20 



Basic- Structure of Vertebrates 



Fig. 14. (Left) Cycloid scale of Salmo fario. (a) Anterior portion covered by 

 overlap of preceding scales; (b) free portion covered only by pigmented epidermis. 

 (denier) Ctenoid scale. (Right) Two ganoid scales. (From "Cambridge Natural 

 History," Vol. VII. By permission of The Macmillan Company, publishers.) 



except some fishes, plates of dermal bone occur, sometimes more or less 

 extensively over the body, as in the alligator, turtle, or armadillo, or at 

 least as part of the skeleton of the head. The cavity of the mouth in all 

 vertebrates is lined by infolded skin whose two layers produce the 

 calcareous teeth. 



Glands of many sorts are produced by the skin. The essential 

 secretory layer of the gland is always of epidermal origin (Figs. 11, 12). 

 Integumentary glands secrete mucus, oily substances, watery perspira- 

 tion, milk, and poisonous or odorous substances. 



Cavities 



The trunk of the vertebrate is hollow. Its wall, the body-wall (pa- 

 rietes), encloses at least two major cavities known collectively as the 



Chromatophores 



Epidermis 



Fig. 15. Bony scales and horny scales. Diagrammatic sections perpendicular 

 to surface of skin. (A) Overlapping bony scales of a fish. (B) Flat horny scales and 

 (C) overlapping horny scales; especially characteristic of reptiles. 



