286 PRACTICAL LESSONS IN SCIENCE. 



fresh water mussel. The fresh-water mussel is abundant in the 

 beds of most of the streams and ponds of the Mississippi valley, 

 and can easily be taken during the low water of the summer. 

 They may be kept for observation in a tub or bucket, in which 

 there are two or three inches of mud and sand covered by six or 

 eight inches of water. A few specimens should be preserved in 

 alcohol for dissection. With a live shell in the hand, notice the 

 two parts or valves of the shell : the edge along which the valves 

 are joined is the dorsal margin, and the other is the ventral mar- 

 gin of the shell. The concentric lines parallel to the ventral mar- 

 gin are lines of growth, the raised portion which forms the center 

 of these lines is the beak or umbo which is nearer the front or 

 anterior end of the mussel. Holding the shell with the dorsal 

 margin upward, and the anterior portion forward, notice the re- 

 lation of parts, the anterior and posterior regions, the right and 

 left valves, the ventral and dorsal margins, and the heavy hinge 

 ligament binding the valves together. 



Drop two or three live mussels into water that is nearly boiling 

 hot. The hot water will kill the animal and loosen it from its shell. 

 When cool enough to handle open the shell by turning back the 

 left valve. Notice the white membrane, mantle, adhering to the in- 

 ner surface of the shell; it can easily be removed with the fingers; 

 then notice the glistening inner surface of the shell, and the rough 

 line parallel with the margin of the valve called the pallia! line t or 

 impression, as it marks the line along which the mantle was at- 

 tached to the shell. Notice that this line terminates in two oval 

 rough-bottomed pits near the extremities of the hinge ligament. 

 With the fingers or a thin piece of wood carefully separate the 

 mussel from the other valve, noting the pits near the dorsal 

 margin, and the bodies which occupied them. They are the ante- 

 rior and posterior adductor muscles' by which the mussel closes 

 his shell. Note their relative length and thickness, and their 

 structure. Notice that the shell does not remain closed, the 

 elasticity of the hinge ligament opens it as soon as the muscles 

 are cut or loosened. Notice that the mantle lobes are continuous 

 across the dorsal portion of the mussel. The hatchet-shaped, 



