CH. VII] SNAILS AND SLUGS 207 



small original shell, i.e. to what is the apex of the fully 

 grown shell. This process is repeated as often as may be 

 necessary and gives rise to the lines of growth already 

 mentioned. By far the greater part of the shell is formed 

 by glands situated near the thickened edge of the "mantle," 

 known as the " collar." The details of the mode of 

 formation have been worked out by Longe and Mer 1 , and 

 independently by Moynier de Yillepoix 2 . In growing 

 snails there may be seen just behind the collar a white 

 band, the pallial gland. In front of this is a groove into 

 which glandular structures open ; it is here that the 

 outer layer or " epidermis " (periostracum) of the shell is 

 produced. This layer is horny and uncalcified and is best 

 seen round the shell-lip in young snails during periods of 

 active growth, where it is almost free from the colours which 

 it subsequently acquires, and is quite flexible, being as yet 

 destitute of the subjacent harder calcareous constituents 

 of the older shell. The white band secretes the dense 

 calcareous matter of the middle layer of the shell, and the 

 surface of the mantle posterior to the band completes the 

 process by adding the innermost or "nacreous" layer. 

 When a snail is fully grown the epidermic groove and the 

 white band gradually disappear, but the general surface of 

 the mantle retains the power of laying down shell material. 

 Normally, and in the absence of accident the additions 

 thus made to the thickness of the shell are comparatively 

 slight. The shell increases in thickness from the apex 

 towards the lip, shewing that the chief additions are those 



1 C. R. Acad. ScL Paris, xc. 1880. 



2 Ibid. cxin. and Bull Soc, ooZ, Fr< xvu. 



