THE MOLLUSC A 



mineral constituent is carbonate of lime, but from 1 to 2 per cent of 

 phosphate of lime is also present, and traces (less than \ per cent) 

 of carbonate of magnesium. The greater part of the thickness of 

 the shell is made up by the ostracum, which consists of two layers : 

 (1) an external layer, frequently coloured, and formed by prisms of 

 calcite ; (2) an internal layer consisting of arragonite, generally in 

 the form of overlapping plates : it is the internal layer that forms, 

 in various species, the nacre or mother-of-pearl. 



The growth of the shell is effected in two ways. Its extent is 

 increased by the addition of new matter, secreted by the thickened 

 edge of the mantle, to the outer or prismatic layer. Its thickness 

 is increased by addition of new matter, secreted by the whole 

 surface of the mantle, to the inner or nacreous layer. In addition 



it 



vt. 



. 



Young embryo of Purjwra lapillus, from the left side, x 16. a, anus ; /, foot ; m, mouth ; 

 of, otocyst ; yi.a.c, post-unal cilia ; re, embryonic kidney ; sh, shell : ve, velum ; vi, vitellus. 



to the two layers of the ostracum already mentioned, there is a 

 third layer, called the hypostracum, on the areas of attachment of 

 the muscles which serve to fasten the animal to its shell. Finally, 

 there is the periostracum or shell epidermis, forming an external 

 covering to most shells: it is formed by the pallial fold at the 

 edge of the mantle. In some Polyplacophora, Gastropoda, and 

 Lamellibranchia, and in most of the Cephalopoda, the free edges of 

 the mantle are reflected over the shell, so as to cover a greater or less 

 part of its outer surface. In some species in which this special 

 feature is developed to its greatest extent, the reflected mantle 

 edges form a completely closed sac round the shell, so that the 

 latter is internal and concealed. In rare cases the pedal integument 

 may secrete a calcified " shell," which may be adherent (Hipponyx) 

 or free (Argonauta). 



The shell was for a long time the criterium of Molluscan 

 classification. But the progress of anatomical study showed that 



