184 COMPARATIVE ANATOMY want of symmetry on the two sides, are an ex- ception in having rough scales with soft fin rays. Cycloids, embracing fishes with smooth scales, like the cod, herring, trout, and eel, are the most numerous types in the present epoch ; the rays are soft, and the bones of the head are smooth and simple ; the mackerel has hard rays, and both serrated and smooth scales, and seems intermediate between cycloids and ctenoids. It appears, therefore, that this di- vision into four orders according to scales is not perfect, and it is not now much insisted upon by its author, except for the first three. So intimate is the relation between the scales of fishes and their general organization, that Agassiz was enabled to restore a fossil fish from isolated scales, as is shown in his work on "Fossil Fishes;" in like manner Cuvier re- peatedly restored fossil genera of mammals, giving the entire skeleton and outline of the form, from single bones found in the gypsum near Paris. In testudinata, the corneous in- tegument is applied directly to the bony box which encloses their soft parts ; the epidermis is covered with large scales, adherent by all their lower surface, except in the species which produces the tortoise shell of commerce, in which they overlap each other ; these plates grow by all their adherent surface and at their circumference, as indicated by the concentric lines of increase, and become larger in propor- tion to the body ; on the neck, tail, and limbs the epidermis is like that of mammals, except in being thicker and rougher. The scales of other reptiles are implanted on the dermis, and vary from horny in most genera to bony in the crocodile ; contiguous by the borders on the head, and generally imbricated on other parts of the body ; of various shapes and sizes, and arranged in serpents in long bands, moved by muscles, serving the purpose of limbs by their contact with the ground at their free posterior edges ; the epidermis covers even the scales, and at certain seasons of the year is shed ; in serpents the change is so complete that the cast skin comes off in a single piece, including even the covering of the eyes; the coloring matter, often of the most brilliant hues, is placed immediately under the epidermis. In batrachians, the skin has no corneous appen- dage, except the nail-like processes of the limbs in a few species. The feathers of birds are analogous to the hairs of mammals, but more complicated in structure ; they are believed to be not the simple product of a secretion, as is frequently maintained, but developed from bulbs formed from ceDs and supplied with ves- sels; when the feather is formed the vessels disappear, and it gradually becomes dry and dead from the summit to the base, and finally is not susceptible of further living changes, resembling in this respect the horns of the stag. Each principal feather may be moved by means of the greatly developed cutaneous muscles, which send slips to them. A feather usually presents at its lower extremity a cor- neous tube open at the end, continuous with the shaft, which is webbed on each side, with fringed barbules. According to F. Cuvier, the capsule which forms the feather grows during the whole development of the latter ; the bulb, after it has fulfilled its office, forms in drying a series of membranous cones in the tube, generally called the pith. The new feather is at first covered by the investing capsule, which often extends several inches from the skin ; it gradually becomes free, and the barbs, at first rolled up, spread laterally ; the end of the tube is implanted lightly in the skin, and at every moulting season is displaced for a new feather ; moulting takes place generally every year after the season of incubation, and sometimes twice a year, and is a period during which the bird loses its voice, and appears more or less unwell. Some feathers 'esemble the quills of the porcupine, as four or five of those in the wing of the cassowary ; in the eagle the barbs are stiff and united by hooked barbules into a broad lamina for retaining better hold on the air ; in the ostrich the plumes of the tail and wings are of great softness and lightness, and in the marabout the feathers resemble the soft- est down; in the turkey's breast they are transformed into bristly hairs, and in some cuckoos into corneous plates at the ends. It is unnecessary to more than allude to the mag- nificent and varied 'colors of these appendages, which are usually the finest in the males and in adults. In a few mammals the skin is naked, but in the greatest number it is pro- tected by hairs, characteristic of this class. Hairs are produced, like feathers, from cu- taneous follicles or capsules lined with a cell membrane, and containing at the bottom a conical bulb supplied with blood, the soft in- terior constituting the pulp. Various as are the forms of hairs, they consist essentially of an external corneous cortical substance, and a medullary matter in the interior ; the quills of the porcupine are only magnified and modified hairs, their cortical substance being very dense, and the medullary matter a pithy aggre- gation of very large cells, without any evident fluid portion in the perfect quill ; so in birds, the cortical substance is found alone in the quill, while the cellular pith is confined to the shaft; in some animals the cortical substance is strongly imbricated, and the medulla made up of rounded cells. Pigmentary matter is devel- oped in the central portion. Hair is, therefore, the product of epidermic cells, developed in abundance in the follicles, and it grows by the addition of new matter at the base. Hairs may become spines, bristles, wool, or down, accord- ing to softness and fineness ; their color varies much, though less than that of feathers, being generally some modification of white, black, reddish brown, or yellowish. Recent observa- tions seem to connect the supra-renal capsules with the regulation of the amount of pigmen- tary matter in man and animals. Hairs, like feathers, are usually shed once a year, with or
