248 ZOOLOGY. 



chief organ used in attacking the plant structures is 

 the swollen tongue, covered by a rasping plate 

 (radula), on the surface of which regular rows of teeth 

 are arranged. The rest of the tongue consists of 

 muscle, and two small cartilages are also found within 

 it. The teeth stick up when the tongue is protruded, 

 and the whole apparatus forms a kind of rasp, by the 

 backward and forward movements of which the food 

 is reduced to small fragments. Marine snails breathe 

 by gills, which are found in the mantle- cavity, while 

 most of the freshwater and all the land forms 

 (" Pulmonata ") breathe by means of the inner wall of 

 the mantle-cavity, which thus serves as a lung. Some 

 members of the group are hermaphrodite, while the 

 sexes are distinct in others ; the first is the case in all 

 terrestrial forms. Such snails pair, however, mutually 

 fertilizing each other. They are not able to fertilize 

 themselves. It is only among land snails and slugs 

 that injurious kinds are found. Species of the genus 

 Helix, e.g. the Edible or Roman Snail (if. pomatia) 

 and the Field Snail {H. neinoralis), may do harm in 

 fruit-culture ; Slugs are very injurious to agriculture 

 (Liinax and Avion) ; in Limax, the respiratory open- 

 ing is situated behind the middle of the right margin 

 of the mantle, while the genital opening lies behind 

 the right feeler ; in Arion, the respiratory opening is 

 situated on the front edge of the mantle, and the 

 genital opening immediately in front of it. Any 

 kind of snail or slug may be harmful, especially if it 

 attacks young plants. Some kinds, however, live 

 chiefly in woods, feeding upon toadstools and dung, 

 perhaps also on bark and weeds (e.g. dandelion). This 

 is especially true of the larger kinds, but these too, if 

 they multiply very rapidly, may efiect much damage 

 in gardens and fields. The commonest slugs are — 



The Common Black Slug {Arion ater). — About four 

 inches long, and generally black. 



The Grey Field Slug {Limax agrestis, Fig. 144). — 



