PHYLUM MOLLUSCA 



723 



surface of the body of the animal ; the opposite side has no 

 prominent edge, but is rounded off to form a smooth inner lip ; 

 a couple of ridges on this inner lip towards the apical end aid the 

 animal in drawing itself out after it has become retracted into the 

 interior of the shell. The outer lip is in relation 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 opcrculum 

 (Fig. 610) which fits ac- 

 curately across the passage 

 some distance internal to 

 the peristome. The oper- 

 culum 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. 683). Its 

 outer surface is covered 

 with a thin layer of uncal- 

 cified chitinoid material, the 



periostracum, beneath which is a thick prismatic layer, and, lining 

 the inner surface, a layer of nacre. 



External Features of Soft Parts. The Triton is able to 

 extend itself to a considerable degree beyond the mouth of the 

 shell ; but a portion of the body always remains concealed in the 

 interior, even when the animal has extended itself to its utmost, 

 the body being, like .that of the fresh-water Mussel (and of nearly 

 all the Mollusca) organically connected with the shell. In Triton 

 the connection is by means of a strong muscle the columellar 



FIG. 609. Longitudinal median section of the shell 

 of Triton nodiferus. 



