THE BONY FISHES II3 



selection pressure to reduce or even to obliterate the 

 glomeruli, as so many of the highly specialized forms 

 have done. If they have all had a continuous marine his- 

 tory of approximately equal length, and of as long a 

 duration as, say, from the Carboniferous, why have the 

 majority retained fair to good glomeruli? It is in better 

 agreement with the evidence to suppose that our pres- 

 ent marine forms have been estabHshed as permanent 

 residents of salt water only during Cenozoic time, an 

 interval possibly too short to permit universal adaptation 

 to the physiological stresses of a marine habitat. 



A few aglomerular fishes have secondarily left the sea 

 to reinvade brackish or fresh water— the toadfish {Op- 

 sanus) of the North Atlantic coast (closely related to 

 the typically deep-sea Lophiidae) enters the tidal creeks 

 of the Chesapeake Bay, while the pipefishes (Syngna- 

 thidae) are represented by several species that live and 

 breed in the fresh-water rivers of Siam, Malaya, the 

 Philippines, Panama, and the West Indies. The toadfish 

 and the East Indian fresh-water pipefish, Microphis 

 boaja, however, remain aglomerular— it is not in the na- 

 ture of evolution that the clock can be turned back to 

 give them glomeruli again. But how this or other forms 

 maintain water balance in fresh water has not been 

 studied. 



Some glomerular fishes, such as the IdUifish (Fundu- 

 lus), the stickleback (Gastrosteus) , and the common eel 

 (Anguilla)y can tolerate rapid transfer from fresh to salt 

 water, quickly making the necessary adjustments in wa- 

 ter excretion. It is an oft-told tale how the common eel 

 breeds in the Sargasso Sea in the mid-Atlantic and how 

 the yoxmg elvers return to fresh water in Europe and 

 America where they reach maturity and spend five or 

 six years before returning to the sea to spawn. In salt 

 water the eel drinks sea water, but in fresh water the only 

 water to enter the gastrointestinal tract is apparently ac- 

 cidental, though other fresh-water fishes, such as the 

 goldfish, may ingest water when feeding on microscopic 



