Ill] THE PEARL OYSTER 35 



set up. This sets the water in motion. The current 

 of water enters the shell between the two mantle 

 lobes at a region about the middle of the ventral 

 margin. It passes over the gill plates and through 

 them. Passing through them continually, the water 

 is to a certain extent robbed of its oxygen and leaves 

 the shell at what is known as the exhalent opening, 

 situated posteriorly. If however food particles enter 

 with the water of the inflowing current, they are 

 wafted along the gill margins anteriorly, until they 

 reach the two flaps known as the palps. There are a 

 pair of these on each side of the body; the outer- 

 most of each is really a prolongation of the upper lip 

 of the mouth and the innermost a prolongation of 

 the lower lip. Consequently the gutter or groove 

 between either pair of palps leads to the mouth. 



Like the gills, the palps are covered with cilia, 

 hence the food particles are wafted along between 

 them up to the mouth opening. The particles may 

 be accepted as food (and then they pass into the 

 alimentary canal) or they may be rejected. In the 

 latter case they are picked up by a current of water 

 running along parallel and close to the margin of the 

 mantle and are carried back until the exhalent 

 opening is reached. Here they leave the shell 

 together with the current of water that has passed 

 through the gills. In this manner the pearl oysters, 

 and indeed also the cockle, mussel, and other bivalves 



3—2 



