40 EVENINGS AT THE MICROSCOPE. 



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 substance of which I 

 mean now to speak. 



The possessor of this structure is a member of the 

 numerous class MOLLUSCA,* which are generally charac- 

 terised by being inclosed 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 

 mantel-piece, and observe its great weight and compact- 

 ness. 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 de- 

 finite form. We will presently examine other examples. 

 The "cuttle-bone" is a shell, not indeed inclosing the 

 animal, but inclosed 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. 

 Wonderful to tell, the calcareous f shell is made not only to 

 be no hindrance 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 buoy- 

 antly as if it were actually carved out of cork. 



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

 and, fixing it on the end of the revolving stage-needle, 

 apply a low power, say seventy diameters, using reflected 



* From the Latin mollis, soft; a name given by Cuvier to this 

 class from their bodies being always soft, whether bearing shells or 

 not. 



f From calx, Latin for lime. 



