54 EVENINGS AT THE MICROSCOPE 



the hollow of which is filled with a white substance, 

 which can be scraped away even with the finger-nail, 

 and which is sometimes used as pounce, to rub on paper 

 from which writing has been erased. It is this sub- 

 stance of which I mean now to speak. 



The possessor of this structure is a member of the nu- 

 merous class MOLLUSCA, which are generally characterized 

 by being enclosed in shells. Now shell, as we all know, 

 is a solid, stony substance, much heavier than water; take 

 into your hand that large Cassis on the mantelpiece, and 

 observe its great weight and compactness. It is, in fact, 

 real limestone; differing from that of the rocks only in 

 this, that it has been deposited by the living organic cells 

 of an animal, and arranged in a definite form. We will 

 presently examine other examples. The *' cuttle-bone" is 

 a shell, not indeed enclosing the animal, but enclosed by 

 it; being contained within a cavity in the substance of the 

 fleshy mantle ; cut open the mantle, and the shell instantly 

 drops out. 



The Cuttle is a rapid swimmer through the open sea. 

 A shell so large as this, if solid and compact like that of 

 the Cassis, would condemn it to grovel on the bottom, and 

 frustrate all the instincts of its nature. On the other hand, 

 it needs the strength and support of a solid column. Won- 

 derful to tell, the calcareous shell is made not only to be 

 no hinderance to its swimming, but to contribute greatly to 

 its buoyancy : it is what the string of corks is to the bather 

 who cannot swim it is a float. Throw this entire cuttle- 

 shell into water; it floats on the surface as buoyantly as 

 if it were actually carved out of cork. 



I cut with a keen knife a little cube out of the 

 44 pounce," and, fixing it on the end of the revolving 



