THE FRESH WATER MUSSEL I4I 



end. Is the upper part still separate from the lower ? Note 

 the method of union. Find two openings, or siphons. With 

 a needle or bristle trace the openings of each backward 

 toward the body. The water enters at the incurrent siphon, 

 bathes the gills, and passes out at the excurrent siphon. 

 Which is the incurrent orifice? Which the excurrent? 



(c) The gills have perhaps already been observed lying 

 flatly on the body under the mantle. Observe them carefully. 

 What is their general appearance? How many on the right 

 side? Are they attached to one another? Can you find 

 out how the water which enters at the siphon reaches them ? 



Remove a piece of gill with the scissors and lay it on a 

 slide in water. With a needle try to find whether it is 

 single or double. What shape would a section across the 

 gills of one side be? Put a small piece under the micro- 

 scope and find the minute structure. The water tubes may 

 be seen, and if the specimen is fresh the action of the cilia 

 is also visible. Draw the structure as seen through the 

 microscope. 



Under the gills may be seen the body. Follow it an- 

 teriorly and find the fleshy, muscular foot, and the labial 

 palpi surrounding the mouth. How many labial palpi? 

 Examine the mouth. Has it jaws or teeth? What kind of 

 food does the mussel eat? 



Internal Structure. — (i) On the dorsal side near 

 the hinge the stringlike heart may be seen through the 

 thin mantle wall. Locate it carefully and then sever the 

 mantle wall and remove it from the body. This exposes 

 the ventricle of the heart with a tube, the intestine, running 

 through it. On either side of the heart find a white mem- 

 brane extending from the gills to the heart. This is the 



