340 NATURE-STUDY REVIEW [16:8— Nov., 1920 



Points of interest concerning these animals are best understood 

 if we recall more important points of their structure. A real 

 teacher among Zoologists has compared them to a book in which 

 the covers are represented by the shells, and the fly leaves by the 

 lobes of the mantle, the latter being a membranous covering of the 

 animal's body which both lines the shell and secretes it. The 

 halves of the mantle, like the halves of the shell, are curved and 

 thus enclose a space, the mantle cavity, which is partly filled by the 

 gills, foot, and the rest of the body. But two structures besides 

 the shell and foot appear to have regular contact with the outside 

 world. These we shall term the inhalant and exhalant siphons, 

 and they take, as seen in the illustration, the form of two tubes, one 

 above the other, which the animal is able to protrude or contract 

 from the posterior, (larger) end of the shell. These portions of the 

 creature's body as well as the lower edge of the mantle seem well 

 provided with sense organs. Irritation of them or disturbance of 

 the water causes the -shell to close up "like a clam." It is addi- 

 tionally known that the senses of smell and possibly taste also exist 

 while some are sensitive to light. Through the lower siphon, a cur- 

 rent of water may be drawn in which bears not only oxygen for 

 respiration, but also the minute animals, plants, organic debris 

 upon which these creatures feed. The gills remove this oxygen 

 from the water, while the food material is swept into the mouth at 

 the opposite end. Material useless for food leaves the body through 

 the exhalent siphon, assisted by the current of water leaving the 

 gills. 



When it so happens as has often been inexcusably the case with 

 so many of our finest rivers, that the surroundings of the mussels 

 become polluted with wastes preventing the development of the 

 small organisms on which they feed, or with organic matter which 

 absorbs the oxygen from the water, it follows that such conditions 

 will neither permit the clams nor any but the hardiest forms of 

 animal life to survive. The clam can only temporarily close its 

 shell, a protection it is soon forced to relinquish from lack of the 

 necessities of life when the elastic hinge of the shell overcomes the 

 resistance of the muscles keeping it closed. Clearly there is need 

 of a different sentiment toward our streams and all of their natural 

 beauty. However, more natural enemies of the fresh water mus- 

 sels are the muskrat, before whose burrows piles of dead shells may 

 be found and certain birds which have learned to fish for them and 

 drop the shell from on nigh to break it to obtain the soft parts. 



