264 PISCES 



In the case of the eeU a catadramous (down running) form in 

 which adults from Europe and America seek the same spawning 

 grounds, we have the most remarkable case of " homing instinct " 

 known in animals. The larvae of two different species find their 

 way back to fresh water streams of the proper continent without 

 guidance by older fishes. The eel migrates from fresh waters for 

 over 2,000 miles into the ocean and the American and the European 

 eels have been discovered by Dr. Johannes Schmidt of Copenhagen 

 to have a common spawning ground in the West Atlantic Ocean off 

 Florida, about equidistant between the Leeward Islands (West 

 Indies) and Bermuda. 



The time for maturity is different in the two species of eels and 

 the American eel terminates its larval stage in about one year and so 

 is not far enough away from the American coast to go to Europe. 

 The European eel takes three years to pass through its larval devel- 

 opment and thus is near the coast of Europe when it is time for it to 

 move up into fresh water. It is believed that when adult eels 

 migrate down from fresh water streams where they have lived 

 perhaps 20 years, seeking a spawning ground in salt water, they are 

 all maturing sexually and have that stimulus in common. Their 

 metabolic condition may determine the optimum salinity and tem- 

 perature. Eels take from 5 to 20 years to mature their eggs and 

 sperms and then they pass down the brooks and rivers to the ocean 

 for their last voyage, for they die after spawning. 



Fossil Relatives of Fishes. — The Elasmobranchii arose in the 

 Upper Silurian, and were extremely abundant in the Carboniferous. 

 The development of a bony skeleton can be traced in the fossil forms. 

 Some of the 2inc\&nt' Selachii had dorsal and anterior spines. The 

 fossil Holocephali show in their skulls certain similarities to the 

 Dipnoi. Rocks of the Devonian Age (Age of Fishes) show spines 

 and teeth of the Holocephali. Physostomous Teleosts (those with 

 the duct of the swim-bladder open) are said to be the most ancient. 

 Herring-like forms appeared in the Jurassic period. Dipnoi, or 

 lung fishes of the type now found in Australia (Ceratodus), are very 

 ancient, appearing in the Triassic period. The Dipnoi are inter- 

 mediate types between Fishes and Amphibians, but not necessarily 

 to be considered the " connecting links." 



