EDIBLE SNAIL. 109 



cells. The cuticula is secreted by these two sets of gland cells. It is possible 

 that the glands may be developed in Helix, as in Zonites, in the spring of the year, 

 and that the atrophy of the organ in the adult (supra) merely marks a period of 

 rest. (See Nalepa, SB. Akad. Wien. Ixxxvii. Abth. i. 1883.) 



A layer of nacre or mother of pearl is not generally found in the calcareous 

 shells of Gastropoda. The calcareous matter is usually arranged in three layers 

 composed of parallel lamellae, those of the outer and inner layer usually disposed 

 parallel to the suture, of the middle layer at right angles to it. The direction of 

 the layers is sometimes reversed. The lamellae are vertical to the surface of the 

 shell, and each lamella is composed of calcareous prisms, all parallel to one 

 another, but forming an oblique angle with the surface of the shell. The prisms of 

 adjoining lamellae are disposed in contrary directions to one another. The 

 calcareous matter is in the form of Arragonite (Ca CO 3 ). Splinters of the shell are 

 hard enough to scratch Calc-spar. 



The Gastropod shell is composed chemically of about 1-5 per cent, of an 

 organic substance (Conchiolin) ; of about 95-98 per cent, of Lime carbonate ; with 

 small quantities of Magnesium carbonate ; phosphates ; silica and alumina. 



The inner whorls of the shell are in some Gastropoda partially absorbed. In 

 some cases the animal withdraws from the older region of the shell. The part 

 forsaken is closed off by a calcareous lamina secreted by the surface of the mantle. 

 The process of withdrawal and closing off may be repeated several times. 



Technical terms, structure, &c. Woodward, Manual of the Mollusca (ed. 3.), 

 1875, p. 28, p. 204. Keferstein, Mollusca, Bronn's Klass. und Ordn. des Thier- 

 reichs, iii. 2. p. 899, p. 1181. Zittel, Handbuch der Palaeontologie, Abth. i, ii. p. 153. 

 Leydig, Haut-decke und Schale der Gastropoden, A. N. 42. 1876. 



Structure and formation of Shell in Helices. Longe and Mer, C. R. xc. 1880, 

 cf. A. N. H. (5) v. 1880. 



Epiphragm. Keferstein, op. cit. supra, p. 1186. Binney, Terrestrial Mollusca 

 of United States, ii. 1851. Analysis and source of lime. Barfurth, A. M. A. xxii. 

 1883, p. 509. 



21. EDIBLE SNAIL (Helix pomatia) ) 



Dissected so as to show the position of the heart and the respiratory cavity. 



THE shell, with the exception of a part of the columella or spire left in 

 situ, has been removed. The foot is completely expanded and the left upper 

 tentacle is partially protruded. A depression on the animal's right side 

 anteriorly above the foot, marks the position of the generative aperture. 

 The thickened collar is seen crossing the animal's dorsum. In it, on the right, 

 is the pulmonary aperture, and below the aperture, again to the right, the 

 columellar lobule. The pulmonary chamber has been opened by a trans- 

 verse incision, and its cavity exposed by turning forward the anterior flap. 

 Its roof is for the most part thin, and is formed by the branchial fold of the 

 mantle, the anterior edge of which has fused, as in most pulmonate Molluscs, 



