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. [Illustration: Fig. 20.--Cycloid scale from the lateral line of Labrichthys laticlavius (magn.)] The scales, their structure, number and arrangement, are an important character for the determination of fishes; in most scaly fishes they are arranged in oblique transverse series; and as the number of scales in the lateral line generally corresponds to the number of transverse series, it is usual to count the scales in that line. To ascertain the number of longitudinal series of scales, the scales are counted in one of the transverse series, generally in that running from the commencement of the dorsal fin, or the middle of the back to the lateral line, and from the lateral line down to the vent or ventral fin, or middle of the abdomen.[4] [Illustration: Fig. 21.--Arrangement of scales in the Roach (Leuciscus ratilus): _Ll_ = Lateral line; _tr_ = Transverse line. _a_, Transverse line from lateral line to ventral fin.]