22 



ZOOLOGY 



SECT. 



or less measure of non-protoplasmic matter formed between the 

 cells. Tissues composed entirely 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. 8) ; they 

 may consist of a single 

 layer of cells (a-h) or 

 may be many-layered 

 (i) ; the former are 

 termed non-stratified, the 

 latter stratified, epithelia. 

 The cells of an epithe- 

 lium may be flattened 

 (c, d\ their edges being- 

 cemented together, so as 

 to form a continuous 

 membrane ; or they may 

 be cubical or cylindrical 

 or prismatic (a, &) ; in 

 the case of a stratified 

 epithelium the cells 

 may be of different 

 forms in different strata 

 (i). The epidermis.whicli 

 covers the outer sur- 

 face of the body of an 

 animal, is an example 

 of an epithelium ; some- 

 times it is stratified, 

 sometimes unstratih'cd : 

 its cells sometimes pos- 

 sess cilia, sometimes are 

 devoid of them. Lining 

 the internal cavities of 

 the body are layers of 

 cells, or epithelia, some- 

 times in a single layer, 

 sometimes in several 

 layers, sometimes cili- 

 ated, sometimes non- 

 ciliated. 



Glands < Fi-. <)) are formed for the most part by the modifica- 

 tion of certain cells <>f epithelia, In many cases a single cell of the 

 epithelium f..rms a -hind, which is then termed a unicellular gland 

 '- !) - 4 'I' 1 "' secretion (or substance which it is the function of 



FIG. 8. Various forms of epithelium, a, ciliated epi- 

 thelium ; />, columnar ; ,l, surface view of the same ; 

 i'lted : , the same from the surface ; f, flagel- 

 late epithelium with collars ; ; i, flagellate epithelium 

 without collars; /,, epithelium of intestine with 

 peeudopodiai i, stratified epithelium; /-, deric epi- 

 thelium of a marine planarian with pigment cells 

 Us, arid sub-epithelial giancU. (From Lang's 



