CHAPTER VI. 



THE SKELETON OF THE DOGFISH 1 . 



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 with- 

 out 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 con- 

 sisting 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 See Marshall and Hurst's Practical Zoology, 4th ed. London, 1895, 

 p. 214. 



