V 



THE APPEARANCE OF JAWS. 

 THE ORGANIZATION OF THE HEAD 



1 . The elasmobranchs : introduction 



In all parts of the sea there are to be found members of the class of 

 the elasmobranchs (literally 'plate-gilled' fishes), including sharks 

 ranging from monsters of 50 ft long to the common dogfish Scylio- 

 rhinus caniculus of 1-2 ft. Nearly all the fishes in the group are carni- 

 vorous or scavengers: the skates and rays are bottom-living relatives, 

 feeding mostly on invertebrates. Although they are not quite so fully 

 masters of the water as are the bony fishes, they are yet well enough 

 suited to that element to survive in great numbers in all oceans. 

 Perhaps the skill and cunnning of a shark is exaggerated by the 

 frightened boatman or bather, who is apt to mistake a keen nose and 

 the persistence of hunger for intelligence, especially when he is faced 

 at intervals with a well-armed mouth; but the sharks have a large 

 brain and their active, predacious habits enable many of them to 

 live by eating the more elaborately organized bony fishes. 



Evidently such active creatures have changed considerably if they 

 have been evolved from the heavily armoured and probably slow- 

 moving agnathous vertebrates that shovelled up food from the bottom 

 of Palaeozoic seas. It used to be supposed that these elasmobranch or 

 cartilage fishes represent a very primitive stock, but we now realize 

 that there have been great changes since the biting mouth was first 

 evolved; we cannot be sure that any features we find in the elasmo- 

 branchs were possessed by the earliest gnathostomes. 



The typical shark is a long-bodied fish, swimming by the passage of 

 waves of contraction along tne metamerically arranged muscles. As in 

 the lampreys and eels, the wave that passes down the body is of short 

 period, relative to the length of the fish, and is therefore evident as it 

 travels along. This is probably a less efficient system than is provided 

 by the longer period waves of the most highlv developed bonv fishes; 

 the sharks are good swimmers, but except for the mackerel sharks 

 (Isuridae) not among the swiftest. Stability and control of direction 

 are ensured by the upturned tail and the fins. The tail, with its dorsal 

 lobe larger than the ventral, is called heterocercal, and tends to drive 

 the head downwards. This is corrected by the flattened shape of the 

 head itself and by the pectoral fins, which act as 'aerofoils', allowing 



