136 



UNDERWATER GUIDE TO MARINE LIFE 



2'"' DORSAL FIN 



PRE CAUDAL PIT 



LATERAL LINE 



LABIAL FUHROy 



CAUDAL (tail) fin 



PELVIC FIN 



Fig. 47 . The external anatomy of sharks and rays. 



edge, about some species, particularly sharks, one has to be careful in his 

 judgment about what to believe. 



This is a marine group with the exception of very few species. The entire 

 skeleton is made up of gristle or cartilage. An enormous advance has been made 

 over agnathans in that sharks and rays possess true jaws and paired fins (the 

 pectorals and pelvics), characteristics which give great versatility in habits and 

 motion. The best field characteristics are (1) the possession of several gill slits, 

 (2) the tail in which the upper lobe is much the largest (heterocercal tail), (3) 

 the position of the mouth under a pronounced snout, and (4) the lack of true 

 scales of the type seen in bony fishes. 



Sharks and rays possess scales of a verv interesting and basic type. These are 

 called the "dermal denticles" or "placoid scales" and are anatomically very 

 similar to the teeth of all vertebrates (with the exception of agnathans) in that 

 they possess enamel, dentine, and pulp cavity. The scales are scattered over the 

 body of sharks and ravs and are usually small, giving, however, a certain 

 roughness to the skin. Very large sharks have large scales which can produce 

 cuts on a diver. Formerly, carpenters capitalized on this roughness and used 

 sharkskin, or shagreen, as sandpaper until the advent of more modern abrasives. 

 In some parts of the body, these scales are greatly enlarged, on the bodies of 

 skates, Raja, for instance. The sting ray's spine, the sawfish's saw teeth, and, 

 most notably, the teeth in the mouth of all sharks and ravs are all evolutionarily 

 modified and enlarged placoid scales. Even we humans owe what few teeth 

 we have left to their origin as placoid scales. 



The teeth of sharks and rays are modified greatly for a wide variety of 

 functions and are constantly replaced from the inside of the jaws as the functional 

 teeth on the outside of the jaws are worn down or lost Qfig. 48). A few species 

 have several rows of teeth functioning at once, particularly the shell-crushing 

 kinds such as Port Jackson sharks and eagle rays. 



