Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/658

640 640 ICHTHYOLOGY [OSTEOLOGY. 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. The scales, their structure, number, and arrangement, constitute an important character for the determination of Fio. 2-2. Cycloid scale from the lateral line of Labrichthys laticlavius (magn.). 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 Fio. 23. Arrangement of scales in the Roach (Leuciscus rutilus). L /^Lateral line; t r = transverse line; a, tranverse line from lateral line to Tentral fin. 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 the middle of the abdomen. OSTEOLOGY. In order readily to comprehend the following account of the modifications of the skeleton in the various sub classes and groups of fishes, the student should acquaint himself with the terms used for the numerous bones of the fish skeleton, as well as with their relative position. For this purpose we commence this section with an account of the skeleton of the Teleostei, which is composed of the greatest number of specialized bones, and is most readily accessible. The skeleton of any of the more common kinds of osseous fish may serve for this purpose ; that of the perch was chosen by Cuvier, and is employed here (fig. 24), as it was in the last edition of the present work. In the Teleosteous fishes the spinous column consists of y er t, completely ossified amphiccelous vertebrae ; its termination is homocercal that is, the caudal fin appears to be more or less symmetrical, the last vertebra occupying a central position in the base of the fin, and being united to a flat fan-like bone, the hypural (70 in fig. 24), on the hind margin of which the fin-rays are fixed. The hypural is but a union of modified haemapophyses which are directed backwards, and the actual termination of the notochord is bent upwards, and lies along the upper edge of the hypural, hidden below the last rudimentary neural elements. In some Teleosteans, as the /Salmonidce, the last vertebrae are conspicuously bent upwards : in fact, strictly speaking, this homocercal condition is but one of the various degrees of heterocercy, different from that of many Ganoids in this respect only, that the caudal fin itself has assumed a higher degree of symmetry. The neural and hsemal arches generally coalesce with the centrum, but there are many exceptions, inasmuch as some portion of the arches of a species, or all of them, may show the original division. FIG. 24. Skeleton of the The vertebra are generally united with one another by zygapophyses, and frequently similar additional articu lations exist at the lower parts of the centra. Parapo- physes and ribs are very general, but the latter are inserted on the centra and the base of the processes, and never on their extremities. The spinal column consists of abdominal and caudal vertebrae, the coalescence of the parapophyses into a com plete htemal ring and the suspension of the anal fin generally forming a sufficiently well-marked boundary be tween the abdominal and caudal regions (fig. 24). In the perch there are twenty-one abdominal and as many caudal vertebrae. The centrum of the first vertebra or atlas is very short, with the apophyses scarcely indicated; neither the first nor the second vertebra has ribs. All tbe other abdominal vertebrae, with the exception of the last or the two last, are provided with ribs, many of which are bifid (72). A series of flat spines (74), called interneurals, to which the spines and rays of the dorsal fins are articulated, are supported by the neural spines, the strength of the neurals and inter neurals corresponding to that of the dermal spines (75). The caudal vertebrae differ from the abdominal in having the hsemapophyseal elements converted into spines similar to the neurals, the anterior being likewise destined to sup-