68 LAND AND FRESH-WATER MOLLUSKS. 
body is an aperture leading into tlie respiratory 
orifice. 
The lingual ribbon of the slugs is characterized 
by its numerous transverse rows, containing a 
large number of very similar teeth; in general, 
the central tooth has a long central point with 
a small denticulation on each side of it; the 
lateral teeth, as they approach the margin, be¬ 
come spine-like, having but a long projecting 
point. 
All molluscous animals excrete a mucous fluid 
to lubricate the skin, furnished by glands situated 
in it; the slugs copiously exude this slime, more 
especially when irritated. The “ silvery slimy 
trails ” of slugs and snails are depositions of 
mucus left in their tracks. The slugs are divided 
into four genera, characterized principally by the 
relative positions of the mantle, shell, and respi¬ 
ratory orifice. Thus in Arion the shell is repre¬ 
sented by mere calcareous granules in the mantle; 
the respiratory orifice is near the fore part of 
the shield; the body is truncated behind, and 
terminated by a mucous gland. In Limax the 
shell is of the nature of a thin oblong, or slightly 
concave plate, the mantle-shield marked with 
concentric lines, %nd the respiratory orifice near 
the hind part of ^he shield. Geomalacus has a 
gland at the extreihity of the tail like Arion , the 
respiratory orifice nd^rer the front than in Limax , 
