74 



GENERAL PRINCIPLES OF ZOOLOGY. 



Simple Epithelium. — Exclusively one-layered epitlielia are 

 foiuul ill all iiivertebrated animals and in Ampliioxus; in the 

 vertebrates, on the other hand, they are limited to the internal 

 cavities of the body, and even here are occasionally, as always in 

 the skin, replaced by a many-layered epithelium. According to 



Fig. 2!") — Vinous forms of epithelia. a, flattened epithelinm of Sifcandra rapli(inu.% 

 11/ in cross section, n" in siirfaoe view; b and r, ruboidal and columnar epithe- 

 lium of i moUusf (Haliiiti.^ tvfirrfiihitti); J, fla.y:ell;Ucd epithelium of an actinian 

 (('nUi<if li-^ }Kii(i^ttua\; r, ciliated epithelium fi-oiu the intestine of the f resh- water 

 mussel, / ( pithelium (c) with cuticle (ci of <'i)iihr.i' cnrniial it.^ (a wasp). 



the shape of the cells we distinguish cuboidal or pavement, flat, 

 and columnar epithelium. In the case of pavement epithelium 

 (fig. 25, ti) the cells are all developed about equally in all direc- 

 tions of space, and because tliey have become compressed by 

 lateral pressure have the appearance of cubical blocks or paving- 

 stones. In columnar epithelium the long axis, the distance from 

 tile deeper to the peripheral end of the cell, is especially great 

 (fig. 25, (■); finally, in flat or squamous epithelium this is greatly 



