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 Puimonates 

 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- 

 lariidae) 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 aerated, and hence the necessity for 

 being able to extract proportionately more oxygen 



