24 



ZOOLOGY 



(Fig. 11, a); sometimes there is on each cell a single, relatively long, 

 whip-like cilium, which is then termed a fiagdlum (/, g). Cells 



provided with cilia are 



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termed ciliated, such as 

 bear flagella flcujdlaU 

 cells. 



Some tissues are com- 

 posed entirely of cells. 

 Others, though originat- 

 ing from cells or by the 

 agency of cells, consist in 

 greater or less measure of 

 non-protoplasmic matter 

 formed between the cells. 

 Tissues composed en- 

 tirely of cells take the 

 form, for the most part, 

 of membranes covering 

 various surfaces, external 

 and internal. Such mem- 

 branes are known under 

 the general name of 

 epithelia (Fig. 11) ; they 

 may consist of a single 

 layer of cells (a-K) or 

 may be many-layered 

 {%) ; the former are 

 termed non-stratified, the 

 latter stratified, epithelia. 

 The cells of an epithe- 

 lium may be flattened 

 (c, e), their edges being 

 cemented together so as 

 to form a continuous 

 membrane ; or they may 

 be cubical or cylindrical 

 or prismatic (a, b); in 

 the case of a stratified 

 epithelium the cells may 

 be of different forms in 

 different strata (i). The 

 epidermis, which covers the outer surface of the body of an animal, 

 is an example of an epithelium ; sometimes it is stratified, some- 

 times unstratified ; its cells sometimes possess cilia, sometimes are 

 devoid of them. Lining the internal cavities of the body are 

 layers of cells, or epithelia, sometimes in a single layer, sometimes 

 in several layers, sometimes ciliated, sometimes non-ciliated. 



Fig. 11 — Various forms of epithelium, a, ciliated epi- 

 thelium ; h, columnar ; d, surface view of the same ; 

 c, tesselated ; e, the same from the surface ; /', flagel- 

 late epithelium with collars ; ff, flagellate epithelium 

 without collars ; h, epithelium of intestine with 

 pseudopods ; i, stratified epithelium ; A', deric epi- 

 thelium of a marine planarian with pigment cells, 

 rod-cells, and sub-epithelial glands. (From Lang's 

 Comi^arative Anatomy.) 



