4 2 ZOOLOGY. 



range of variation both as to form and function. Besides its 

 primary work as a protective layer, the epithelium may have 

 a glandular function, being favorably situated for the final 

 elimination of products from the body. Since it is especially 

 exposed it is the layer best adapted by position to receive those 

 external stimuli which we know to play such an important role 

 in the life of all organisms. The position of the epithelium 

 also renders it specially liable to destruction. To compensate 

 for this its primitive or undifferentiated character makes it 

 particularly capable of regenerating portions of itself which 

 may have been lost. Epithelium is often especially active also 

 in the regeneration of other than simple epithelial structures. 

 In close connection with this latter regenerative quality is to 

 be considered the fact that epithelium gives rise to the re- 

 productive or sexual cells by which new individuals are pro- 

 duced. The foregoing enumeration of functions suggests the 

 physiological classification of epithelia : bounding, glandular, 

 sensory, and reproductive epithelia. The same layer may 

 fulfill several of these functions at once. 



66. Bounding Epithelium. The ordinary protective epithelium may 

 be made up of cells cuboidal in shape (Fig. 14, B), or columnar (Fig. 14, 

 A), or much flattened (Fig. 14, C). In extreme cases of flattening and 

 hardening we have squamous epithelium, e. g. the outer cells of the human 

 epidermis. Motile protoplasmic projections often extend from the free 

 surface of the epithelium. Flagellate epithelium (Fig. 77, D) has one such 

 projection from each cell, whereas ciliate epithelium (Fig. 14, D, ) has 

 numerous small ones. Cilia are more common in the lower groups of 

 animals, but are found even in mammals, in the moist internal passages, as 

 in the nose, trachea, etc. 



Membranes bounding the body cavity are called serous membrane 

 (endothelium} . The lining of the digestive tract is described as a mucous 

 membrane. 



Epithelial cells often secrete upon their outer surface a layer of mate- 

 rial (cuticula), which serves to protect the cells beneath and the organism 

 as a whole from external influences (as the covering of the cray-fish). 

 From the epithelium arise various outgrowths, as scales, hair, feathers, 

 and the like. 



67. Glandular Epithelium. The ordinary columnar or pavement epi- 

 thelium may here and there present cells or areas of cells which are spe- 

 cially active in producing and pouring out on their free surface certain 



