THE MANTLE-COVERED ANIMALS. 121 
tacles or feelers below, and here we find a beautiful 
machinery. If you watch a snail drawing in its horns 
you will see that the eye disappears down the tube, 
just as the tip of a glove-finger does, when you draw it 
down from inside the glove. These horns are in fact 
hollow tubes, and a special muscle pulls them in from 
the top downwards, and when the eye is wanted again, 
it is only necessary for the muscles round the tube to 
contract, and so to squeeze the tip gradually out. 
Most of the land-snails have lost the horny door, 
not having any need for it ; but in winter, when they 
sleep without food in the cracks of old walls, under 
the bark of trees, and in other sheltered spots, they 
pour out a layer of slime, which hardens and shuts 
them into their shell till spring returns. 
Slugs (C, Fig. 46), on the other hand, bury them- 
selves in the ground for winter safety. At first sight 
you might imagine that a slug had no shell at all, 
but if you examine carefully you will find a small 
shell (s] under its black skin, just behind the neck, 
and the small breathing hole (b] at the side will show 
you that this shell covers the breathing organs. This 
is in fact the only part of a slug's body which is 
covered by the mantle, and if you alarm him you 
will see him draw his head in under it, as though he 
expected it to shield him from danger. No doubt 
the absence of a large shell enables the slug to creep 
into many places where a snail cannot go, and the 
havoc worked by these creatures in our gardens 
shows how rapidly and successfully they feed. The 
great gray slug"" has a supply of 28,000 teeth, so 
that he can use them without scruple ; and if it 
* Limax maximus. 
