The parts of the skeleton are frequently grouped under two heads: the _axial skeleton_, comprising the skull and vertebral column; and the _appendicular skeleton_, including the pectoral and pelvic girdles and the skeleton of the fins. VERTEBRAL COLUMN. The vertebral column is divided into two regions, thoracic and caudal, distinguished by the slightly different character of the vertebrae. Remove the muscle and connective tissue from the vertebral column for a short distance anterior to the first dorsal fin. Care is required not to cut away small cartilages occupying the positions of ribs. Now remove from the body about two inches of the portion of the column exposed with any cartilaginous parts which may be attached to the vertebrae. The vertical column is made up of segments, called _vertebrae_. Each vertebra consists of a large ventral mass, the centrum, and an arch, the neural arch, roofing over the dorsal surface of the centrum; the arch is composed of several small plates of cartilage. The opening enclosed by each centrum and its neutral arch is the vertebral foramen; the joined vertebral foramina form the neural canal, which is occupied by the spinal cord. Separate one of the vertebrae from the rest. The centrum is deeply concave at each end; such a centrum is termed amphicoelous. At the middle of the centrum the concavities meet and thus a canal is formed through it. This canal and the spaces between the ends of adjoining vertebrae are filled by the remains of the notochord, a rather pulpy structure extending from end to end of the vertebral column. The concave faces of the vertebrae consist of much firmer cartilage than the remaining portions, sometimes even calcified. Make a transverse section through the middle of a centrum and observe the relations of the parts. On each side of the centrum, near the ventral edge, is a plate-like projection, the transverse process. Attached to the extremity of this is a slender cartilaginous _rib_. Each _neural arch_ is made up of two distinguishable sets of plates. The first consists of a pair of broad _neural plates_ extending upward from each side of the centrum and uniting with each other dorsally. Between the neural plates of two successive vertebrae is a pair (one on each side) of _intercalary plates_ which also unite over the neural canal. The intercalary plates are over the joint between the centra. Neural and intercalary plates together make the lateral and dorsal walls of the neural canal. The relations of these plates can sometimes be seen best when the neural arch is cleaned, then cut away from the centrum, and looked through toward the light. In the lower part of each neural plate is a small foramen which allows the passage of the ventral root of the spinal nerve. A foramen for the dorsal root is found at about the middle of the intercalary plate.