704 ZOOLOGY SECT. 



impressed spiral line or suture, parallel with which are numerous 

 fine ridges and depressions the lines of growth ; the increase in 

 size of the shell takes place in the direction of these lines, not at 

 right angles to them as in the shell of the fresh-water Mussel, 

 and the lines that more strictly correspond to the lines of growth 

 of the latter are excessively fine striae which run transversely to 

 the stronger lines. At certain points, usually three in a full-grown 

 shell, the spiral is interrupted by a transversely directed edge 

 which appears to overlap the succeeding portion ; this edge marks 

 the position which the mouth of the shell has occupied 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 siphoned process. The promi- 

 nent edge of the peristome is in relation to the dorsal 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 re- 

 tracted 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 

 FIG. 620. opercuium process is for the lodgment of a spout-like 

 cundus ton rub process of the edge of the mantle the 



siphon,. 



When removed from the water, or disturbed in any other way, 



the animal becomes completely withdrawn into the interior of 



the shell, when the latter is observed to become closed by a plate 



-the opercuium (Fig. 620) which fits accurately across the passage 



some distance internal to the peristome. The opercuium 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. 666). Its outer surface is covered 

 with a thin layer of uncalcified 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 

 intv.lor, even wlien tV ' nal has extended itself to its utmost, 

 tue 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 



