206 HALF-HOURS IN THE GREEN L.LNES. 



snails that have left their shells, and that they have 

 the power of returning to them whenever they 

 please. So far from this being the case, slugs make 

 no shells at all, whilst the true snail cannot leave 

 its shell, being attached to it by certain muscles. 

 The Large Spotted slug (Limax maximus) is exceed- 

 ingly common, and this may serve as an example of 

 the general structure of the rest. Over what would 

 be its shoulders if it had feet, is the oval-shaped 

 mantle. Beneath this, you may find a thin shelly 

 plate, so that the chief difference between snails and 

 slugs is that the mantle of the former deposits 

 shelly matter on the outside, and the latter inside. 

 In both cases, the object is to protect the visceral 

 organs. The shelly plates of the slugs were formerly 

 called " snail stones," and were believed to possess 

 a medicinal virtue when taken for the disease called 

 " gravel." Like the fresh- water snails already re- 

 ferred to, the mouth of the slug is armed with rows 

 of curved teeth, placed on a moveable ribbon. The 

 mucus which slugs secrete so abundantly, and 

 which is one reason for their being so generally 

 disliked, is formed by glands which are situated in 

 the skin. Our native slugs are divided into four 

 genera, according to the relative position of the 

 mantle, the shell, and the breathing orifice. The 

 shell often exists in a very rudimentary state, as in 

 the Black slug (Arion ater], where it is represented 

 merely by granules of shelly matter diffused through 

 the mantle. The latter species is perhaps the most 



