418 COMPARATIVE ANATOMY. 



tlie dermis, wliicli vary greatly iu form, and unite to form a dermal 

 skeleton. Lastly, there are glandular organs connected with the 

 cutis, which are developed from the epidermis, and are therefore to 

 be regarded as epidermal organs. 



§ 320. 



In Amphioxus the epidermis is a single layer; in other forms 

 it is made up of a number of layers of cells, which invest the 

 corium, with its elevations and depressions. Even in the Verteb rata 

 we find a ciliated epithelium as a heritage from a lower condition ; 

 but it is limited to the embryonic stages in Fishes, and to 

 certain parts of the body in the larval stages of the Amphibia. 

 The lower layers of the epidermis, which lie closer to the 

 corium, are seen to be younger, and these replace the effete 

 portions of the superficial layers. The cells of the epideimiis vary 

 greatly in consistency, form, and mode of connection. Pigmented 

 cells are not unfrequently found between the rest. They are some- 

 times able to produce a change of colour by the movement of their 

 protoplasm (chromatophores) ; this has been observed in Fishes and 

 Amphibia. In the aquatic Anamnia the whole of the epidermis is 

 less consistent, and, owing to the softness of its elements, the whole 

 layer is often gelatinous ; so much so, indeed, that it was for a long 

 time considered to be a mucous layer secreted by glands. 



There is another arrangement which forms a contrast to the con- 

 dition of the epidermis in the Anamnia ; this is brought about by the 

 cornification of the cells which is first seen in the Amphibia, and 

 is commonly found in the Amniota. The cells form resisting plates 

 or fibres, which give rise to firm structures becoming overlapped 

 by, and being more or less marked off from, one another. The 

 process of cornification never affects any but the superficial layers, 

 the deeper-lying parts being always indifferent. When the cornified 

 layers increase in thickness, various kinds of plates, knobs, and 

 scale-like structures (Reptilia) are developed. The corium takes 

 part in the formation of these structures, for it is almost always 

 provided with elevations which correspond to those epidermal 

 formations, which are developed from enlarged papillas. The scales 

 of the Saurii and Ophidii are therefore processes of the whole cutis. 

 In Birds this cornified covering is retained in some parts only of 

 the body ; as beaks on the jaws, and as scales, plates, knobs, and so 

 on, on the feet. Larger horny plates are connected with a bony 

 dermal skeleton in the Chelonii, and in some families of the 

 Edentata among Mammals. The cornification of the epidermis, 

 which obtains in some divisions or in still smaller groups, cannot be 

 directly referred to the organisation of the Reptilia ; it is rather due 

 to adaptations to definite external conditions. However, we do meet 

 with horny epidermal structures in many parts of the body, which 

 must be regarded as acquired arrangements in consequence of their 

 wide distribution and constancy of character. These are the nails 



