CHAPTER VI 



RESPIRATION AND CIRCULATION — THE MANTLE 



The principle of respiration is the same in the Mollusca 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 land 

 '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^ Planorhis)^ 

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

 visits to the surface, and take down a bubble of air, return- 

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

 hand many marine Mollusca which live between tide-marks 

 (e.g. Patella^ Littorina^ Purpura^ many species of Oerithium^ 

 Planaxis, and Neritd) 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 bran- 

 chiae, 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 frequently happen 

 that they are untouched by the sea for several' weeks together, 

 while they are frequently exposed to a burning sun, which 

 beats upon the rocks to which thej cling. In this case it ap- 

 pears that the respiratory organs will perform their functions 



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