372 TEGUMENTARY ORGANS 



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 Diphyd^, and in all the spines, hairs 

 and scales of the Insecta, Crustacea, and Arachnida. 



The shells and plates of Mollusca and Articulata belong principally 

 to the former division, being simply laminated thickenings of the 

 outer portion of the ecderon. In the vertebrata the integument but 

 rarely possesses appendages 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 portions 4 lines long, whose whole 

 edges adhere to the cutis. They usually consist of a couple of 

 superimposed lamina; identical in structure with the rest of the 

 epidermis" (/. c. p. 168.). The polygonal horny plates of the Chelonia 

 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 we have a transition to the scales 

 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 

 the beak of Birds, all of which are constructed on essentially the same 

 plan, being diverticula of the whole integument, the outer layer of 

 whose ecderon has undergone horny metamorphosis. 



Among these the nails, horns, and hoofs of mammalia present 

 certain complexities of arrangement which entitle them to particular 

 notice. 



Nails are flattened horny plates developed from the upper surface 

 of the phalangeal integument only ; they are free at their distal 

 extremities, but laterally and at their proximal ends they are enclosed 

 within raised ridges of the whole integuments, the nail walls. The 

 enderon beneath them in the space which is called the " bed of the 

 nail " is raised into parallel longitudinal ridges or laminje, which 

 fit into corresponding depressions of the under surface of the 

 ecderon. 



Claws are nails which embrace a larger portion of the phalanx, 

 being developed, not merely from its upper surface, but also from its 

 extremity, and extending far round on its sides. In the dog and cat 

 {fig. 305. A) the bed of the claw is laminated as in man, but presents no 



