INTEGUMENT 25 



entire (e.g. Snakes, Anguis). Most Lizards simply creep out of 

 their cast skin, as out of a sack ; while in Snakes it becomes 

 turned inside out while being shed. 



The horny substance may undergo a variety of modifications, 

 and may give rise to such structures as ridges, prickles, warts, 

 claws, shields (e.g. the " tortoiseshell " of Chelonians l ), and 

 rattles (Rattlesnakes); or it may develop bunches of cuticular 

 hair-like bodies, such as those found on the toe-discs of Geckos. 



In the derm, a superficial and a deeper layer may be dis- 

 tinguished. The latter is composed mainly of strong bundles of 

 connective tissue fibres which as a rule cross one another at right 

 angles, as in Fishes and Amphibians. The superficial or sub- 

 epidermic layer is looser in structure, and in addition to con- 

 nective tissue fibres, encloses smooth muscles and chromatophores 

 (Fig. 19), the degree of development of the latter differing greatly : 

 several rows of them may be present, e.g. in the Chameleon. The 

 power of changing colour, so characteristic of the last-named, is, 

 however, possessed to a greater or less extent by many other 

 Reptiles. 



Birds. 



The skin of Birds is characterised by giving rise to feathers, as 

 well as by the relatively thin epiderm and derm, the connective 

 tissue fibres of the latter being irregularly felted. A uropygial 

 gland, peculiar to Birds, and situated at the base of the rudi- 

 mentary tail (uropygium) is present in nearly all, being wanting 

 only in a few groups (e.g. Ratitae) : its secretion serves to oil the 

 feathers, and it is especially well developed in Water-Birds. A 

 gland is also present in the neighbourhood of the auditory passage 

 in certain Gallinacea?, but otherwise integumentary glands are 

 wanting in Birds. Characteristic of the derm is its richness in 

 sensory organs (tactile corpuscles) and muscle-fibres, most of which 

 latter are inserted into the feather-sacs and serve to erect the 

 feathers (arrectores plumarum). Epidermic scales are present 

 on the feet. 



The feather is foreshadowed in the reptilian epidermic scale, 

 of which it is merely a further modification. That scales and 

 feathers are homologous structures is, at any rate to a certain 

 extent, indicated by their mode of development, which is briefly as 

 follows. 



In the region where a feather is to be formed, the dermal tissue 

 becomes slightly raised up towards the thickened epiderm (Fig. 

 20, A), and thus gives rise to a vascular papilla. As this papilla 

 grows out to form an elongated cone, the feather-germ (B), its 



1 The individual epidermic shields of Chelonians are independent of the 

 underlying bony plates (Fig. 33), and do not correspond with them in arrange- 

 ment. 



