ADDITIONAL ANIMAL STUDIES 183 



end. After cutting the muscle at the other end, we can readily 

 separate the valves. All over the surface of the animal, except 

 where the two muscles were attached to the shell, is a thin cover- 

 ing called the mantle. By raising the body of the mussel from the 

 valve it will be evident that there is a similar structure on the 

 other side. 



Now, if we fold back the mantle, it will be possible to follow the 

 course of the food and water. The first thing that strikes our at- 



muscles that hold 

 > the valves to- 



muscles that .. jSSK. -\! gether 



hold the valves 

 together 



umbo 

 excurrent siphon. 



incurrent'' 

 siphon 



lines of growth 

 FIG. 132. Fresh water mussel with foot extended. 



tention is the contracted foot, and above this is a soft mass called 

 the abdomen. In the abdomen are found the digestive organs. On 

 each side of the abdomen are two broad, thin flaps, the gills, by which 

 the animal breathes. Between the foot and the end that was bur- 

 ied in the sand are found, on either side of the body, two small flaps 

 or palps, and between them lies the mouth opening. To this mouth 

 the food that has been swept into the tube is brought by the wav- 

 ing of thousands of cilia that are found on the surface cells of the 

 gills and palps. 



Let us now return to the study of the mussel partly covered by 

 the sand. The hinge is on the dorsal region of the body, the free 

 edges of the valves on the ventral, while the mouth and foot are at 



