346 COMPARATIVE ZOOLOGY 



complicated of all the modifications of the epidermis. 

 They consist of a " quill " (answering to the bulb of a 

 hair), and a "shaft," supporting the "vane," which is 

 made up of "barbs," "barbules," and interlocking 

 "processes." The quill alone is hollow, and has an ori- 

 fice at each end. The feather is molded on a papilla, 

 the shaft lying in a groove on one side of it, and the 

 vane wrapped around it. When the feather emerges 

 from the skin, it unfolds itself. Thus shaft and vanes 

 together resemble the quill split down one side and 

 spread out. 



The teeth of mollusks, worms, and arthropods are 

 also epidermal structures. Those of vertebrates are 

 mixed in their origin, the dentine being derived from 

 the dermis and the enamel from the epidermis. In all 

 cases teeth belong to the exoskeleton. 



(2) The Endoskeleton, as we have seen, is represented in 

 the cuttlefish. With this and some other exceptions, it 

 is peculiar to vertebrates. In the cuttlefish, and some 

 fishes, as the sturgeon and shark, it consists of cartilage ; 

 but in all others (when adult) it is bone or osseous 

 tissue. Yet there is a diversity in the composition of 

 bony skeletons ; that of fresh-water fishes contains the 

 least earthy matter, and that of birds the most. Hence 

 the density and ivory-whiteness of the bones of the 

 latter. Unlike the shells of mollusks and the crust of 

 the lobster, which grow by the addition of layers to 

 their borders, bones are moist, living parts, penetrated 

 by blood vessels and nerves, and covered with a tough 

 membrane, called periosteum, for the attachrhent of 

 muscles. 



The surface of bones is compact; but the interior 

 may be solid or spongy (as the bones of fishes, turtles, 

 sloths, and whales), or hollow (as the long bones of 

 birds and the active quadrupeds). There are also cavi- 



