88 



GENERAL PRINCIPLES OF ZOOLOGY. 



account of their hardness, can exist, at least for a more or 

 less considerable time, without epithelial covering. 



Glandular Sensory Epithelia. By their superficial 

 position the epithelia are suited for presiding over two other 

 functions: all substances which ought to be removed from 





FIG. 24. Various forms of the different epithelia. a, flattened epithelium of Sycandra 

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

 lium of a snail (Haliotis tufrerculata) ; d, flagellated epithelium of an Actinian (Calli- 

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

 epithelium with cuticle of Cimbex coronatus (a wasp). 



the body some because they have become useless, and con- 

 sequently injurious (excreta), and others, as, for example, 

 the digestive fluids, because they have to perform still 

 further important functions (secreta) must pass the surface, 



