MICROSCOPE AT THE SEA-SIDE. 77 



from the shell being composed of a series of hex- 

 agonal prisms, as seen in the view of a longitudinal 

 section given at figure 166, plate 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, as seen at figure 167. In 

 many shells the inner layer is almost structureless, 

 but in those cases where the smooth white appear- 

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

 consists of a series of waved laminae lying irre- 

 gularly one on the top of the other ; represented at 

 figure 169. In other shells this membranous in- 

 ternal layer is traversed by minute tubes, as is seen 

 in the genus Anomia, seen at figure 168. This 

 structure has been considered clue to the natural 

 form of the shell ; but late investigations lead to 

 the conclusion that these tubules are the borings of 

 some parasitic animal. 



The shells of the Crustacea also present a series 

 of very interesting structural differences. The shell 

 of the common prawn, when mounted in Canada 

 balsam, or examined in water or glycerine, presents 

 a series of bodies looking like nucleated cells. 

 These are seen in figure 170, plate 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 pre- 

 sented by the prawn-shell may be produced by the 

 crystallization 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- 



