8 



there are four tleshy flaps — the palps (P), two ou each side oi 

 the body. They are much like the gills in appearance aud are 

 connected with each other by two ridges which run across the 

 middle of the body, and between these folds is the large, oval 

 mouth, m, which is thus seen to be sUicated, not at the open end^ 

 of the shells, but as far away from it as possible. 



As the 0}ster is inmiovably fixed to some support, aud has 

 no arms or other structures for seizing food and carrying it to 

 the mouth, the question how it obtains its food at once sug- 

 gests itself. To answer this question, as well as to understand 

 tiie relationship of the various parts of the oyster, a thorough 

 understanding of the structure and function of the gills is 

 necessary. 



The gills are not only, as their name implies, breathing 

 organs for the aeration of the blood, but are also the means 

 by which food is gathered up from the water, carried forward 

 t(j the mouth, and by which currents of water are maintained 

 in the mantle cavity. 



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

 body. They are long, flat, thin, leaflike organs 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. 



A single gill may be roughly represented by a sheet of 

 paper folded lengthwise, with the ends glued together, and the 

 opening of the pocket thus formed held uppermost. 



The 'closed portions of the gills hang down into the mantle 

 chamber, side by side, while their upper edges are fastened to 

 each other and to the inside of the mantle, so that they form a 

 folded partition, like two W's, which divides the mantle cavity 

 into two parts : a lower chamber, in which the gills hang, known 

 as the gill chamber, and an upper chamber, into which the 

 pockets open, called the cloaeal chamber. 



The cavity within each gill is divided by a series of vertical 

 partitions into small 'chambers, the water tubes, each of which 

 ends blindly below and opens above into the cloaeal chamber. 

 The walls of the water tubes are made up of a number of min- 

 ute rods, the gill filaments, placed side by side, each ridge being 

 separated from the next by a deep furrow. In the bottom of 



