the 

 lodgment of a spout-like 

 process of the edge of the 

 mantle the siphon. 



When removed from the 

 water, or disturbed in any 

 other way, the animal be- 

 comes completely with- 

 drawn into the interior of 

 the shell, when the latter 

 is observed to become 

 closed by a plate the oper- 

 culum (Fig. 573) which 

 fits accurately across the 

 passage some distance in- 

 ternal to the peristome. The operculum is an oval plate of 

 chitinoid material hardened by calcareous deposits ; like the 

 shell itself, it exhibits lines of growth marking what has been 

 its edge at successive stages in the development of the shell. 



The minute structure of the shell is in the main similar to that 

 of the fresh-water Mussel (p. 634). The outer surface of the 

 shell is covered with a thin layer of uncalcified chitinoid material 

 the periostracum ; beneath which is a thick prismatic layer, and, 

 lining the Jimer surface, a layer of nacre. 



VOL. i x x 



FIG. 572. Longitudinal median section of the shell 

 of Triton iiodiferus. 



