92 THE OYSTER. 



own shell. As long as the shell is soft and thin, the 

 danger from enemies is very great, and this danger is 

 greatly diminished as soon as the shell becomes thick 

 enough to resist attack. It is, therefore, very necessary 

 that the shell should be built up as rapidly as possible, 

 and an abundant supply of food in general will be of no 

 advantage unless the supply of lime is great enough 

 for the growth of the shell to keep pace with the 

 growth of the body. All sea water contains lime in 

 solution, but the percentage is, of course, greatest near 

 the sources of supply. It is well known that on 

 coral reefs, which are entirely made of lime, all kinds 

 of shelled molluscs flourish in unusual abundance, and 

 have very strong and massive shells ; and our common 

 land and fresh-water snails are much larger and more 

 abundant in a limestone region than in one where 

 the supply of lime is scanty. In such regions it is not 

 unusual to find the snails gathered around old decay- 

 ing bones, to which they have been drawn in order to 

 obtain a supply of lime for their shells. 



From all these causes combined it results that a 

 young oyster which settles upon a natural oyster-bed 

 has a much better chance of survival than one which 

 settles anywhere else, and a natural bed thus tends 

 to perpetuate itself and to persist as a definite, well- 

 defined area ; but there is still another reason. As the 

 flood-tide rushes up the channels it stirs up the fine 

 mud which has been deposited in the deep water. The 

 mud is swept up on the shallows along the shore, 

 and if these are level, much of the sediment settles 



