SNAILS AND SLUGS 



are the openings of the excretory system and the anus. These 

 apertures are so concealed in the fleshy substance of the collar 

 that they are difficult to see. 



When a snail crawls along it leaves behind it a slimy trail. 

 This slime is partly derived from the general surface of the body, 

 but a large portion of it is discharged from a special opening 

 situated between the anterior edge of the " foot " and the lower 

 lip. From this aperture a bed of slime is laid down over which 

 the animal crawls, partly by means of cilia covering the sole and 

 partly by a series of waves, some 30 to 50 per minute, of muscular 

 contraction and expansion which sweep over the " foot " from 

 before backwards. The fine undulatory movements of the 

 ventral surface impress upon the slime a series of transverse 

 wavy ridges and furrows. These can be made evident if a snail 

 is made to crawl up an inclined sheet of glass viewed from below ; 

 or the animal may be enclosed in a lamp chimney for the purpose. 

 A track of a different kind is often left by a snail which has been 

 feeding as it slowly moves along ; for instance, at times the 

 " green " is removed in this way from the bark of a tree or surface 

 of a paling, or again the whitening may be eaten off the glass 

 roof of a greenhouse. If the slime-trail of a snail be followed it 

 will generally be found that the animal has returned to the hiding- 

 place whence it set out ; 

 thus showing that a 

 good sense of direction 

 is possessed. 



The food of snails and 

 slugs consists of veget- 

 able matter as a rule, 

 either leaves or young 

 shoots or fruit; but some 

 slugs are carnivorous, e.g. 

 the shell - bearing slug 

 Testacella, which eats 

 earthworms, while some devour fungi, and others are not averse 

 to the droppings of birds. In all cases the food is attacked by 

 means of a hard crescent-shaped horny jaw upon the roof and 

 a rasp-like band upon the floor of the mouth ; the latter is termed 





FIG. 31. Section of jaw apparatus of.snail. KF, jaw ; 

 KN, cartilage on which radula is carried ; M, mus- 

 cles of radula ; MH, mouth cavity ; O, mouth ; 

 OE, gullet ; RD, radula ; Z, sheath of radula. 



