34 



GENERAL PABT. 



atozoon is hat-shaped ; while in Crustacea it has the form of a cell, 

 with long radiating processes (fig. 20). 



Epithelial tissues consist of aggregations of cells which as simple 

 or stratified layers cover the external and internal surfaces of the 

 body, and which line its closed spaces (Endothelium). According to 

 the different shape of the cells composing it, we distinguish cylin- 

 drical, ciliated, and pavement epithelium. In the first case the cells, 

 in consequence of the elongation of the long axis, are cylindrical 

 (fig. 21, c) ; in the second, the free surface of the cells is beset with 

 vibratite cilia or flagella (fig. 21, d), which are continuous with the 

 living protoplasm of the cell. If only one flagellum projects from 

 the cell (sometimes a flat cell fig. 21, 6-) then the name flagellate cell 

 is applied (collared cell of sponges, fig. 21, e). Finally, in the case of 

 pavement epithelium (fig. 21, a) the cells are flattened; and if there 



FIG-. 21. Various kinds of epithelial cells, a, Flat cell*. I, flat cells with flagella (from a 

 Medusa), c, cylindrical cells. <?, ciliated cell, e, flagellate cell with collar (from 

 sponge), f, cylindrical cell with porous border (intestinal epithelium). 



is more than one layer the superficial cells are flat, while those in ' 

 the deeper layers are more and more rounded. 



While the cells of the lower layers retain their semi-fluid character, 

 and are occupied in continual cell division and growth ; those of the 

 upper layers possess a firmer consistency, gradually become horny, 

 and are thrown off as scales or continuous flakes, to be replaced by 

 the continuous growth of the lower layers. Thick stratified layers 

 of cornified cells, almost fused with one another, give rise to indurated 

 or horny structures (nails, claws, hoofs), which may form a more or 

 less complete coat for the body and function as protective exoskeleton 

 (fig. 21, a to/). 



There are also cells the free sui-face of which is distinguished by a 



