SHELL OF EDIBLE SNAIL. 107 



des cdtes de France, de Lacaze Duthiers, A. Z. Expt. iii. 1874; vi. 1877. Heller, 

 Untersuchungen, &c., Dk. Wien. Akad. xxxiv. 1875 ; xxxvii. 1877 ; and SB. 

 Wien. Akad. Ixxvii. Abth i. 1878. 



Degeneration. Ray Lankester, Nature Series, 1880. 



Various points. R. Hertwig, J. Z. vii. 1872. 



Test. O. Hertwig, J. Z. vii. 1872. Semper, Verhandl. Phys. Med. Gesellsch. 

 zu Wurzburg, vii. 1875. 



Tunicin Cellulose. Cf. Watt's Dictionary of Chemistry, v. p. 918, and 

 Suppl. 2, 1875, p. 271. 



Nervous System. E. van Beneden et Julin, Archives de Biol. v. 1884. Neural 

 gland. Julin, Archives de Biol. ii. 1881. Herdman, Nature, xxviii. 1883. 



Endostyle. Fol, M. J. i. 1876. 



Tubular gland opening into stomach. Chandelon, Bull. Acad. Royale Belgique 

 (2) xxxix. 1875. Ulianin, on Doliolum, Fauna and Flora des Golfes von Neapel, 

 x. 1884. 



20. SHELL OF EDIBLE SNAIL (Helix pomatid). 



WHEN the shell has its apex directed upwards, and its aperture down- 

 wards and towards the reader, its spire, as is the case with the great 

 majority of Gasteropod spiral shells, ascends obliquely towards the right. 

 It is in consequence termed ' dextral.' In the living animal inhabiting such 

 a shell, the heart is on the left, the generative, respiratory and anal aper- 

 tures on the right of the body. The aperture of the shell in Helix as in all 

 vegetable-feeding Gastropoda is entire, i. e. forms an unbroken curve : in 

 carnivorous Gastropoda on the contrary it is notched or produced into a 

 canal which lodges a process of the mantle or siphon : e. g. in the Whelk. 

 These two varieties of aperture are known respectively as ' holostomatous ' 

 and ' siphonostomatous.' The columella or pillar formed by the contact of 

 successive whorls of the shell lies on the left side of the aperture in the 

 angle between the first whorl and the peristome or free margin of the 

 aperture. It is hollow and the external opening or umbilicus is partially 

 hidden by the peristome, not wholly as it is in the Garden Snail, Helix 

 aspersa. The smooth rounded off edge of the peristome shows the animal 

 to have been adult. The shell is coarsely striated in a direction parallel to 

 the margin of the aperture, i. e. corresponding with the lines of growth, and 

 more delicately in a spiral direction, parallel to the five coloured bands with 

 which it is marked. The ' apex ' or ' nucleus/ the tip of the shell or the 

 part first formed, is smooth and semi-porcellanous in appearance, thus 

 differing, as is so often the case, from the rest of the shell. The spiral line 

 marking the point of contact between one whorl and its successor is known 

 as the ' suture.' The shell increases in thickness towards the peristome, and 

 it is probable that but little addition to its substance takes place except in 



