110 



PRINCIPLES OF ANIMAL BIOLOGY 



axis. The cells adhere after division and in this manner several layers 

 of cells (Fig. 72, H) arc built up and the epithelium is spoken of as 

 stratified. The outer layers of cells in such epithelia become flattened 

 until the outermost layer is composed of very thin flat cells which 

 have the shape of ])olygonal tiles (Fig. 72, D) accurately fitted together. 

 Stratified epithelium forms the lining of the mouth and rectum of mam- 

 mals and some other vertebrates. The outer skin of adult vertebrates 

 is also made up of stratified epithehum. In the frog and other am- 

 phibians and certain reptiles, as snakes, the hardened outermost layer 

 of skin {stratum corneuvi) is shed as an extensive layer. In most terres- 

 trial vertebrates these outer cells are shed in minute flakes. 



Fig. 72. — Types of epithelium. A, columnar; B, cubical; C and D, squamou.s (side 

 and surface views, respectively); E, ciliated; F, flagellated; G, collared; H, stratified; vac, 

 vacuole. 



Glands. — Many epithelial surfaces are smooth or plain with all the 

 cells lying in a single layer. In such a tissue scattered cells may be differ- 

 entiated from the others for the production of a secretion. The substance 

 produced may be mucin, from which mucus is derived, some digestive 

 secretion, or any one of many other products. Such a layer of epithelium 

 is shown in Fig. 73, A, and examples of it are found in the hypodermis 

 of the earthworm or the intestinal epithelium of the frog. This is the 

 simplest form which secreting surfaces take. In slightly modified secre- 

 tory epithelia the bodies of some large secretory cells may drop below 

 the general level, as in Fig. 73, B. Examples of such enlarged and 

 displaced secreting cells may be found in the epithelia covering the ton- 

 gue of the frog and the foot and mantle of freshwater mussels. In other 



