4 
32 EPITHELIA. 
Fig. 7.—TyPEs 
gills of Archicwlomata, Annelida, and OF EPITHELIUM. 
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, Rane se apr nee of 
like columns. Their upper or outer sur- le 
face usually differs from the rest of the 
cell and may be clear and hyaline, or 
show striations, or it may be in an amee- 
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. Squamous Epithelium 
Each touches its fellows at its edge, and Geetend 
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 squamous Epithelium 
the outer limiting surface of these latter (surface view). 
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. 
Columnar Epithelium. 
ame 
(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 of 
