THE SKIN AND SKELETON. 



137 



feathers, which differ from hairs only in splitting up into 

 numerous laminae. They are the most complicated of all 

 the modifications of the epidermis. 

 They consist of a " quill " (answer- 

 ing to the bulb of a hair), and a 

 "shaft," supporting the "vane," 

 which is made up of " barbs," " bar- 

 bules," and interlocking " process- 

 es." The quill alone is hollow, and 

 has an orifice at each end. The 

 feather is moulded 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 be- 

 ing 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 Cuttle- 

 fish. With this and some other 

 exceptions, it is peculiar to Verte- 

 brates. In the Cuttle-fish, and some Fishes, as the Stur- 

 geon 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- 



vane, or beard : d, accessory 

 plume, or down : , /, lower 

 and upper umbilicus, or ori- 

 fice, leading to the interior 

 of the quill. 



