TEGUMENTARY ORGANS. 



cated cavities in which the apparatus of the vane is moulded. 

 On examining even the minutest parts of the barbules of 

 feathers, the constituent elongated compressed angular cyto- 

 blasts, compactly and symmetrically arranged, and provided 

 with persistent central nuclei, are distinctly perceptible. A 

 second shaft, furnished with all the apparatus of the vane, is 

 generally more or less developed from the superior umbilicus or 

 distal opening of the quill, and this supplementary shaft some- 

 times, as in the emeu, equals in length that of the primary feather. 

 The rudiment even of a third shaft is sometimes developed 

 from the feather, and the entire plumage of a bird is some- 

 times renewed once or twice in a single season. But not- 

 withstanding the endless diversity of form and the intricate 

 structure of these organs, and the remarkable changes they 

 undergo during their development, growth, and moulting, 

 they present only a more complex form of the ordinary insen- 

 sible, extravascular, epidermic tissue, forming the exterior 

 integument of most organized bodies. 



In the simple organizations of the lowest animals, the dif- 

 ference is less marked, between the exterior cutaneous and 

 the interior mucous coats, and between the epidermic and 

 the epithelial developments they form on their surface; and 

 as most of them are inhabitants of an aquatic medium, their 

 epidermic covering generally retains the soft condition of a 

 rete mucosum, or of a mucous deposit, as seen in the naked 

 forms of radiated, helminthoid, and molluscous animals. It 

 is shed in flocculi from poriphera, and in larger pellicles from 

 the surface of many zoophytes, as lobularia, and its con- 

 stituent cytoblasts were observed by Gade in acalepha. In 

 many vaginated forms of polygastric and polypipherous ani- 

 mals, it composes a firm, elastic, often articulated, almost 

 horny sheath, over the entire surface of the true skin ; and 

 in the entomoid and testaceous animals, it becomes consoli- 

 dated by the addition of various earthy materials, to compose 

 their enveloping extravascular skeletons. Its granular nuclei 

 are constantly pouring from the vascular secreting surface of 

 the cutis, its condensed accumulations adhere to the skin by 

 means of the cytoblastema, and these accumulated epidermic 

 masses are periodically thrown off from the body in the 

 articulata, but are consolidated, collected, and permanently 

 retained in the testaceous mollusca and radiata. The resplen- 



