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PRINCIPLES OF ANIMAL BIOLOGY 



of any animal, the peritoneum which covers the intestine and Hnes the 

 abdominal cavity (coelom) of vertebrate animals, the outer layer (epi- 

 dermis) of the skin, and the inner or secreting layer of any gland. 



An epithelium is designated cubical, columnar, or squamous, according 

 to the shape of its component cells (Fig. 71A-D), the last term meaning 

 flat and tilelike ; ciliated, flagellated, or collared, if the free ends of the cells 

 bear any of the structures indicated by these words (E-G) ; and stratified, 

 if the layer is several cells thick and the cells at different levels have 

 different shapes (H). 



Fig. 72. — Types of secreting surfaces and glands. A, scattered gland cells (two 

 goblet cells containing secretion in the darkly stippled goblets) ; B, gland cell enlarged and 

 dropped below general level; C, group of secreting cells dropped slightly below the general 

 level; D, a simple multicellular gland; E, alveolar gland with neck; F, tubular gland; G, 

 compound alveolar gland; H, compound tubular gland; I, lumen; m, mouth; n, neck; v, 

 acini. Secreting portions of the glands are stippled. 



Epithelia on the outer surfaces of organs are usually in some degree 

 protective. When they line a cavity, they often have the function of 

 secretion. The lining membrane of the intestine in vertebrate animals 

 is secretory, and in all glands the secreting portion is epithelium. If a 

 gland consists of a single cell, that cell is in an epithelial layer (Fig. 72A, 

 B). If the gland is multicellular, its secreting cells may dip below the 

 general level of the surface, but still it is part of the epithelium (C, D). 

 When the secreting cells thus indented form a channel of nearly uni- 

 form diameter, the gland is said to be tubular; if the deepest portion is 



