lo8 DESCRIPTIONS OF PREPARATIONS. 



that region. In this particular, as well as in being univalve and possessing 

 a much smaller amount of organic matter, the shell of a Gastropod 

 contrasts with that of a Lamellibranch. 



There is no operculum in the Ptdmonata^ the order of Gastropoda to 

 which the Snails and Slugs belong. When the snail hibernates it closes 

 the aperture of its shell by a whitish disc, the hibernaculum or epiphragma 

 a structure containing much organic matter and calcareous granules of 

 variable shape irregularly scattered. It appears to be secreted by the 

 margin of the mantle or collar (see next preparation) rather than by the 

 foot, as has been supposed. There are sometimes two such epiphragmata, 

 one within the other. The moveable clausilium of the pulmonate Clausilia, 

 which covers the aperture of the shell when the animal is retracted, appears 

 to be a structure homologous with the epiphragma. 



The majority of Gasteropod spiral shells are dextral. When the spire of a 

 shell turns in a direction opposite to what is normal, it is said to be 'reversed.' In 

 some species this is a common occurrence, e. g. in the Common Whelk ; in others 

 it is so usual that the type of shell commonly met with is sinistral, e. g. Clausilia. 

 The lower whorls of the shell may, in a few instances, become loose and straggling, 

 e.g. Vermetus, Siliquaria. 



The reflection of the peristome over the umbilicus in Helix is due to the 

 presence of a ' columellar lobule ' or projection of the collar. When the shell is 

 injured at the peristome before the animal is adult, both the outer non-calcareous 

 layer ' periostracum ' or ' epidermis ' and the calcareous layers internal to it are 

 repaired : when at any other spot, or after the animal is adult, only the latter, and 

 in this case the shell is thickened internally. The colouring matter is secreted by 

 unicellular glands of the collar. The spiral shell, as in all Gastropoda with few ex- 

 ceptions, is a secondary shell, i. e. formed on the outer surface of the mantle, and 

 not from the shell-gland of the embryo. A primary shell, i. e. one formed by this 

 gland, rarely persists. It is found, however, as the ' nucleus ' of the shell in the 

 pulmonate Clausilia, and as the internal shells of the Slugs Arion and Limax. 



The shell of the Helices is stated by Longe and Mer to consist of an external 

 organic cuticle or periostracum ; of an outer calcareous layer containing colouring 

 matter, and composed of (i) a thin external division with confused striation, (2) a 

 thicker internal division which consists of vertical prisms ; and of an inner nacreous 

 layer including several laminae of prisms arranged horizontally, the axes of the 

 prisms in the different laminae being at right angles to one another. The cuticle is 

 formed by a cutogenic apparatus situated just behind the collar. It is composed 

 of a pallial groove into which glandular caeca open, and an epithelial organ con- 

 sisting of long bottle-shaped cells which secrete granules. The epithelial organ 

 disappears in the adult, and the glands of the pallial groove gradually undergo 

 atrophy. The rest of the shell appears to be formed by the edge of the mantle ; 

 the nacre from its surface in general. In Zonites algirus, according to Nalepa, the 

 cells of the pallial groove develope in spring the period at which growth of the 

 shell takes place into long unicellular glands, and the cells of a ridge (? = epithelial 

 organ of Longe and Mer) just behind the groove enlarge into flask-shaped gland 



