CHAPTER VI 



RESPIRATION AND CIRCULATION THE MANTLE 



The principle of respiration is the same in the MoUusea as in 

 all other animals. The blood is purified by being brought, in 

 successive instalments, into contact with pure air or pure water, 

 the effect of which is to expel the carbonic acid produced by 

 animal combustion, and to take up fresh supplies of oxygen. 

 Whether the medium in which a mollusc lives be water or 

 air, the effect of the respiratory action is practically the 

 same. 



Broadly speaking, Mollusca whose usual habitat is the water 

 ' breathe ' water, while those whose usual habitat is the laud 

 'breathe' air. But this rule has its exceptions on both sides. 

 The great majority of the fresh-water Mollusca which are not 

 provided with an operculum (e.g. Limnaea, Physa, Flanorhis), 

 breathe air, in spite of living in the water. They make periodic 

 visits to the surface, and take down a bubble of air, retm^ning 

 again for another when it is exhausted. On the other hand 

 many marine Mollusca whicli live between tide-marks (e.g. Patella, 

 Littorina, Purpura, many species of CerWiium, Planaxis, and 

 Nerita) are left out of the water, through the bi-diurnal recess 

 of the tide, for many hours together. Such species invariably 

 retain several drops of water in their branchiae, and, aided by 

 the moisture of the air, contrive to support life until the water 

 returns to them. Some species of Littorina {e.g. our own L. rudis 

 and many tropical species) live so near high-water mark that at 

 neap-tides it must ixequently happen that they are untouched 

 by the sea for several weeks together, while they are frequently 

 exposed to a bm-ning sun, which beats upon the rocks to which 

 they cling. In this case it appears that the respiratory organs 



