Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/657

639 PARTS.] ICHTHYOLOGY (539 FIG. 18. Ventrala of Gobius. wards the right and left, resembling the locomotion of snakes. In the Synguathi (pipe-fishes) and Hip pocampi (sea-horses), whose body admits of but a slight degree of lateral curvature, and whose caudal fin is generally small if present at all, locomotion is very limited, and almost wholly dependent on the action of the dorsal fin, which consists of a rapid undulating movement. The Skin and Scales. The skin of fishes is either covered with scales, or naked, or provided with more or less numerous scutes of various forms and sizes. Some parts, like the head and fins, are more frequently naked than scaly. All fishes provided with electric organs, the majority of eels, and the lampreys are naked. Scales of fishes are very different from those of reptiles, the latter being merely folds of the cutis, whilst the former are distinct horny elements, developed in grooves or pockets of the skin, like hairs, nails, or feathers. Very small or rudimentary scales are extremely thin, homogeneous in structure, and more or less imbedded in the skin, and do not cover each other. When more developed, they are imbricated (ar ranged in the manner of tiles), with the posterior part extruded and free, the surface of the anterior portion bein ; usually covered by the skin to a greater or less extent. On their surface (fig. 1 4) may be observed a very fine striation concentric with and parallel to the margin, and coarser stride radi ating from a central point towards the hind margin. Scales without a covering of enamel, with an entire (not denticulated) posterior margin, and with a concentric stria tion, are called cycloid scales. Ctenoid scales (figs. 15, 16) FIG. 14. Cycloid scale of Scopeltu resplendent (magn.). FIG. 15. Ctenoid scale of Gobius ommaturus (magn.). FIG. 16. Ctenoid scale of Lethrinus (magn.). are generally thicker, and provided with spinous teeth on the posterior edges of the layers of which the scale con sists. In some species, only the layer nearest to the margin is provided with denticulations (fig. 15). Scales the free surface of which is spiny, and which have no denticulation on the margin, have been termed sparoid scales ; but their distinction from ctenoid scales is by no means sharp, and there are even intermediate forms between the cycloid and ctenoid types. Both kinds of scales may occur, not only in species of the same genus of fishes, but in the same fish. Ganoid scales are hard and bony, covered with a layer of enamel ; they are generally rhombic or quadrangular, rarely rounded and imbricate, and are 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 Pvlypterus only. Finally, in sharks, the Balistidte, and others, true scales are absent, and are replaced by the ossified papilla? of the cutis (fig. 18), which give the surface the appearance of fine- FIG. 17. Ganoid scales of Dapedius. grained shagreen. These generally small bodies, as well as the large osseous scutes (figs. 6 and 20) of the rays, stur- FIG. 18. Dermal papillsc of Monacanthus trof stilus. geons, &c., have been comprised under the common name pla- coid scales, a term which is being deservedly abandoned. FIG. 19. Dermal papillse of Monacanthus hippocrepis (magn.). Along the side of the body of osseous fishes runs a series of perforated scales, which is called the lateral line (fig. 23). The perforating duct is simple at its base, and may be simple also at its outer opening (fig. 21), or (a frequent case) the portion on the free surface of the scale is ramified (fig. 22). The lateral line runs from the head to the tail, sometimes reaching the caudal fin, sometimes stopping short of it, sometimes advancing over its rays. Some species have several lateral lines, an upper one following the dorsal, a lower the abdominal outline, while a J.-L- i i 1.1, -j n i rr&amp;lt;i FIG. 20.-Devmal spines third runs along the middle as usual. The O f a male Thorn- scales of the lateral line are sometimes back Raia clavata - larger than the others, sometimes smaller; sometimes they are modified into scutes; sometimes there are no other scales besides these, 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 prajopercular margin and mandible. FIG. 21. Cy&amp;lt;&quot;o &amp;lt;l scale from the lateral line of The muciferous system is abundantly provided with nerves, and has therefore been considered to be the seat of a sense peculiar to fishes,