46 Shell Life 



through which the blood flows and takes up oxygen 

 through their tissues. In the bivalves, as will be 

 more fully described later, the gills vary greatly in 

 the degree of complexity. They are usually placed 

 on either side of the body between it and the mantle 

 which thus form the branchial cavity. In most cases a 

 couple of tubes or siphons lead to this chamber from 

 the exterior, one admitting fresh supplies of water, 

 the other carrying off that which has passed over the 

 gills and been robbed of its oxygen. 



In the land and fresh-water snails (excepting the 

 few whose shells are closed by an operculum) respira- 

 tion is effected in a " lung-chamber," though there is 

 no lung in the sense of a spongy mass of tubes as we 

 know it in the higher animals. The chamber, which 

 opens on the right side of the snail, more or less 

 towards the front part, is lined with a network of 

 branchinof vessels throuo^h which the blood circulates. 

 Air is taken into this cavity frequently when the 

 mollusk is active, and then the opening is closed 

 until all the oxygen has been taken up by the blood, 

 when the chamber is refilled and again closed. The 

 Pond-snails (Lhnncea) w^hen living in shallow water 

 frequently come to the surface to recharge this cavity 

 with fresh air, but when living at considerable depths 

 they appear to absorb the oxygen from the water, 

 whether by filling this lung-chamber or through the 

 general skin-surface is not quite clear. 



In intimate relation with the varied forms of gills 

 or lungs there is, of course, a heart ; and where there 

 is only a single gill there will only be one auricle and 

 one ventricle to the heart, but there may be two or 

 four auricles if there are as many gills. From the 



