iO THE ANATOMY OF VERTEBRATED ANIMALS. 



formed connective tissue. An exoskeleton maj be developed 

 by the hardening of either the epidermis, or the dermis. 



The epidermal exoskeleton results from the conversion into 

 horny matter of the superficial cells of the epidermis. The 

 horny plates thus formed are moulded upon, and follow the 

 configuration of, areee, or processes, of the dermis. AVhen the 

 latter are overlapping folds, the horny epidermic investment 

 is called a scale, squama. When the dermic process is papilli- 

 form, and sunk in a pit of the dermis, the conical cap of modi- 

 fied epidermis which coats it is either a hair ot ^feather. To 

 become a hair, the horny cone simply elongates by continual 

 addition of new cells to its base ; but, in a feather, the horny 

 cone, which also elongates by addition to its base, splits up, 

 for a greater or less distance along the middle line of its undei 

 surface, and then spreads out into a flat vane, subdivided into 

 harbs^ harhules^ etc., by a further process of splitting of the 

 primary horny cone. 



The epidermis remains soft and delicate in Fishes and 

 Amj)Jiihia. In Reptilia it sometimes takes the form of plates, 

 which attain a great size in many Ghelonia ^ sometimes, that 

 of overlapping scales, as in OpMdia and many Laoertilia ; 

 but, sometimes, it remains soft, as in some Chelonia and in the 

 Chamreleons. Epidermic plates in the form of nails appear 

 upon the terminal phalanges of the limbs. 



All Aves possess feathers. In addition, the beak is partly 

 or completely ensheathed in horn, as in some Meptilia. Corni- 

 fied epidermic tubercles or plates are developed on the tarsi 

 and toes, the terminal phalanges of which (and sometimes 

 those of the wing) have nails. Besides these, some birds pos- 

 sess spurs, which are ensheathed in horn, on the legs or 

 wings. 



In Mammalia^ the horny exoskeleton may take all the 

 forms already mentioned, except that of feathers. In some 

 Cetacea it is almost absent, being reduced to a few hairs, pres- 

 ent only in the foetal state. The Pangolin {Manis), on the 

 other hand, is almost completely covered wnth scales, the 

 Armadillos with plates, and most terrestrial mammals with a 

 tliick coat of hair. The greater part of the mass of the horns 

 of Oxen, Sheep, and Antelopes, is due to the epidermic sheath 

 which covers the bony core. Where the horny epidermis be- 

 comes very thick, as in the hoof of the Horse, and in the horn 

 of the Rhinoceros, numerous long papillas of the dermis extend 

 into it. These papillfB, however, are comparable to tlie ridges 

 of the bed of the nail, not to the papilla? of the hairs. 



