MOLLUSCA 



The largest oysters * procurable are of course the 

 best for teaching purposes ; but small ones can be used, 

 though less effectually. 



They should be cleaned with a sponge, just enough 

 to remove the dirt, without destroying the brown, horny 

 skin covering the shell, which sometimes is quite per- 

 fect, though generally more or less worn off. They 

 can be killed by laying them in fresh water, which is 

 kept at a blood heat, or about 90° Fahr. This is a 

 slow process, however, twenty-four hours being re- 

 quired sometimes. It may be quickened by nicking 

 or filing the edges until two small holes are opened, 

 admitting the water directly into the interior. It will 

 often be found advisable to put wedges between the 

 valves when they open, after the death of the animal, 

 and leave the shells for some hours. 



The first operation is to seek on every shell for the 

 marks left on one of the valves which show where it has 

 been attached to some rock, or stick, or other oyster, f 



The greater size and convexity of this valve will at 

 once attract attention; and that it is also the lower 



* Cohasset oysters are especially good, and can be obtained 

 from wholesale dealers. 



t Occasionally these are wanting, but as a rule they will be 

 found. 



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