bb 



GENERAL TRINCirLES OF 7x)OLOC,\' 



among other examples, liy the cak-areous shells of molluscs and the chit- 

 inous integument of insects (fig. 2O, /). 



Stratified Epithelia. — The protection furnished liy the cuticle in the 

 case of simple epithelium, may in the stratitied he obtained immediately 

 through a change of a part of the cells themselves. In the stratitied 

 epithelia the cells of lire \arious layers can ]>e distinguished L>y their form. 



Fig. 26— \ r f r f , I U u 11 It 1 C| ll 1 u is lniraf>hanus, 



a' in cros. sect ( , <; in surfatL \ie\\ , b and r, (.uliDulal and tolumnai qiilhclium i f a 

 mollusc (Haliotis luhercuhil a) ; d, llagclUUcd epithelium (if an aclinian (Calliiulis 

 parasitica): r, ciliated epithelium from the intestine of the fre.sh-waler mussel;/, epithe- 

 lium (e) with cuticle (c) of Ciiiibcx coruiialiis( a wasii). 



The deepest layer consists of c)-lindrical cells; the superficial, of more or 

 less llattened dements; between lie several la>ers of trajisitional forms, 

 so that starling from (he 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 stale of active multiplication; their descendants, with i^radual 



