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]
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.
The scales of many fishes are modified for special purposes, especially to form weapons of defence or a protective armour, but the details of such modifications are better mentioned under the several families in which they occur. All scales are continually growing and wasting away on the surface, and it seems that some fish, at least,—for instance, Salmonoids—“shed” them periodically; during the progress of this shedding the outlines of the scales are singularly irregular.
CHAPTER III.
TERMINOLOGY AND TOPOGRAPHY OF THE SKELETON.
In order to readily comprehend the subsequent account of the modifications of the skeleton in the various sub-classes and groups of Fishes, the student has to acquaint himself with the terms used for the numerous bones of the fish skeleton, as well as with their relative position. The skeleton of any of the more common kinds of osseous fish may serve for this purpose; that of the Perch is chosen here.
The series of bones constituting the axis of the body, and destined to protect the spinal chord and some large longitudinal blood-vessels, is called the vertebral or spinal column; the single bones are the vertebræ. The skull consists of the bones surrounding the brain and organs of sense, and of a number of arches suspended from it, to support the commencement of the alimentary canal and the respiratory organs.
The vertebra (Fig. [22]) consists of a body or centrum (c), with a concave anterior and posterior surface, and generally of several processes or apophyses, as—1. Two neurapophyses (na), which, on the dorsal side, rising upwards, form the neural arch over the canal, in which the spinal chord is lodged. 2. Two parapophyses (pa) usually projecting from the lower part of the sides of the body, or two hæmapophyses (ha) which actually coalesce to form on the ventral side the hæmal canal for a large trunk of the vascular system. 3. A neural spine (ns), which crowns the neurapophyses, or is interposed between their tips. 4. A hæmal spine (hs), having the same relation to the hæmapophyses. 5. Two pleurapophyses or floating ribs, suspended from, or from the base of, the parapophyses. 6. In most fishes the neural arches are connected together by articular or oblique processes, zygapophyses (za), which are developed from the base of each neurapophysis.
