92 
LAND AND FRESH-WATER MOLLUSKS. 
Family Helicid^ (Land Snails ). 
The appellation of this wide-spread family of 
the Land Snails is derived from Helix , a snail, 
the typical genus of the group. The Helicidce 
are provided with an external spirally-coiled 
shell, capable of containing the entire animal. 
The body is conformable with the shell, and is 
distinct from the foot. The head of the snail is 
short and retractile, furnished with two pairs of 
cylindrical tentacles; the tentacles of the upper 
pair are the longest, and bear the eyes at their 
summits. 
The snail and slug have both the power of 
drawing in their horns on being touched. We 
shall now proceed to explain “ how this is done 
for the snail can be examined more conveniently. 
On lifting our snail, the head and tentacles are 
retracted; but not doubting the efficacy of the 
words of the old doggerel, we admonish the 
snail “to come out of his hole.^ And we ob¬ 
serve that the tentacle is lengthened by gradu¬ 
ally unfolding itself, and not by being pushed 
out from the base. Each tentacle is a hollow 
cylinder (see fig. 17), to the apex of which is 
attached a muscle (g), arising from the retractor 
muscle (m) of the foot, and by it§ contraction 
the tentacle is simply inverted and retracted. 
