SLUGS. 67 
between these two familiar animals. Firstly, one 
afforded by the nature and position of the shell. 
Observe the oval prominence on the back of that 
large spotted slug (Limax maximus) ; this is the 
mantle which in the snail forms a sack through 
which the head and foot protrude, but in the 
slug covers but a small part of the body. Be- 
neath the shield-ike mantle hes a thin shelly 
plate which protects the viscera. A snail may 
be viewed in the light of a slug whose visceral 
matter and mantle are elongated upwards, 
and then spirally coiled, the mantle secreting 
an external shell, instead of depositing shelly 
matter from its mner surface. The shelly 
plate of the slugs has been called the snail’s 
stone, and was formerly esteemed a valuable 
medicine in cases of gravel and strangury. This 
internal shell varies in size, structure, and 
position in the different species and genera, a 
fact overlooked by Swammerdam ; for to account 
for large slugs having “ very small membranous 
plates, while the smaller ones had them often 
much larger, and formed of solid stone,” he was 
inclined to think “that the snails change this 
their little stone yearly, in the same manner 
as crawfish change those two semiconvex and 
plain stones which are likewise placed in their 
thorax, and are improperly called crab’s eyes.”’ 
Beneath the mantle on the right side of the 
F2 
