56 A HALF-HOUR WITH THE 



form a very thin layer, it may be examined with 

 transmitted light, and its hexagonal structure will 

 be much more apparent. If a portion be examined 

 lengthwise, it will be seen that the hexagons result 

 from the shell being composed of a series of hex- 

 agonal prisms (Fig. 162, PL 6). 



All bivalve shells partake, more or less, of this 

 character ; and if a portion of the outer coating of 

 the shell of the oyster be examined, it will be 

 found to present a general resemblance to that of 

 the shell of the pinna (Fig. 163, PL 6). In many 

 shells the inner layer is almost structureless, but 

 in those cases where the smooth white appearance 

 is presented which is called mother-of-pearl, it 

 consists of a series of waved laminge lying irre- 

 gularly one on the top of the other (Fig. 165, PL 6). 

 In other shells this membranous internal layer is 

 traversed by tubes, as is seen in the genus Anomia 

 (Fig. 164, PL 6). 



The shells of the Crustacea also present a series 

 of very interesting structural differences. The shell 

 of the common prawn, when a thin section is made, 

 presents a series of bodies looking like nucleated 

 cells (Fig. 166, PL 6). Many shells present this 

 appearance, and it was at one time supposed to 

 indicate clearly that the shell originates in cell- 

 growth as well as other parts of the structure of 

 an animal. It has been, however, recently shown, 

 that such appearances as that presented by the 

 prawn-shell may be produced by the crystalliza- 

 tion of inorganic salts in contact with organic 

 substances in solution, independent of a living 

 organism. 



Surprising as it may seem to some persons, the 

 teeth of mollusca afford beautiful objects for mi- 

 croscopic examination. All that is necessary to 

 examine these organs is, to take the palate, or 



