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 sys-
