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SOFT-BODIED ANIMALS— THE MOLLUSKS 



the posterior end of the animal there is a pair of openings which allow 

 water to enter and leave the mantle cavity, which is the cavity lying be- 

 tween the mantle and the visceral hump and foot. The ventral opening 

 is the incurrent siphon, through which water enters the mantle cavity, 

 and the dorsal opening is the excurrent siphon, through which the water 



Photo by Cornelia Clarke 



Fig. 19.3. Pearl buttons from fresh-water clam. This photo shows a clam shell that 

 has been drilled out for buttons and a series of different kinds of buttons that are 



produced from such shells. 



leaves. There are a pair of gills on either side of the mantle cavity 

 which bear cilia that keep up the circulation of water through the cavity. 

 Respiration is carried on by the gills as the water flows through them. 

 The water coming into the cavity will contain bits of organic matter 

 which can be used as food. There are a pair of palps on either side of 

 the mouth at the anterior end of the body which sort out edible particles 

 and carry them into the mouth. Digestion takes place in a small stom- 



