412 



THE MICROSCOPE. 



The simplest form of shell occurs in the rudimentary 

 oval plate of the common Slug, Limax rufus, (Fig. 206); it 

 is imbedded in the shield situated at the back and near the 

 head of the animal. 



When a molluscous or conchiferous shell is composed 

 of a single piece, it is then termed univalve; when of two 



