372 SPECIAL PHYSIOLOGY. 



highly sensitive to the touch. Amongst the Annuloida, the revolving-wheels 

 of the Kotifera generally, and the proboscis of some species, are probably tac- 

 tile. The marine parasitic species have a soft, sensitive integument. The 

 succulent feet of the Echinodermata are also remarkably irritable, if not 

 sensitive. 



In the Coelenterata, the ectoderm, especially over the oral tentacles, possesses 

 keen excitability ; but they do not apparently exhibit much discriminating 

 sense, seizing all objects alike. The Protozoa, destitute of a nervous system, 

 exhibit no tactile organs. 



In all cases, the integument of the Yertebrata consists of a vascular cutis, 

 or true skin, covered by a non-vascular epidermic layer, and is moreover, 

 often provided with various appendages. Some of these, such as the hairs, 

 spines, nails, claws, hoofs, and even the horns of the Mammalia, are epidermic 

 structures, formed on papillae or matrices, developments of the cutis or true 

 skin. In a few instances, the dense dermal plates of the rhinoceros, and the 

 bony plates of the armadillo, are formed apparently, beneath the epidermis, 

 on the surface of the true skin, and are partly vascular tissues ; they belong 

 to the so-called dermal skeleton or exo-skekton, as distinguished from the endo- 

 skeleton or skeleton proper. 



In Birds, feathers take the place of hairs, being, like these, epidermic for- 

 mations, developed upon papillae at the bottom of follicles, and having, like 

 hairs, a root sheath, one layer of which, however, closes the young follicle, 

 and for a time invests the growing feather, but ultimately is broken through, 

 and falls away. The quill of the feather consists of fibres and flat scales, and, 

 for a time, contains a portion of the vascular papilla or pulp ; the shaft, barbs, 

 and barbules, consist of a pith composed of polyhedral cells, and of an outer 

 firmer layer, composed of flattened epidermoid scales. 



Amongst Reptiles, the thick coriaceous integument of certain saurians, the 

 osseous plates of the crocodiles, the scales of serpents, and the horny cover-" 

 ings of the Chelonia, are also epidermic formations, beneath which, in the 

 crocodiles, bony matter is formed, constituting a dermal skeleton ; but in the 

 Chelonia, the bony case beneath the horn, named the plastron and carapace, is 

 formed by the expansion and coalescence of parts of the internal skeleton. 



The soft integument of the Amphibia, in many cases, almost resembles a 

 mucous membrane. 



In Fishes, the integuments are either soft, as in the eels, in which they are 

 still provided with minute scales, or they are covered with the characteristic 

 larger dermoid scales, the pattern and formation of which, have led to impor- 

 tant distinctions in this large class ; sometimes they present numerous dermal 

 plates or spines. The scales consist of an outer laminated, and sometimes 

 canaliculated, shining layer, composed of the so-called enamel or ganoin, and 

 of a deep layer, which may be horny, fibrous, or even bony ; in the latter case, 

 it sometimes contains Hayersian; canals. The bony scales, and also the bony 

 plates and spines, of certain fishes, are partly epidermoid structures, but are 

 probably also in part, formed, by conversions of the outer layer of the dermis ; 

 they have great analogy to teeth, especially to the teeth of fishes themselves. 

 The spines of the fins of fishes, are also dermal structures, belonging to the 

 exo-skeleton, and not to the internal skeleton. 



The cutaneous glands, both sebaceous and sudoriferous, are found in all 

 Mammalia, except when the integument is covered with horny or bony plates. 

 In Birds, the chelonian and ophidian Reptiles, and Fishes, the cutaneous 

 glands appear to be wanting ; in the Saurian Reptiles, they are small and few ; 

 in the Amphibia, cutaneous glands of a peculiar structure are very abundant. 

 The so-called glands, or mucous canals and follicles, along the lateral line of 

 fishes, corresponding with the Savian bodies of the torpedo, are believed by 

 Leydi^, to be really sensory organs, contained in depressions, or canals, formea 

 in the integument. They are lined with epithelium, and often contain a knob- 

 like projection, abundantly supplied with nerves derived from branches of 

 the fifth pair, or of the pneumogastric nerve. Some of the so-called cutaneous 

 glands of the Amphibia, just described, may be of a similar nature. They 

 must be acted on by irritating solids, fluids, or gases, present in the water, 



