78 A FRESH-WATER MUSSEL. 



This is the oldest and thickest portion of the shell and is the initial 

 point from which growth proceeded. 



c) The anterior and the posterior ends of the animal, determined 



by the nearer proximity of the umbo to the former. 



d) The dorsal and ventral margins of the valves, the former hinged, 



the latter gaping. 



e) The hinge ligament connecting the right and left valves of the 



shell. 



f) The foot, a tough, tongue-shaped, muscular body protruding from 



the anterior end of the gape between the valves, if the animal is 

 alive. In the preserved specimen it will be withdrawn within the 

 shell, but can still be seen. 



g) The mantle, a delicate membrane one lobe of which lines internally 



each valve of the shell. Normally the mantle lobes extend to or 

 project quite beyond the margin of the valves, but, being muscular 

 and sensitive at the border, they contract when irritated, 

 h) Two oval openings, the siphons, formed by the close approxi- 

 mation of the edges of the right and left mantle lobes at the 

 posterior portion of the gape between the valves. The lower is 

 called the inhalant siphon, the upper the exhalant siphon. 

 Note the delicate projections, fimbriae, around the edges of the 

 former. 



If you have a living specimen, watch for the protrusion of the foot 

 and the forward movements of the animal. How is this progression 

 accomplished ? Can the mussel stand alone ? The currents of water 

 at the posterior end of the gape in the shell can easily be detected by 

 lowering a few drops of ink through a tube to a point opposite the 

 siphonal openings. The water thus taken in at one opening and 

 exhaled at the other serves the double purpose of conveying oxygen 

 and food to the animal, the latter in the form of microscopical organ- 

 isms. What is the probable function of the fimbriae ? Touch them. 



Technical Note. In a preserved specimen carefully remove the left 

 valve of the shell. This is accomplished by freeing it from the 

 corresponding mantle lobe by means of a thin knife, and then severing, 



