168 MANUAL OF ANIMAL BIOLOGY 



are in close contact with each other to form a tile-like surface. 

 The cells of the succeeding under layers of skin epithelium gradu- 

 ally become thicker and appear like tiny cubes — hence the term 

 cuboidal epithelium. Finally, the columnar epithelium is 

 reached in which the individual cells are more or less tubular in 

 shape. Epithelium, as in the skin, consisting of several layers in 

 which a gradual variation in shape is noted, is spoken of as strati- 

 fied epithelium, and may be found in various regions. Inside 

 the body we find that the abdominal cavity is lined by peritoneal 

 epithelium. Again the alimentary tract is covered both outside 

 and inside with epithelium. The inside lining of the alimentary 

 tract is derived from the endoderm, and it is these cells which 

 function in digestion and absorption. (W. f. 32, A.) 



Another important type of epithelium is known as ciliated 

 epithelium, and, as the term indicates, cilia are present on the 

 cells. This type of ciliated epithelial cell is found in the lining of 

 various tubes and ducts where, by the coordinated movements of 

 the cilia, they cause the movement of fluids and other materials. 

 Examples of ciliated epithelium in the Frog may be found in the 

 roof of the mouth and throat, in the kidneys, and in the oviducts. 



Many types of glands which manufacture and secrete important 

 substances are formed from epithelial cells so that we may speak 

 of the glandular epithelium. Such glands may be unicellular 

 (goblet-cell), as found in the epithelium lining various regions 

 of the alimentary canal, which synthesize and secrete the essential 

 digestive enzymes. Or multicellular glands of many different 

 varieties are present, such as was noted above in the case of 

 the mucus and poison glands in frog skin. In the human skin 

 abundant sweat glands are present and also the sebaceous 

 glands, at the root of each hair, which secrete an oily substance. 

 In the mammalian female, the large, paired mammary glands which 

 secrete an abundant supply of milk to nourish the newly born 

 offspring are, of course, very important. In certain domesticated 

 Ungulates, notably the goat and cow, these glands are particu- 

 larly large and highly developed, and the milk secreted, since it is 

 well-suited for human consumption, forms the basis of the world- 

 wide dairy industry. (W. fs. 33, 34, 96.) 



Finally, the surface epithelial cells are, in numerous instances, 

 modified for peripheral sense organs in association with the nervous 

 system. Thus we may speak of sensory, or nervous, epithe- 



