CHAPTER VI.
THE SKELETON OF THE DOGFISH[34].
Scyllium canicula.
I. EXOSKELETON.
The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel. The scales are larger on the dorsal than on the ventral surface, and on the jaws they are specially large and regularly arranged in rows, there forming the teeth. The margins of the jaws or lips are without scales.
A second exoskeletal structure is found in the fins, all of which, both paired and unpaired, have, in addition to their cartilaginous endoskeleton, large numbers of long slender horny fibres, the fin-rays, which are of exoskeletal origin.
II. ENDOSKELETON.
The endoskeleton of the dogfish consists almost entirely of cartilage, which however may become calcified in places, e.g. the centrum of each vertebra is lined by a layer of calcified tissue.
The endoskeleton is divisible into an axial portion consisting of the vertebral column, skull, and skeleton of the median fins, and an appendicular portion consisting of the skeleton of the paired fins and their girdles.
1. The Axial Skeleton.