32 PEARLS [CH. 



in position. It is partially hidden by two lips which 

 are produced at the sides to form flat plates — known as 

 labial palps. Since each corner of the mouth is 

 produced laterally to merge into the gutter between 

 the two palps on each side (a feature not easily 

 shown in a diagram), the two pair of palps act as 

 directing walls and guide food to the mouth. The 

 mouth leads into the alimentary canal, a more or less 

 simple tube, which, with the exception of a swollen 

 region, the stomach, is much the same in structure 

 and appearance throughout its length. 



It coils once and finally runs over the adductor 

 muscle to open by the anus into the cavity between 

 the two shell valves. Tlie stomach is embedded in, 

 and surrounded by, the so-called liver. It is very 

 difficult, however, to compare any organ in Inverte- 

 brates with organs in Vertebrates, and this so-called 

 liver possesses many functions that are not performed 

 by the human liver. It is better to call it a digestive 

 gland. It secretes juices that help to break up and 

 alter the food and it absorbs this digested nutriment. 



Now let us see how these organs do their work 

 and by what means the pearl oyster obtains its food. 

 Starting at the very beginning of the story, the food 

 of the pearl oyster consists of minute animals and 

 plants which occur everywhere floating in the sea- 

 water. These same microscopic organisms are found 

 in the fresh water of ponds, streams, rivers and lakes 



