THE OYSTER. 69 



ble lid. Its gills are very complicated organs, adapted 

 for drawing into the fixed shell a steady current of 

 water, and they pour into the open mouth of the animal 

 a constant stream of food, so that eating goes on as un- 

 interruptedly as breathing, and is just as much beyond 

 the control of the animal. The adult oyster makes 

 no efforts to obtain its food, it has no way to escape 

 from danger, and after its shell is entered it is per- 

 fectly helpless and at the mercy of the smallest enemy. 

 So far as active aggressive life goes it is almost as 

 inert and inanimate as a plant, and its life is purely 

 vegetative. This is the adult oyster. The young 

 oyster is very different. It is an active animal, swim- 

 ming from place to place. Its gills are not leaf-like, 

 and they do not divide the mantle-chamber into two 

 parts. They are nothing but breathing organs, and 

 are simple finger-like tentacles which hang down into 

 the water. There is no gill-current as there is in the 

 adult, and the young oyster must find its own food by 

 swimming through the water. Its two shells are also 

 exactly alike, and therefore quite different from those 

 of the adult. 



The egg therefore tends, at first, to build up an 

 animal which differs greatly from the adult, in struc- 

 ture as well as in habits, and naturalists believe, as I 

 have already said, that our modern oysters are the 

 descendants of an ancient form which was not seden- 

 tary, and the egg at first exhibits a decided tendency 

 to build up this ancestor rather than an oyster. 



Some of you may ask how we know that the remote 



