334 MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 



moUuscan shell differs from the skeletal parts of other animals 

 in the fact that where once formed it is not subject to change 

 and the removal of effete portions, either interstitially as in the 

 Vertebrata, or en masse as in the Crustacea^ although the 

 structure is to some extent organised. 



As I have just stated, the shell itself is secreted by the 

 mantle or cloak, although there is reason to suppose in some 

 Gasteropods with expanded bases, such as Cassis, that the 

 upper part of the foot is also concerned in its formation. The 

 shell is really an excretion, and the body of the animal moves 

 freely over its polished inner surface, except at the insertion of 

 the muscles which are inseparable from the shell during life. 

 The muscles are attached to the shell in different ways. In 

 bivalves there are either one or two transverse muscles which 

 pierce the body, and when contracted overcome the elasticity 

 of the hinge and close the interval between the valves. 



In the MoUusca which are covered with a shell in the form 

 of a case or sheath, as in some Pteropods and Gastropods, the 

 animal is connected to the base of the shell by a large dorsal 

 retractor muscle. In the limpet the muscular fibres are attached 

 all round the shell, and inserted into the edge of the foot, where 

 they interlace with its circular fibres. 



The spiral shells are attached to the snail by two muscles 

 attached to the columella, which traverse the mantle and take 

 their oiigin in the foot. By the examination of the inner side 

 of shells much variety may be seen in the depressions which 

 mark the site of the attachment of the muscles, these being 

 apparently in some cases much stronger than in other examples 

 of the same species, as may be seen in Capulus. 



I have called the shell an excretion, and so it is, although 

 it exhibits a distinctly organised structure, due to the fact that 

 it is formed by epithelial cells of the mantle and consolidated by 

 the withdrawal of carbonate of lime from the circulating fluids 

 of the animal, but at the same time the living inhabitant exerts 



J.C., vii., Jan. 1894, 



