66 



GENERAL PRINCIPLES OF ZOOLOGY 



among other examples, by the calcareous shells of molluscs and the chit- 

 inous integument of insects (fig. 26, f). 



Stratified Epithelia. The protection furnished by the cuticle in the 

 case of simple epithelium, may in the stratified be obtained immediately 

 through a change of a part of the cells themselves. In (he stratified 

 epithelia the cells of the various layers can be distinguished by their form. 



FIG. 26. Various forms of epithelia. a, flattened epithelium oiSycandra raphanus, 

 a' in cross-section, a" in surface view; b and c, cuboidal and columnar epithelium of a 

 mollusc (Haliotis tuberculata); d, flagellated epithelium of an actinian (C alii act is 

 parasitica); e, ciliated epithelium from the intestine of the fresh-water mussel;/, epithe- 

 lium (e) with cuticle (c) of Cimbex coronjtus( a wasp). 



The deepest layer consists of cylindrical cells; the superficial, of more or 

 less flattened elements; between lie several layers of transitional forms, 

 so that starting from the cylindrical cells we gradually pass through cubical 

 cells to the flat cells of the surface. As this arrangement shows, there 

 exists a genetic connection between the layers of cells: the lower cylindrical 

 cells are in a state of active multiplication; their descendants, with gradual 



