THE OYSTER. 33 



themselves. They are also organs for gathering up 

 food from the water, and after it has been gathered 

 they become organs for carrying it to the mouth. 

 They are also reproductive organs, adapted for secur- 

 ing the fertilization of the eggs, and thus providing for 

 the propagation of the species. In the European oyster 

 and in the mussel they are also brood-chambers, in 

 which the young are held and protected and nourished 

 during their early stages of growth, until they are large 

 enough to care for themselves. 



An organ which is at once a gill, a pump for sup- 

 plying the gills with water, a food-collector, an organ 

 for carrying the food into the mouth, a reproductive 

 organ, and a nursing-chamber, must, of course, be 

 complicated. The oyster's gill does all these things, 

 and does them all well. It is not a jack-of-all-trades, 

 but a machine which is beautifully adapted for carrying 

 them all on at the same time, in such a way that each 

 use helps the other uses, instead of hindering them. 

 This is the more remarkable since an ordinary mol- 

 lusc, such as the conch, has distinct organs for all 

 these purposes, although the oyster's gill does every- 

 thing just as well and just as readily as the various 

 organs of the conch. 



There are four gills in the oyster, two on each side 

 of the body. They are long, flat, thin, leaf-like organs, 

 Plates I and II, g f placed side by side, and nearly 

 filling the mantle chamber, in which they hang. Each 

 gill is made up of two leaves, so that there are in all 

 eight gill-leaves. 



