654 TEGUMENTARY ORGANS. 



dent hairs of halithea, the setae of armelides, and the agglu- 

 tinating matter of their tubes, the down and hairs of larvse, 

 and those common on adult insects, arachnida, and Crustacea, 

 the byssus of coiichifera, the horny opercula of gasteropods, 

 the horny mandibles of cephalopods, the lingual spines of 

 many mollusca, the gastric teeth of aplysia, the spines of the 

 gizzard of insects, the gastric plates of bulla, and the animal 

 matter of all testaceous coverings, may be considered as parts 

 of the same epidermic or epithelial tissue, having the same 

 extravascular and cytoblastic character, and the same or- 

 ganization and independent vitality. 



The tegumentary organs of the vertebrata are closely re- 

 lated to the temperature of the body, and to the density of 

 the surrounding medium, those of the warm-blooded classes 

 being slow conductors of caloric, in order to preserve the 

 high temperature of their body, and those of the cold-blooded 

 being indifferent in their conducting power, as they are also 

 in most in vertebrata. The sensitive vascular skin of fishes, 

 is thick, soft, gelatinous, and closely connected by tendinous 

 intersections with the subjacent muscular system. The 

 cuticle, as in the naked aquatic mollusca, forms a thin, soft 

 layer ; and by its periodical shedding, the lively colours of the 

 inferior loose strata of cytoblasts, or rete mucosum, are 

 allowed to shine more distinctly through the pellucid scales, 

 and this increased brilliancy of colour is most marked at the 

 spawning season. The imbricated scales of fishes, which are 

 wanting in the cyclostome and very minute in the plagios- 

 tome species, but generally cover their surface, are consoli- 

 dated by phosphate of lime, detected also in the hairs of 

 mammalia ; and they grow, like the human nails, or the wing 

 scales of lepidopterous insects, by successive layers added 

 by the squamiferous follicles of the cutis, in which they are 

 fixed. 



From the soft, thin, and granular condition of the newly 

 formed epidermic and epithelial coverings of membranes, the 

 secretion of glandular tubuli, and the respiration of pul- 

 monary ceils, or branchial laminae, are easily effected through 

 these coverings. And from the necessity for free respiration 

 by the entire cutaneous surface of the body in the amphibia, 

 their highly sensitive and vascular cutis is destitute of scales ; 

 it is covered only with a thin, soft epidermis, which is cast 



