34 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the skin, but some of the fossil groups (ichthyosaurs, pterodactyls, 

 some dinosaurs, possibly plesiosaurs) had a naked skin. Correlated 

 with this cornification of the epidermis, glands are rare. Some turtles 

 have scent glands beneath the lower jaw and along the line between 

 carapace and plastron; snakes and crocodilians have them connected 

 with the cloaca, while the latter have others, of unknown function, 

 between the first and second rows of plates along the back, as well as 

 protrusible musk glands on the lower jaw. These latter are not true 

 glands as they produce no secretion but cast out the lining cells. 



The corium presents two layers, the outer rich in chromatophores, but, aside 

 from some snakes and lizards, the color changes are not remarkable. Claws 

 are common on the toes. The so-called "femoral pores" on the undersurface 

 of the legs of lizards are not glands, but are epidermal structures composed 

 of horny cells, and possibly have a sexual function. 



BIRDS have both layers of the skin very thin, the epidermis de- 

 veloping both scales and feathers. Correlated with this extensive 



Fig. 25. — Diagram of base of contour feather, a, aftershaft; b, barbs; bl, barbules; 

 h, hooks on ends of barbules; lu, lower umbilicus; u, umbilicus; q, quill; s, shaft; v, vane. 

 A, portion of a barb showing the barbules and hooks. 



4^ 



aucitv of 



development of cornified structures is a striking paucity of glands. 

 There are none in the ostriches, but others have the familiar oil (uro- 

 pygial) glands at the base of the tail, the secretion of which is used in 

 dressing the feathers. The only other dermal glands in birds are 

 modified sebaceous glands near the ear in some rasores. The scales 

 on the legs and the claws on the feet and occasionally on the wings, 

 are derivatives from reptilian ancestors. The feathers are also 

 derived from scales, but are greatly modified. 



