Ganoid scales are hard and bony, covered with a layer of enamel; they are generally rhombic or quadrangular, rarely rounded and imbricate; and arranged in oblique rows, those of one row being linked together by an articulary process. This type of scales, common in fossil Ganoid fishes, occurs among recent fishes in Lepidosteus and Polypterus only.
Finally, in Sharks, the Balistidæ, and others, true scales are absent and replaced by the ossified papillæ of the cutis, which give the surface the appearance of fine-grained chagreen. These generally small bodies, as well as the large osseous scutes of the Rays, Sturgeons, etc., have been comprised under the common name Placoid scales; a term which deservedly is being abandoned.
Fig. 17.—Dermal papillæ of Monacanthus trossulus.
Fig. 18.—Dermal papillæ of Monacanthus hippocrepis (magn.)
Fig. 19.—Cycloid scale from the lateral line of Odax lineatus (magn.)
Along the side of the body of osseous fishes runs a series of perforated scales, which is called the lateral line (Fig. [21]). The perforating duct is simple at its base, and may be also simple at its outer opening (Fig. [19]), or (and this is frequently the case) the portion on the free surface of the scale is ramified (Fig. [20]). The lateral line runs from the head to the tail, sometimes reaching the caudal fin, sometimes stopping in front of it, sometimes advancing over its rays. It is nearer to the dorsal profile in some fishes than in others. Some species have several lateral lines, the upper one coasting the dorsal, the lower the abdominal outline, one running along the middle as usual. The scales of the lateral line are sometimes larger than the others, sometimes smaller, sometimes modified into scutes, sometimes there are no other scales beside them, the rest of the body being naked. The foramina of the lateral line are the outlets of a muciferous duct which is continued on to the head, running along the infraorbital bones, and sending off a branch into the præopercular margin and mandible. In many fishes, as in many Sciænoids, Gadoids, and in numerous deep-sea fishes, the ducts of this muciferous system are extraordinarily wide, and generally filled with mucus, which is congealed or contracted in specimens preserved in spirits, but swells again when the specimens are immersed in water. This system is abundantly provided with nerves, and, therefore, has been considered to be the seat of a sense peculiar to fishes, but there cannot be any doubt that its function is the excretion of mucus, although probably mucus is excreted also from the entire surface of the fish.
