Page:The New International Encyclopædia 1st ed. v. 07.djvu/729

FISH. diversity of form, though this diversity is mostly concerned with detail, leaving, as stated above, a characteristic fish form as a whole. The typical symmetry of a fish is embodied in such forms as the trout.

FORMS OF SCALES.

. Fishes are usually covered by scales 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 catfishes. Scales 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 backward, 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, which is derived from the epidermis, while the bony base arises from the derma. The differences of character in the scales have been made the basis of a classification of fishes by Louis Agassiz, according to whom all fishes are distributed into four orders—, ,, and (qq.v.). This classification 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 natural classification. The dermal plates may become variously specialized, giving rise to spines, teeth,

etc. The teeth vary greatly in size, shape, and arrangement. They may be flat, plate-like, as in the rays, or long and sharp, as in certain sharks. The conditions in the sharks, and in certain other groups, show in the clearest way by their structure and transitional forms that they are merely modified dermal plates or denticles. In the more recent fishes they are not restricted to the edge of the mouth, but may occur in the roof and floor, and on the tongue, gill-bars, and pharynx. The epidermis of fishes contains unicellular glands, which secrete the mucus covering their body, and pigment cells giving rise to the colors of the body.



VERTEBRA OF A BONY FISH.

. The skeleton of fishes consists of the skull with its visceral skeleton; the vertebral axis with its processes; the pelvic and pectoral girdles; and the supporting elements of the various fins. This in the lower cartilaginous fishes consists only of cartilage, no true bones being present. The skull, which in the higher fishes is a complicated structure, in the elasmobranchs consists of a rather simple cartilaginous hollow case, the chondrocranium, inclosing the brain, and not composed of distinct pieces. As one ascends the scale, bones are added to this chondrocranium from the outside, arising as dermal ossifications; these are probably merely highly modified dermal plates. In the ganoids the chondrocranium generally persists with centres of ossification present, and the whole head is incased in dermal bones. In the higher bony fishes the chondrocranium is usually replaced by cartilage bones with many dermal bones added. To the lower part of the skull in all fishes a series of arches are attached. These form the lower jaw and the hyoid and gill arches. The backbone generally consists of a series of vertebræ which, with the exception of Lepidosteus, are biconcave. Dorsally they bear neural arches inclosing the spinal cord, and these are prolonged dorsally as a neural spine, varying in length. Ventrally, the vertebræ bear ribs in the anterior portion, and in the caudal region there are hæmal arches inclosing the caudal artery and vein. These arches are prolonged ventrally as a hæmal spine. In some elasmobranchs, in the chimæras, in the lung-fishes, and in some ganoids, there are no such definite vertebræ developed, but