JO THE ANATOMY OF VERTEBRATED ANIMALS. 



formed connective tissue. An exoskeleton may 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, areas, or processes, of the dermis. "When 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 or a. feather. ^ To 

 become u, hair, the horny cone simply elongates by continual 

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

 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 

 barbs, barbules, etc., by a further process of splitting of the 

 primary horny cone. 



The epidermis remains soft and delicate in Fishes and 

 jimpMbia. In Heptilia it sometimes takes the form of plates, 

 which attain a great size in many Ohelonia / sometimes, that 

 of overlapping scales, as in Ophidia and many Lacertilia ; 

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

 Chamseleons. Epidermic plates in the form of nails appear 

 upon the terminal phalanges of the limbs. 



All A.ves possess feathers. In addition, the beak is partly 

 or completely ensheathed in horn, as in some Heptilia. Oorni- 

 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 fcetal state. The Pangolin (Ifanis), on the 

 other hand, is almost completely covered with scales, the 

 Armadillos with plates, and most terrestrial mammals witli a 

 thick 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 papillae, however, are comparable to the ridgea 

 of the bed of the nail, not to the pajpillos of the hairs. 



