THE RIVER MUSSEL. 



143 



Observe a soft, whitish membrane, with a narrow, dark- 

 colored, ruffled border lining each valve. It is the mantle. 

 Loosen the edge of the mantle from the edge of one valve 

 by pushing a knife blade between the two, and drawing 

 it entirely around the free border of the valve. Keep 

 the point of the blade close to the shell. Observe that 

 the mantle clings to the valve along a line within and 

 parallel to its margin. Observe that the point of the 

 knife blade meets with an obstruction near the dorsal sur- 



DIAGRAM OF UNIO: u, umbo; h, hinge; s, siphons; small arrows indicate 

 the direction of the water currents; large arrow indicates direction of 

 travel, and also depth to which the animal is usually found buried in the 

 sand in locomotion ; /, foot ; e, edge of the valves ; I, concentric lines of 

 growth ; m, mantle margins extruded in front. 



face, at both anterior and posterior ends : it encounters 

 the strong muscles which hold the valves together. Cut 

 these off close to the valve. Notice the valves spring 

 apart when these are severed. Free the one valve from 

 all connection with mantle and muscles, and turn it back 

 like the lid of a watch case. 



I. The Different Parts. Study the following parts, 

 doing no more dissecting than is absolutely necessary : 



1. The mantle. Observe the thinness and transpar- 



