THE INCREDIBLE SHARKS 341 



mathematics applied, for the Ggg laying forms, being subject 

 to greater loss, must and do produce a larger number of spawn 

 to maintain themselves. 



The eggs of sharks, and the affiliated skates and rays, are 

 encased in tough leathery covers, usually rectangular in shape 

 with long fibrous tendrils which are caught about marine vege- 

 tation or solid objects to prevent them from being drifted away. 

 Others, however, are formed in a peculiar ridged spiral resem- 

 bling somewhat the curled forms of certain sea shells. Every 

 Atlantic beach at certain seasons of the year is strewn with the 

 black remnants of these cases. 



Not the least curious of all fin developments is that belong- 

 ing to those most original of all hoboes, the remoras, itinerant 

 and unwanted companions of the sharks. They attach them- 

 selves to their elasmobranchian freight trains with an oval disk 

 located on the flattened upper surfaces of their heads. These 

 disks are made up of a series of fleshy partitions which by mus- 

 cular retraction create a semi-vacuum. The disk is simply a 

 dorsal fin which by an amazing transformation has become 

 modified into a sucker. 



The thresher sharks, however, have the most unusual fin 

 development of all. The upper lobe of their caudal fin is equal 

 to the length of the remainder of the animal. Its purpose has 

 not yet been discovered, although it is believed by some to be 

 utilized as a flail to herd the small schooling fishes on which it 

 feeds into a compact mass where they can be attacked with 

 ease. The common stories that the blade-like tail is used to beat 

 porpoises to death is utterly without foundation. There is also 

 the possibility that it is one of those freaks of animate existence 

 which has no sensible purpose but which was caused by an 

 outlandish mutation, a going astray of the genes and chromo- 

 somes of a distant ancestor. We will not know much of these 

 creatures of the sea until some way is devised of observing 



