SNAILS 
189 
Figure 174 . — Snail. 
Notice the relation of the animal to its 
shell. The eyes are borne on the ends of 
the tentacles on the top of the head. 
cells from the males taken in with the water through the 
siphon. These sperm cells have reached the water through 
the upper siphon. Thousands of embryos form in the body 
of the female and develop 
into larvae in the outer 
gills, which thus become 
greatly distended. Later 
the larvae pass into the 
water through the upper 
siphon. ( 
The larvae of many 
fresh-water clams have 
hooks on their shells by 
means of which they are 
able to cling to the gills or body of a fish, where they live as 
parasites for several weeks. They absorb food from their 
host and are carried from one place to another and are thus 
scattered. After a few 
weeks they leave the host 
and settle down to lead 
an independent life. 
179. Snails. — Snails, 
having one valve, are 
called univalves as dis¬ 
tinguished from clams, 
oysters, etc., which are 
called bivalves because 
their shells are formed of 
two valves. The greater 
number of snails are ma¬ 
rine (live in salt water), 
although some live in 
fresh water and some on 
land. Snails have a broad 
foot which is used as a 
Figure 175 . — Our Common Pond Snail. 
Notice that they are able to creep along 
the surface of the water like a fly walking 
on the ceiling. They also have special 
paths running in various directions through 
the water. These paths are made of slime 
that the snail gives off. There is a mass 
of eggs attached to one of the plants. 
These eggs are orange color and deposited 
in a mass of jelly. 
