INTEGUMENT. Fishes are usually eovered by sealer or bony plates. These may become very minute, as in eels, or may be entirely wanting, as in the leather carp, in certain eels, and in many of the eat fishes. Seales may be either bony or horny, and are generally imbricated like slates on a roof, the free end being backward. They arise from the deeper layer of the skin, the derma, grow outward and baekward, and remain covered by a thin layer of epidermis. Bony plates are attached by the whole of one surface, and usually have a coat of enamel, whieh is derived from the dermis, while the bony base arises from the derma. The differences of character in the scales have been made the sis of a elassifieation of fishes by Louis Agassiz, aecording to whom all fishes are distributed into four orders — Ctenoidei, Pla eoidai, and Ganoidei (qq.v.). This elassifiea tion was very artificial and did not admit many intermediate cases, or the cases where more than one kind of scale was possessed by the same fish, and has long been disused, but it has been found very convenient in the study of fossil fishes. Here also it is giving way to a more natu
ral elassifieation. The dermal plates may beeome variously specialized, giving rise to spines, teeth, etc. The teeth vary greatly in size, shape, and ar rangement. They may be fiat, plate-like, as in the rays, or long and sharp, as in pertain sharks. The conditions in the sharks, and in certain other groups, show in the clearest way by their struc ture and transitional forms that they ire merely modified dermal plates or denticles. In the more recent fiches they are not restricted to the edge of the month, hut may occur in the roof and floor, and on the tongue, gill-bars, and pharynx. The epidermis of fishes contains unicellular glands, whieh secrete the mucus covering their body, and pigment-cells giving rise to the colors of the body.