38 Shell Life 



attached themselves to rocks or weeds, or burrowed 

 slightly in sand or nuid, and lay with their mouths 

 open to receive such good things as might chance 

 to fall in. They lay over on one side, the mantle 

 became divided into two lobes, and consequently 

 the shell moulded upon its surface was in two 

 valves hinged together, the head was lost, though 

 the toothless, tongueless mouth remained, hidden 

 by the lobes of the mantle. 



That the general tendency of development was 

 in the direction of improving upon the simple 

 ancestral form is indicated by the fact that of the 

 enormous number of distinct species known in a 

 fossil or recent condition, only one-fifth are bi- 

 valves, whilst three-fourths have a single shell. 



To get back to the adaptation of the shell to the 

 conditions of life : pelagic or floating species have 

 the shell reduced to a thin glassy consistency, serv- 

 ing the double purpose of rendering them light and 

 transparent, the transparency preventing their easy 

 detection by enemies who are likely to regard them 

 instead as part of the jelly-fishes upon which such 

 pelagic mollusks largely subsist. Some species that 

 dwell in deep water out of the reach of surface 

 convulsions have very thin shells also. At the 

 other extreme some deep-water forms that have 

 evidently been much sought after by the larger 

 fishes for their food secrete very thick and solid 

 shells that may be proof against the equally hard 

 and stony palate-teeth of the fishes. In all the 

 fresh- water forms it is evident there can be little 

 need for thick and heavy shells, and as a matter 

 of fact they are light and thin. Especially is this 



