Habits and Adaftations 259 



top of the head, which has been modified into a mechanically 

 quite beautiful suction disk. With this its owner attaches it- 

 self to the skin of a shark, and is given a free ride from place 

 to place, letting go to feed when the shark does, returning to 

 its carrier when the shark moves on. The sucker is extremely 

 powerful, so much so that the natives of some of the islands 

 in the Pacific use the fish to catch turtles with. They fasten 

 a line to the remora, release it close to a turtle, and then 

 let it cling inexorably to the shell while they tow it to 

 shore. 



And fins are also used in some species in mating. The tiny 

 guppy uses his anal fin to direct balls of sperm at the female's 

 vent; the great shark uses his ventral fins to introduce the 

 sperm into her body. 



In fact, mating and reproduction being the most important 

 functions of fish, as of all animals, it is not to be wondered 

 at that the most numerous and most interesting adaptations 

 and habits are those which have to do with these processes. 

 Most famous are the spawning of the salmon and the eel. 

 The salmon's migrations, which we described in an earlier 

 chapter, are more widely known because of the fish's sport 

 and commercial importance, but the spawning activities of 

 the eel are in some ways even more remarkable. They were 

 for centuries a complete mystery, and the discovery of their 

 breeding-grounds is one of the outstanding scientific achieve- 

 ments of the first quarter of this century. 



The eel which we are speaking of here is not the moray 

 nor the conger, but the common eel found in fresh water on 

 both sides of the North Atlantic, the snaky fish with no 

 ventral fins. In this country fishermen are inclined to look 

 upon it as a nuisance when they find it on the end of their 

 line, as a menace when they learn that it eats young bass, 

 pickerel and even trout. They overlook the fact that, in ex- 



