18 

 MOLLUSCS 



SNAILS 



Several species of snail are found in Britain, some living 

 amongst land vegetation and others in fresh water. The two most 

 frequently studied are the garden snail, Helix aspersa, and the 

 larger Roman or edible snail, H. pomatia. The following description 

 applies, except for certain points, to both. H. aspersa is wide- 

 spread in thick vegetation, although it is rare in or absent from 

 many districts in which the soil is deficient in lime ; H. pomatia 

 is much more local in its distribution, and although it was 

 previously thought to have been introduced by the Romans, 

 who are known to have cultivated it for food, its shells have now 

 been found in pre-Roman deposits. Both species are in fact edible, 

 and eaten ; aspersa was especially sought after in the glass- 

 blowing districts, where it was considered good for the wind, 

 while pomatia is the escargot of French menus. 



When a snail is moving (Fig. 195) there are three obvious 

 divisions of the body ; an anterior head, not sharply marked off, 

 but bearing the mouth and two pairs of tentacles, of which the 

 posterior pair are longer and bear eyes ; a long muscular foot, 

 on which the animal moves ; and the dorsal shell, inside which is 

 the visceral mass or hump. Below the edge of the shell is a thick, 

 fleshy rim called the collar ; this is the edge of the mantle, which 

 is referred to below. A number of openings can be seen. Below 

 the shell on the right side is a conspicuous pulmonary aperture, 

 which opens not into the body but into a space called the mantle 

 cavity, within the shell. Just inside the pulmonary aperture is 

 the opening of the excretory duct, and just behind this is the 

 anus. On the right side of the front part of the foot a groove runs 

 forward to the common genital aperture just behind the second 

 tentacle. The foot consists of longitudinal muscle fibres, and if 

 a snail be watched crawHng on a glass plate, waves of contraction 

 can be seen running from tail to head along its length. The 

 contact between the foot and the surface on which it moves is 

 lubricated by mucus discharged by a pedal gland which opens 

 just below the mouth. 



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