I 12 



CHORDATE ANATOMY 



layer of cells. Gland cells are numerous in the epidermis of Amphioxus. 

 The layer secretes a thin cuticle like that of annelids. The corium in 

 Amphioxus is gelatinous. 



Although the epidermis is stratified in all vertebrates, such low forms 

 as cyclostomes do not have the outer dead horny layer, and they do have 

 the thin cuticular layer of the invertebrates and Amphioxus. The skin 

 of fishes is like that of cyclostomes, except for differences in gland secre- 

 tions. See Fig. io6. 



A. AMPHIOXUS 



a PETROMYZON 



C SQUALUS. 0. RANA. 



Fig. io6. — Cross sections of the skin of four chordates, Amphioxus, Petromyzon, 

 Squalus, and Rana, showing the fundamental differentiation of the skin into corium and 

 epidermis. The difTerentiation of the epidermis into a dead outer layer and an inner liv- 

 ing layer began in aquatic animals. (Redrawn mainly after Plate and Schimkewitsch.) 



The outer dead horny layer of the epidermis, the corneum, appears 

 first in Amphibia, correlated apparently with the land habit, since most 

 land animals have it. As the amphibian skin is fundamentally like that 

 of higher vertebrates, the evolution of the skin beyond the amphibians 

 presents no serious difficulties. The striking differences are in the secre- 

 tions of the glands. It is indeed difficult to imagine how the skin mucus 

 of amphibians could have evolved into the milk of mammals. It should, 

 however, be remembered that sUght chemical differences often result in 

 striking differences in properties; so that we should not be surprised to find 

 chemical differences in the skin secretions of vertebrates that are far greater 

 than any morphological differences in the glands themselves. 



