16 . LAND AND FRESH-WATER SHELLS. 



by the mantle, not by the shell-gland of the embryo. In structure, 

 according to the. researches of Longe and Mer, it may be said to 

 consist of three layers, a cuticle (epidermis, periostracum\ a honey- 

 comb layer, and a nacreous or mother-of-pearl layer. The first of 

 these layers is formed by a cutogenic apparatus composed of an 

 " epithelial organ. " of bottle-shaped cells secreting granules, and 

 of numerous glandular caeca which open into a line the " pallial 

 line " situated just behind the collar of the mantle. Nalepa has 

 found that in Zonites algirus the cells of this "epithelial organ" 

 develop in spring, the period of greatest growth, but after that /.*., 

 during summer, autumn and winter they gradually atrophy, and, 

 according to Longe and Mer, they are entirely wanting in the 

 full-grown animal. The colour of the shell is secreted by the uni- 

 cellular glands of the mantle, the second layer by the edge of the 

 mantle, and the third layer by the whole surface of the mantle. 

 The honeycomb layer is composed of a superficial and a deeper 

 portion ; the former shows a confused striated appearance, the 

 latter consists of vertical prisms. The nacreous layer is arranged 

 in three super-posed series of calcareous lamellae, those of the outer 

 and inner layers running parallel to the suture, and the middle 

 layer at right angles to it. Splinters of the shell are hard enough 

 to scratch calc-spar; the form in which the calcareous matter 

 exists is represented by arragonite (Ca Co 3 ) in the mineral 

 kingdom. In chemical composition the shell is made up of 

 about 95-98 per cent, of carbonate of lime, of about 1*5 

 per cent, of an organic substance termed conchiolin, and of 

 magnesium carbonate, silica, and alumina, with phosphates, in 

 small quantities. 



The mouth of the shell is closed in some species by an 

 epiphragm, in others by an operculum, and in others still by 

 a clausilium. Each of these structures requires a few words. 

 The epiphragm is a film of mucus impregnated by calcareous 

 salts secreted over the aperture of the shell in some species, as 

 the Helices, previous to hibernation. Barfurth has shown that the 

 calcareous salts are a secretion of the cells of the digestive gland. 

 The adhesion that takes place between the epiphragm and the 

 peristome of the shell is dissolved away in the spring by an 

 excretion of fluid mucus from the mantle, after which it is easily 



