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.
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FORMS OF SCALES.
| 1, 2, Cycloid scales; 3, 4, 5, 6, ctenoid scales; 7, ganoid scales; 8, 9, dermal papillæ (from Monacanthus); 10, 11, cycloid scales from lateral line. |
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| OPEN MOUTH OF A SALMON. |
| Shows arrangement of maxillary, palatine, and vomerine teeth in fishes. |
Integument. 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—Cycloidei, Ctenoidei, Placoidei, and Ganoidei (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.
| Front and side views. c, Centrum; na, neurapophysis; pa, parapophysis; ha, hæmapophysis; ns, neural spine; hs, hæmal spine; za, zygapophysis. |
Skeleton. 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
