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 foot^ and in the mid-dorsal region is the 



shelly containing 'a part of the body called the 



■visceral hump. The whole body is soft, and has 



no cuticular exoskeleton as in the Arthropoday 



nor is there any trace of metameric segmentation. 



The shell is a right-handed spiral. Its central axis is 

 ^ctM.d the columella^ with a hollow cavity, the umbilicus^ in 

 its centre. The apex of the shell represents its first formed 

 portion or nucleus. The shell consists of three layers, the 

 outer chitifious and coloured part, the middle white calcareous 

 layer, and the inn^r thin smooth nacreous layer. Round its 

 edge may be seen the collar or thickened edge of the mantle 

 which secretes the shell. The anterior end of the body 

 forms the head, which bears tw^o 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 pedal 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 anus 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 mantle 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 d^Ters from the great majority of Gastropoda^ which 

 are aquatic and breathe by gills under the mantle. 

 / If the thickened edge of the mantle [collar) 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 « hite tube running down to the anus), the mantle 



