74 GENERAL PRINCIPLES OF ZOOLOGJ. 



Simple Epithelium. Exclusively one-layered epithelia are 

 found in all invertebrated animals and in Amphioxus; in the 

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

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

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



TIG. 25 Various forms of epithelia. a, flattened epithelium of Sycandra raphanw, 

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

 lium of a mollusc (Haliotis tuberculata) ; d, flagellated epithelium of an actinian 

 (Calliactis parasitical; e, ciliated epithelium from the intestine of the fresh- water 

 mussel; /, epithelium (e) with cuticle (c) of Cimbex coronatus (a wasp). 



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

 and columnar epithelium. In the case of pavement epithelium 

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

 tions of space, and because they have become compressed by 

 lateral pressure have the appearance of cubical blocks or paving- 

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

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

 (fig. 25, c)', finally, in flat or squamous epithelium this is greatly 



