THE OYSTER. 31 



the soft mud of our bays and estuaries, in order to 

 avail itself of the rich vegetation ; that it has lost its 

 symmetry in order to fit it for this mode of life. The 

 oyster is a very ancient animal, and its sedentary habits 

 belong to the more modern part of its history ; although 

 this change took place very long ago, so far as human 

 chronology goes, for fossil oysters are found in many 

 parts of the world. 



In order to understand the anatomy of the oyster, a 

 clear conception of the structure and significance of 

 its gill is most important. In all the bivalve mol- 

 luscs the gills are very complicated, and they domi- 

 nate the whole structure of the body in such a way 

 that an anatomical sketch of the animal is of necessity 

 little more than an account of the gills. A thorough 

 knowledge of the oyster-gill will not only throw light 

 on the purpose and use of all its other organs : it will 

 at the same time help us to understand the great value 

 of the animal as a means for making the microscopic 

 inhabitants of our waters useful, and it will also show 

 how well it is adapted for cultivation, and why it is 

 impossible for natural oysters to stock the whole bay 

 without aid from man. 



The labor which is necessary before we can have a 

 clear, accurate picture of the gills ; of their complicated 

 structure; their relation to other parts of the body; 

 their use and their origin, is considerable, but it is 

 well worth while ; for the gills give us the key to the 

 whole significance of the oyster. The oyster's gill 

 cannot be understood without close attention to all the 



