CLAMS, MUSSELS, AND OYSTERS. 45 



the pupil may get these clusters. Before studying a speci- 

 men, it is best to clean the shell thoroughly in water by means 

 of a coarse brush. 



44. Instead of having two adductor muscles, it has but 

 one (though this muscle, it seems, is composed of two ele- 

 ments). A single dark-purple mark on the inside of each 

 valve shows the point of attachment of the adductor muscle. 

 When the oyster is opened, the mantle contracts somewhat, 

 so that the edge of the mantle is some way from the margin 

 of the shell, as shown in Fig. 47. 



The left valve is the larger, and is the one that becomes 

 attached ; the right valve is flattened, and somewhat smaller. 

 The mantle has its margins free ; that is, the edges are not 

 united as in the common clam, where they are not only 

 united, but greatly thickened. Neither is the mantle pro- 

 longed into siphons as in other species ; consequently, the 

 water flows in at one portion of the shell, and pours out of 

 another portion, not being definitely conducted by special 

 channels, as in those forms heretofore given. The oyster 

 can be readily studied, as specimens may be got in almost 

 every village in the country. 



In looking over canned specimens, be sure and pick out a 

 large one, and one that does not appear to be mutilated, as 

 they frequently are when taken out of the shell by the oys- 

 terman, or jammed, as they often are in packing. 



To those who can get them alive, it is well to have the 

 oysterman open the specimen, being sure that he removes 

 the larger valve, leaving the oyster attached, and resting in 



