120 LIFE AND HER CHILDREN. 
and devoured by other animals, the remainder settle 
down and feed on the sea-bottom. 
This is the history of the sea-forms, and we have 
now to glance at those on the land. First, we must 
notice, in passing, the water-snails in the ponds and 
rivers, feeding on decayed leaves and travelling often 
from place to place, floating shell downwards on the 
surface of the water, or hanging from the water-plants 
by slimy threads. Some of these have and some have 
not the horny door, while some breathe by gills, and 
others are air-breathers. Then we have not much 
difficulty in recognising the land-snails as being very 
like the periwinkle, only breathing by air instead of 
by water. The way this is done is very simple. If 
you watch a snail when its head is out of its shell, 
you will see a little slit opening and shutting steadily 
in the top of the neck, and through this hole air is 
passing into a closed chamber made by a fold of 
the mantle. The walls of this chamber are covered 
with a network of blood-vessels, through which the 
blood flows, taking oxygen this time from the air 
instead of from water. By this simple arrangement 
the snail, no longer confined to the sea and rivers, is 
able to spread over the fields, and woods, and gar- 
dens, feeding on the delicate juicy leaves of plants, 
on mosses, and fungi, and all the rich vegetation of 
the country. But it has many dangers, for birds and 
hedgehogs, and even insects, prey upon it greedily. 
Therefore it feeds chiefly in the dusk of the evening ; 
while it has sharp eyes (e, Fig. 46) set upon long stalks, 
which can see on all sides when it is out of its shell. 
Now in order to retire safely into its shell, it must 
be able to draw in these eyes, and also the two ten- 
