212 SNAILS AND SLUGS [CH. VII 



they live dry up ; and this in addition to burying 

 themselves in the mud. A similar but permanent and 

 moveable contrivance for closing the aperture of the 

 shell is found in the pulmonate gastropods Glausilia. 

 This structure is known as the " clausilium." It consists 

 of a white plate attached by an elastic spirally-twisted 

 stalk to the columella. When the animal comes out of 

 the shell it presses the clausilium back against the inside 

 of the shell, leaving the exit free. On the retreat of the 

 animal the elastic stalk pulls the clausilium into position 

 across the aperture. The whole may be compared to a 

 door shut with a spring. There can be no doubt that in 

 all cases the purpose of the epiphragm is not only, nor 

 perhaps chiefly, to form a protection against climatic 

 conditions, but also to prevent the entry of carnivorous foes 

 such as beetles, crustaceans, and parasitic worms. The 

 hibernation of slugs generally takes place below the 

 surface of the earth within a slimy cocoon. 



The body of Helix is attached to the shell by means of 

 the columellar muscle which runs along the inner side of 

 the -spiral of the shell and is fastened by tendons to the 

 columella in the upper part of the first turn. From 

 the tendinous origins strong bundles of muscle fibres pass 

 down into the foot : it is by the contraction of these that 

 the animal is retracted within the shell. Slugs, as is well 

 known, have not a similar protective shell and, indeed, are 

 popularly supposed to have none. Nevertheless the shell-sac 

 of Arion (A. ater, the large black slug) contains numerous 

 crystals of calcium carbonate, and in Limax (L. flavus, the 

 yellow cellar slug, and L. maximus, a large pale grey slug 



