. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. TEGUMENTARY ORGANS. 477 embryonic state of all animals, and in the adult condition of many of the lower forms, the in- tegument, constituted as above defined, forms a continuous investment over the surface of the body without any important processes or irregularities. Such is the case in many of the Worms, Polypes, and lower Mollusca. From such simple forms of integument as these the most rudimentary kinds of appendages or tegumentary organs are produced in one of two ways,— either the outer portion of the ecderon is thi
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. TEGUMENTARY ORGANS. 477 embryonic state of all animals, and in the adult condition of many of the lower forms, the in- tegument, constituted as above defined, forms a continuous investment over the surface of the body without any important processes or irregularities. Such is the case in many of the Worms, Polypes, and lower Mollusca. From such simple forms of integument as these the most rudimentary kinds of appendages or tegumentary organs are produced in one of two ways,— either the outer portion of the ecderon is thickened, and as a spine or as a plate projects beyond the common surface — e. g. cells of Hydroid and Polyzoic Polypes ; or the whole integument is developed into a spine-like or plate-shaped process, as in the so-called " bracts " of the Diphyclae, and in all the spines, hairs, and scales of the Insecta, Crustacea, and Arachnida. The shells and plates of Mollusca and Arti- culata belong principally to the former division, being simple laminated thickenings of the outer portion of the ecderon. In the Vertebrata the integument but rarely possesses appen- dages of so simple a nature. Simple plates of this kind, however, coat the surface of the beaver's tail, in which animal, according to Heusinger, " the epidermis is divided by a great number of clefts into hexagonal por- tions 4 lines long, whose whole edges ad- here to the cutis. They usually consist of a couple of superimposed laminae identical in structure with the rest of the epidermis " (I. c. p. 168.). The polygonal horny plates of the Chelbnia are of the same nature. The scales on the under surface of the tail of the rat and other rodents, and on the tarsi of birds, are similarly constituted ; but here one edge is thrown up, and wehaveatransitiontothescales of the Pangolin, — to those of Ophidia and Sauria,— and to the nails, claws, hoofs, and hollow horns of Mammals, and the horny sheaths of t
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