130 THE LIFE OF THE MOLLUSCA 
mantle. So, too, are the plume-like gills of many of 
of the Nudibranchs (Plate XII., Fig. 20-28), but in 
others of this group those curious outgrowths known 
as “cerata,” as well as the general mantle surface, 
also take on the function of respiratory organs. 
In the terrestrial Snails and freshwater Pulmonates 
respiration is carried on by means of the complex 
network of blood-vessels that overspread the roof of 
the pallial cavity, which thus acts as a “lung.” In 
one family of marsh-frequenting Snails (Ampul- 
lariidz) the pallial chamber is divided into two, 
the right-hand containing a gill, while the left is 
converted into a pulmonary chamber, whose opening 
is transformed into an extrusible siphon which the 
animal uses if near the surface of the water; or it 
can breathe by its gill under water and by its lung 
when out of water (Plate II., Fig. 1). 
All these changes appear to be made in response 
to the demands of the environment, even the highly 
complicated structure evolved in the Pelecypoda is 
a question of the requirements of respiration solely. 
|The majority of the gill-bearing Gastropoda and the 
less specialized Pelecypoda live in waters that are 
constantly in a state of more or less agitation, and 
where, consequently, entangled oxygen is compara- 
tively abundant ; whereas the Bivalves that burrow 
do not get the water in their lurking-places so fully 
or so frequently aérated, and hence the necessity for 
being able to extract proportionately more oxygen 
