68 LAND AND FRESH-WATER MOLLUSKS. 
body is an aperture leading into the bie cai a 
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 iritated. The “ silvery slimy 
trails’? of slugs and snails are depositions of 
mucus left in their tracks. Theslugs 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 Limaz the 
shell is of the nature of a thin oblong, or slightly 
concave plate, the mantle-shield marked with 
concentric lines, and the respiratory orifice near 
the hind part of the shield. Geomalacus has a 
gland at the extremity of the tail ike Arion, the 
- respiratory orifice nearer the front than in Limaa, 
