20 COMPARATIVE ANATOMY 



as the colour becomes modified according to the surroundings of 

 the animal, it may serve as a protection (e.g. Hyla). 



Calcifications may occur in the dermis, or, as in Ceratophrys 

 dorsata, definite bones may be formed (see p. 33) : the dermis also 

 encloses numerous smooth muscle-fibres. 



Reptilia. The characteristic peculiarity of the skin of Reptiles 

 is its capacity of producing scales (these are very simple in Geckos 

 and Chameleons), warts, prickles, shields (e.g. the '' tortoiseshell " 

 of Chelonians), claws, rattles (Rattlesnake), and other epidermic 

 structures (Fig. 13). All these are due in the first instance to 

 the formation of dermal papilla?, the markedly stratified epidermis 

 covering which becomes cornified secondarily. The horny layer of 

 the epidermis may be periodically cast off either entire (Snakes) 



FIG. 13. DIAGRAMMATIC SECTIONS THROUGH VARIOUS RINDS or EPIDERMIC 

 SCALES OF REPTILES. (From Boas's Zoology.} 



A, rounded scales ; B, shields ; C, imbricating scales ; D, overlapping scales with 

 bony scutes in the underlying dermis; h, horny layer; *, Malpighian layer of 

 the epidermis ; /, dermis ; o, bony scutes. 



or in shreds: it is renewed from the Malpighian layer. The 

 integument of Hatteria retains the most primitive characters 

 amongst Reptiles. 



Pigment-cells occur in the integument, rendering a change of 

 colour possible in many cases (e.g. Chameleon). 



Ossifications in the dermis are very common in Reptiles, and 

 there is great variation in the degree of their development, from 

 the small bony scutes present in Geckos (Ascalabota) to the large 

 exoskeletal plates of Chelonians (see p. '33). Muscles are also 

 present in the dermis. In contrast to the skin of Amphibians, 

 that of Reptiles is entirely wanting in glands. 



In Lizards, the so-called femoral glands occurring along the ventral side 

 of the thigh are said to be merely solid cones of epidermic cells, which form 

 a series of papilla? or warts and serve as clasping organs during copulation. 



Birds. Birds possess a thinner dermis than any other Ver- 

 tebrates, and it is less plentifully supplied with blood-vessels. 



