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 



V j 



layer of cells (a-h) 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, d), their edges being 

 cemented together, so as 

 to form a continuous 

 membrane ; or they may 



i/ , 



be cubical or cylindrical 

 or prismatic (((,b); in 

 the case of a stratified 

 epithelium the cells 

 may be of different 

 forms in different strata 

 (*). The epidermis, which 

 covers the outer sur- 

 face of the body of an 

 animal, is an example 

 of an epithelium ; some- 

 times it is stratified, 

 sometimes unstratified : 

 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 (Fig. !') :nv formed for the most part by the modifica- 

 tion of certain cells of epithelia. 'In many cases a single cell of the 

 epithelium forms a gland, which is then termed a unicellular <//f/i// 

 (Fig. 9, A}. The secretion (or .substance which it is the function of 



Fie.. 8. Various forms of epithelium, u, cilititerl epi- 

 thelinm ; /., ci.lunnuiv; /, suvfaeu view of the same ; 

 r, tesselated ; t 1 , the same from the surface ; /, flagel- 

 late epithelium with enllars ; 11, flagellate epithelium 

 without ei >llars; A, epithelium f intestine with 

 pseudopodia ; <', .stratified epithelium; /,-, ilerie e]>i- 

 thelium of a marine planarian with pigment cells, 

 rod-cells, ami sul.-epitheli-il glands. ' 



COK ''' Aim*" i 



