32 



E PI THE LI A. 



Fig. 7. — Types 

 OF Epithelium. 



Ciliated Epithelium of 

 Frog's Mouth. (Howes. 



Columnar Epithelium. 



Squamous Epithelium 

 (section). 



gills of Archicmlomata^ Annelida^ and 

 Mollusca. In some cases the outer limit- 

 ing surface or epithelium of the body is 

 composed of ciliated epithelium. 



(2) Columnar epithelium, — The cells 

 are placed side by side in regular order, 

 usually deeper at right angles to the sur- 

 face than in other directions — in fact, 

 like columns. Their upper or outer sur- 

 face usually differs from the rest of the 

 cell and may be clear and hyaline, or 

 show striations, or it may be in an amoe- 

 boid condition with minute pseudopodia. 

 It is a form of epithelium commonly lining 

 the alimentary canal. 



(3) Squamous epithelium, — Each cell 

 is spread out into a flat, scale-like plate. 

 Each touches its fellows at its edge, and 

 the whole forms a delicate limiting mem- 

 brane. Simple squamous epithelium forms 

 the outer limiting surface of sponges (pin- 

 nacocytes), and the inner peritoneal lining 

 endothelium of many higher types. In 

 the outer limiting surface of these latter 

 the squamous epithelium is not simple but 

 stratified. The surface-cells only are flat- 

 tened, and these gradually pass downwards 

 to columnar. By cell-division the colum- 

 nar produce fresh squamous cells which 

 are lost at the surface by wear or otherwise. 



(4) Glandular epithelium is a special 

 form of columnar epithelium. Glandular 

 secretion collects in the substance of the 

 cell and is then discharged at the surface. 



(5) Lastly, there is Sensory epithelium^ 

 in which the cells are specially modified 



for sense functions. Sensory Epithelium. 



These epithelia may often occur in a mixed condition 

 Thus the endoderm of Hydra is an epithelial mixture o 



Squamous Epithelium 

 (surface view). 



Glandular Epithelium. 



