EPITHELIAL TISSUE 



37 



epithelial cells generally secrete externally layers of varying thickness, 

 which often harden and possess different chemical and physical pro- 

 perties ; these form a cuticle 

 over the epithelium. 



In the simplest cases 

 every epithelial cell pos- 

 sesses nearly all the qualities 

 of a Protozoon. Division 

 of labour, however, soon 

 steps in, so that the epi- 

 thelial cells belonging to 

 different areas undertake 

 different functions. In a 

 simple condition, which we 

 meet with in certain low 

 Metazoa, e.g. the Hydra, and 

 which is also passed through 

 by many higher Metazoa in 

 the course of their develop- 

 ment, the wall of the pouch- 

 like body consists of two 

 contiguous epithelial layers, 

 an outer (eetodermal epi- 

 thelium) and an inner 

 (endodermal epithelium), 

 the latter lining the central 

 cavity of the pouch (arch- 

 enteron). The two layers 

 pass into each other at 

 the opening of the pouch 

 (mouth). This stage is 

 known as the Gastrula. A 

 certain not very sharply 

 defined division of labour 

 occurs between the cells of 

 the inner epithelium and 

 those of the outer epi- 

 thelium. The cells of the 

 outer epithelium appear 

 to be specially suited, in 

 correspondence with their 

 position, to carry on the relations between the animal body and the 

 outer world, and further to promote locomotion and movement ; those 

 of the inner epithelium are adapted more for the ingestion and 

 digestion of the food taken into the gastric cavity. In correspond- 

 ence with these functions, differences in the structure and in the 

 size of the cells of the two layers appear. The higher we rise in 



f . .'- v _.' 

 /% 



'c 





'%> $'* 

 <( -.-'- A - : 





.. 1 



' 



, Ciliated 

 d, surface 



FIG. 34. Various forms of epithelium. 

 epithelium ; b, columnar epithelium in profile ; 

 view ; e, tesselated epithelium ; c, the same from the sur- 

 face ; /, epithelium of collar cells, with flagella (fmni the 

 endoderm of a sponge) ; <j, flagellate epithelium ; h, epithe- 

 lium of intestine with amoeboid processes ; <', multilaminar 

 epithelium ; 7;, body epithelium of a marine plauarian, 

 with pigment cells, rod cells, and sub -epithelial glands. 



