430 TEXT-BOOK OF ZOOLOGY 



shell longitudinally, we see that it consists of a spirally-coiled passage, 

 the inner surfaces of which unite in a hollow " central axis," or 

 " columella," and which gradually widens out towards its aperture. 

 This "house" is sufficiently spacious to include even the protrusible 

 parts of the body. The retraction of these parts is effected by the 

 contraction of a muscle attached to the columella, and extending to 

 the anterior part of the creeping sole-like foot columellar muscle (see 

 Section C, 1). 



(b) Components and Layers of the Shell. If a snail (or mussel) shell 

 be placed in hydrochloric acid, it is almost entirely dissolved, carbonic 

 acid gas being liberated at the same time. This shows that it consists 

 for the most part of calcium carbonate, only a thin membranous structure 

 retaining the shape of the shell being left. This consists of a horn-like 

 substance similar to chitin, and called conchiolin. This substance enters 

 more especially into the composition of the outer layer of the shell, the 

 so-called epidermis, which hence protects the shell from being destroyed 

 by water holding carbonic acid in solution, such water, as is well known, 

 having the power of dissolving calcium carbonate. The central layer of 

 the shell is composed of densely-packed prisms of calcium carbonate, and 

 is known as the prismatic layer. The inner layer is composed of thin 

 undulating lamellae of carbonate of lime, and imparts to the shell the 

 well-known nacreous (or mother-of-pearl) appearance (produced by inter- 

 ference of light), whence it is known as the nacreous layer. In the land 

 snails, contrary to what we find in aquatic species, this layer is always 

 very thin. 



(c) Formation of the Shell. The mode of production and growth of 

 the shell may be distinctly observed in spring. At that time a thin, 

 newly-formed ring is found at its margin, which gradually increases in 

 hardness. It is imposed on the mantle (see Section A), and secreted by 

 the latter, just as the chitinous shell of arthropods is secreted from the 

 subjacent layer of the integument. (What differences, however, exist 

 between these two formations ?) The striation of the shell shows how it 

 has gradually arisen. The lime necessary for its construction is taken 

 in with the food, or obtained by the gnawing of calcareous rocks, the 

 cement of walls, etc. The large quantity of this material which has 

 annually to be secreted explains 



(d) The habitat of this snail and allied species of land snails, which 

 are only to be found in places where there is no dearth of lime. Hence 

 districts with chalky or limestone soil, such as is the case with many 

 vineyards, are frequently a veritable snail's paradise. 



