CHAPTEE XI 

 PHYLUM X.: MOLLUSCA {continued) 



Bivalve Molluscs 



Another group of Molluscs is characterised by the possession 

 of a two-lobed mantle resulting in a bivalve shell. These two 

 valves are equal in size in all fresh-water bivalves, but in 

 manj'- marine forms, e.g. the oyster, one valve is much larger 

 than the other. During life the body can be completely 

 withdrawn into the shell, and the two valves held tightly 

 closed by special muscles which are developed at both ends of 

 the body attaching it to the valves of the shell. In this 

 way very complete protection is gained, except against such 

 enemies as the whelk which can bore a hole right through 

 the shell (see p. 121). 



These bivalves are all aquatic, and obtain their food 

 merely from the current of water which they suck in ; hence 

 they have no need, either for protection or for purposes of 

 nutrition, of active movements or of keen senses, and we 

 find them generally living sedentary lives, often partially buried 

 in the sand or mud at the bottom of the water. Correlated 

 with this inactive life is the necessarily greater development 

 of the respiratory organs (compare also sedentary with free- 

 swimming woi'ms), and at the same time the degeneration of 

 the head with its sensitive tentacles and eyes. There is 

 merely a mouth-opening in the front of the body, and no 

 special head-lobe. The mouth contains no rasp, for none is 

 needed where the food is not to be scraped from the surface 

 of a solid object. 



139 



