THE EXOSKELETON. 41 



completely -formed connective tissue. An exoskeleton may- 

 be developed by the hardening of either the epidermis, or 

 the dermis. 



The epidermal exoskeleton resixlts from the conversion into 

 homy matter of the superficial cells of the epidermis. The 

 homy plates thus formed are moulded upon, and foUow the 

 configuration of, arese, 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- 

 f orm, 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 a hair, the homy 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 

 under-surface, and then spreads out into a flat vane, siib- 

 divided into harbs, barbules, &c., by a further process of 

 splitting of the primary horny cone. 



The epidermis remains soft and delicate in Fishes and 

 Amphibia. In Reptilia it sometimes takes the form of 

 plates, which attain a great size in many Chelonia ; some- 

 times, that of overlapping scales, as in Ophidia and many 

 Lacertilia ; but, sometimes, it remains soft, as in some Che- 

 lonia and in the Ohamseleons. Epidennic 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 Reptilia. 

 Comified epidermic tubercles, or plates, are developed on 

 the tarsi and toes, the terminal phalanges of which (and 

 sometimes those of the wing) have nails. Beside these, 

 some birds possess spiu-s, 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, 

 present only in the fcetal state. The Pangolin {Manis), on 

 the other hand, is almost completely covered with scales, 

 the Armadillos with plates, and most terrestrial mammals 



