FISHES 



259 



from the surface are the sharp points of innumerable small hard 

 structures, closely resembling teeth in structure and usually known 

 as placoid scales. The presence of these causes a peculiar rough- 

 ness suggestive of sand - paper. Numerous sense organs are 

 present in the skin, some of which will be alluded to farther on. 



The first point of general interest to note with regard to 

 the internal skeleton (fig. 159) is that it is entirely made up of 



HYOMANDIBULAR 



Fig. 159. Skeleton of Dog-Fish (Scyllium canicula) 



A, Skull and part of vertebral column. B, End view of a caudal vertebra, and longitudinal section through centra of 

 two vertebrae, c, Pectoral fin (from below). D, Pelvic fin (from below). 



cartilage and fibrous tissue, as generally in what are called 

 " cartilaginous fishes", while in ordinary " bony fishes ", including 

 all the common edible forms, a great deal of bone is present as 

 well as more or less cartilage. 



The skull is extremely simple compared with that of the 

 higher animals so far considered, and consists of a brain-case 

 to which are attached protective capsules for nose and internal 

 ear, the framework of the jaws, and what is known as the 



