HELIX. 263 
part destroyed by the twisting of the portion contained 
in the shell and consequent loss of some organs and 
distortion of others. 
The whole ventral surface is expanded into a flat muscular 
creeping organ or foo¢, and in the mid-dorsal region is the 
shell, containing a part of the body called the 
visceral hump. The whole body is soft, and has 
no cuticular exoskeleton as in the Arthropoda, 
nor is there any trace of metameric segmentation. 
The shell is a right-handed spiral. Its central axis is 
called the columella, with a hollow cavity, the umbilicus, in 
its centre. The apex of the shell represents its first formed 
portion or mucleuvs. The shell consists of three layers, the 
outer ¢hitinous and coloured part, the middle white calcareous 
layer, and the inner thin smooth zacreous layer. Round its 
edge may be seen the cod/ar or thickened edge of the mantle 
which secretes the shell. The anterior end of the body 
forms the Head, which bears two pairs of retractile tentacles, 
the upper of which carry a terminal eye. Just below the 
head is the mouth, with a chitinous upper jaw and a pair of 
soft lateral lips. Below the head and above the foot is the 
wide opening of the fedal gland which secretes the slime on 
which the foot creeps. On the right side of the head is a 
small opening, the genital aperture. Towards the right end 
of the collar is a large opening, the pulmonary aperture, 
leading into the pulmonary chamber, a space below the 
mantle. Close to this aperture are the avws and the 
excretory pore. All the four external apertures last men- 
tioned are therefore asymmetric and on the right side only. 
If the shell be broken off carefully the visceral hump is 
exposed. ‘The lowest inch or so of the coil will be seen to 
be formed of a soft membranous man¢/e in which there are 
numerous pulmonary veins. Air is taken through the pul- 
monary aperture into the pulmonary chamber, hence the 
mantle forms the respiratory organ of the snail. In this 
respect it differs from the great majority of Gastropoda, which 
are aquatic and breathe by gills under the mantle. 
If the thickened edge of the mantle (co//ar) be cut away 
from its line of fusion with the dorsal wall of the body, and 
the cut be carried up the inner spiral just below the rectum 
(seen as a white tube running down to the azus), the mantle 
External 
Features. 
