44 Shell Life 



of others in order to feed upon the occupant, the 

 saliva is found to contain an appreciable proportion 

 of free sulphuric acid, which evidently softens the 

 spot of shell upon which the radula is then brought 

 to bear. 



The molkiscan stomacli, with which we have not 

 space to deal, varies with the habit and food of the 

 mollusk. Some species have it lined with knife-like 

 plates, or with knobs and teeth. The Canoe-shell 

 {Scaphander lignarius) has a gizzard in which the 

 Tusk-shell (Dentaliuni), its favourite food, is crushed 

 up prior to digestion. 



Having glanced at the mechanism by which the 

 mollusks prepare their food for assimilation, let us 

 take a similarly brief and superficial view of the 

 means whereby the blood is aerated in the principal 

 groups. It is evident that one system of respiration 

 will not serve for the whole of the Mollusca, seeing 

 that whilst some spend their lives submerged in fresh 

 or salt water, others live as constantly surrounded 

 by the atmosphere. Many of the fresh-water species, 

 however, breathe air only, and cannot therefore 

 venture far from the surface, their air-chamber 

 requiring to be frequently recharged. On the other 

 hand, marine species like the Limpet and Periwinkle, 

 provided with gills for breathing water, have to 

 spend hours on the dry rocks, and can only do so 

 by storing sufficient moisture to keep their gills 

 damp. 



It is probable that the ancestral mollusk had no 

 special organs for respiration, but simply absorbed 

 oxygen through his surface tissues, beneath Avhich 

 the blood liowed and took up the vital gas. Then 



