256 SNAILS. PHYLUM MOLLUSCA 



THE SHELL 



The shell is secreted by the mantle and consists of an outer 

 periostracum, made of an organic material called conchiolin, 

 and two layers largely made of calcium carbonate. The first of 

 these, whicii is in contact with the periostracum, is the prismatic 

 layer, and inside this is the nacreous layer. The last consists of 

 layers of thin plates of translucent material, set at a slight angle 

 to the surface of the shell, and it is to the interference caused by the 

 reflection of light from these plates that the colours of the inside of 

 the shell are due. They are much more brilliant in some lameUi- 

 branchs (p. 284). The shell is a three-dimensional spiral, built 

 round a central axis or columella ; to this is attached the columella 

 muscle, contraction of which pulls the whole animal inside the 

 shell. Most shells are right-handed (dextral) spirals, but about one 

 in a thousand is left-handed (sinistral) ; in these aberrant forms 

 the asymmetry of the body is reversed. In winter, snails become 

 inactive and torpid, often collecting in masses in holes in walls. 

 The opening of the shell is then closed by the epiphragm, a 

 temporary sheet of mucus containing some lime. In many water 

 snails there is a permanent hinged operculum for the same 

 purpose. 



TORSION 



When the shell is removed, which may sometimes be done 

 merely by unscrewing the animal, although other individuals 

 need to be cut up the shell along the spiral, the mantle and 

 visceral hump are exposed. The latter is covered with a thin skin, 

 from which hangs down the mantle, a thicker extension of skin 

 which is normally raised away from the hump and closely applied 

 to the shell which it has secreted. The lower edge of the mantle, 

 the collar, is fused to the body-'Wall, so that between the two there 

 is a chamber, the pulmonary cavity, which although inside the 

 shell is outside the body. At the pulmonary aperture the mantle 

 and body- wall are not fused. The main parts of the internal 

 organs have undergone both torsion and coiling, which makes 

 their understanding and dissection difficult. The embryo is at 

 first bilaterally symmetrical, with the anus posterior ; suddenly 

 the body twists, so that the gut, nerve cords and many other 

 structures^ are swung ta the right and forwards through nearly 



