190 
THE MOLLUSKS 
creeping disk. There is a head region provided with eyes 
and tentacles. The mouth of the snail is provided with a 
rasping structure known as the lingual 
ribbon (lm'gwal: Latin, lingua, tongue) by 
means of which it is able to cut and bore 
its way, even through rocks. Land snails 
by osmosis get oxygen from the air through 
the mantle, while water snails use gills and 
take their oxygen from the water. 
In the garden slug the shell when present 
is thin and affords small protection. 
180. Pond Snail. — The pond snail, 
gual Ribbon of a known by the scientific name of Physa, is 
kLrged)^ reatly 6n " Ver ^ common fresh-water ponds, where 
it lives among the water plants. It moves 
about by means of muscular movements in its broad foot. 
It is interesting to observe Physa from beneath as it creeps 
over the surface of glass and to note these contraction 
waves. As it 
moves through the 
water a trail of 
slime is left be¬ 
hind. If these 
snails are studied 
in a jar of water, 
some will be seen 
creeping along 
the surface, some 
slowly sinking to 
the bottom and 
others creeping up 
slirqe threads (Fig- Figure 1 77. —Snail Shells. 
ure 175). 
The pond snails live almost entirely on plants. Most of 
these plants are caught in the slime threads and both are 
