ANIMAL LIFE AND THE MICROSCOPE 



which are discovered from time to time have either 

 ganoid or placoid scales, a fact which shows that 

 the sharks, rays and sturgeon are directly descended 

 from creatures which swam the seas thousands of 

 years ago. 



The shells of shell-fish are not easy to examine 

 microscopically, but frequently their plates may bC; 

 detached from the edges of such shells as oysters 

 and mussels and these should be examined. If the 

 outer part of the shell be taken we can easily see its 

 honeycomb structure and, by adding a little acid 

 and waiting till all action has ceased, we shall have 

 a structure remaining which is remarkably like a 

 number of plant cells. The inner layer of many of 

 these shells is composed of beautifully iridescent 

 mother-of-pearl. Now such iridescence is usually 

 caused by surfaces furrowed with many very fine 

 lines and mother-of-pearl is no exception. Under 

 the microscope, with a moderately high magnifica- 

 tion, we can see minute striations all practically 

 parallel to one another. 



The cuttlefish is peculiar in having a skeleton 

 which is a moderately soft plate. These plates can 

 often be found washed up by the tide, may be cut 

 out from a dead cuttlefish or bought from a chemist's 

 as cuttlefish bone. However we secure the material 

 we shall find that one side of the " bone " is hollow 

 and that across this hollow, delicate plates run 

 parallel to one another at intervals. Between these 

 parallel plates there appear to be a number of fibres 

 but, if we cut a thin slice of the structure and ex- 



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