IX MOLLUSC A 107 



usually indicate the limit between the growth of successive 

 years. 



The newly-formed sliell is a light yellow-brown, but it 

 very soon becomes discoloured and dark, in ordinary stagnant 

 water. In texture it is rather delicate and brittle, and it is 

 considerably rougher and darker on the outer side than on 

 the inner, where it is lined with a light-coloured, smooth, 

 glistening layer. A striking difference between the outer 

 covering of the shell and the inner layer, can be demonstrated 

 in a perfect shell, by touching each part in turn with a rod 

 dipped in weak hydrochloric acid (10 per cent or weaker). 

 The inner lining shows an immediate effervescence, indicating 

 that it is calcareous in composition, whilst the outer covering 

 gives no such reaction. 



If left immersed in acid for a sufficient time the whole of 

 the shell will be dissolved away, except for the delicate outer 

 skin, which will still retain the shape of the shell. This outer 

 skin consists of a horny substance, similar to the chitin 

 which forms the armour-like covering of most insects. It is 

 of special value to snails, living as they often do in stagnant 

 water, where there is abundant animal and vegetable life and 

 much decaying organic matter, for such water contains a con- 

 siderable amount of carbonic acid which, were it not for this 

 protective layer, would attack the calcareous shell. 



The size of the full-grown shell seems to depend chiefly 

 upon the volume of the water in which it has grown, the less 

 the amount of water the smaller the shell, other conditions 

 being similar and favourable to growth. 



By the coiling of successive whorls of the shell, each whorl 

 is in contact with the one above it, but their inner margins 

 do not touch, and so a hollow pillar is formed from apex 

 to base of shell. This pillar is known as the columella of the 

 shell, and the lower open end of the cavity of the pillar is 

 the umbilicus.^ In Limnaea, however, the umbilicus is hidden 

 by the lip of the shell growing over it (see Fig. 60). 



Sometimes, as in the whelk, the columella is solid, and so 

 there is no umbilicus. To the columella are fastened the 

 muscles which attach' the snail to its shell, and also the upper 

 end of the muscles which move the foot, and cause its contrac- 

 tion and withdrawal into the shell, when desired. 



^ See diagram of the shell of the land snail, p. 131. 



