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. [Illustration: Fig. 22.--Side and Front view of Fish-vertebra.]