39<5 READINGS IN BIOLOGICAL SCIENCE 



Differing from their salt-water cousins, the lower jaw of both species 

 projects beyond the upper, while the large mouth gapes back to a point 

 even with or somewhat behind the eyes. On the side of the neck are gill 

 slits with upper corners on a line with the center of the base of the pectoral 

 fins. A single fin, soft and without spines, extends along the back, around 

 the tip of the tail, and forward on the underside of the body. There is 

 no separation into dorsal, caudal or anal parts. After the third or fourth 

 year of life, eels develop small scales that are imbedded in the skin. These 

 are covered with a coating of slimy mucous, which has given rise to the 

 simile, "as slippery as an eel." 



Perhaps the most intriguing part of the life cycle of this unusual fish 

 is that neither European nor American elvers have ever been known to 

 appear off the shores of any country but their own. This fact immediately 

 presents two puzzling questions that challenge the imagination. 



1. What causes the immature larvae of the European species to move 

 eastward from the spawning grounds, while its American cousin works 

 toward the west side of the Atlantic? 



2. How does it happen that the timing is perfect for both species to 

 reach the elver stage within a few months after arriving off the coast of 

 their home continent? 



These may be answered in part by the difference in their individual larval 

 histories. 



While the European larva requires from two and one-half to three years 

 to reach the elver stage of development after life begins, the larval stage 

 of the American species is terminated in about one year. This time element 

 not only acts to keep the two species distinct, but makes it practically im- 

 possible for either to survive in waters other than their own after meta- 

 morphosis takes place. 



Should the larvae of the European eels move westward they would reach 

 the American coast line still in an undeveloped larval stage; while the 

 American species traveling eastward would reach the elver stage some- 

 where in the western Atlantic when the time arrived for them to seek 

 fresh-water retreats. 



A geographic cause for their distribution is advanced by Doctor Schmidt 

 who points out that the center of production for the American eel lies 

 farther west and south than the center of the European beds. These, to- 

 gether with the movement of the ocean currents as an aid to the journey 

 in the early stages of larval development, must be considered as causes 

 directing the two species to its own side of the ocean. 



While much has been learned of the habits of these sluggish, sedentary 

 fish, since the turn of the century, much remains unexplained. 



With a singleness of purpose and an unerring instinct that has confused 

 scientists, untold thousands have deserted their home waters each autumn 

 to seek adventure in a tropic sea and to keep their rendezvous with death. 



