I32 A NATURAL HISTORY OF THE SEAS 



of all the fins and may be transformed into a long and for- 

 midable weapon of offence, as in the case of the Thresher 

 Shark {Alopias). This fish swims round and round a shoal 

 of fish whilst lashing the water with its tail, so driving 

 them into a dense mass, when they are easily captured 

 and devoured. 



In the Rays and Skates (Ram) the tail is long and in some 

 cases whiplike and armoured with a sharp spine serrated 

 on both sides like a double-edged sword. With this 

 weapon the fish is capable of inflicting severe lacerating 

 wounds which are aggravated by a poisonous secretion 

 exuded from the skin. When the spine is worn out it is 

 immediately replaced by a new one. This apache of the 

 sea, which has a very wide distribution, was well known 

 to the ancients, who wrote that the wounds inflicted were 

 beyond the reach of remedy and invariably resulted in the 

 death of the victim. At the same time they asserted that 

 the spine when reduced to powder possessed remarkable 

 healing powers. 



According to J. R. Norman the shape of the tail provides 

 an index of speed and agility, and according to this author 

 fishes with deeply forked tails are capable of swimming 

 for long periods at high speeds, whilst those with square or 

 rounded tails, though capable of sudden bursts of speed, 

 are comparatively slow swimmers. 



In the Sea Horse {Hippocampus) and certain Eels the 

 caudal fin is absent. 



The streamline mackerel shape is obviously the ideal, 

 but there are numerous deviations from this form. Thus 

 in the Rays or Skates (Rata) the body is circular and flat 

 from above downwards, and in a few fishes such as the 



