THE BONY FISHES lOQ 



simple diflFusion, probably carrying water with them by 

 osmosis, and the renal-portal blood meets all require- 

 ments in a marine fish in which tlie filtration rate is 

 markedly cmtailed. 



And as the glomerulus degenerates, in the evolution- 

 ary sense, the arterial circulation to the glomeruli is also 

 obHterated, leaving the tubules suppUed only by renal- 

 portal blood, which enters the peritubular capillaries at 

 a pressure too low to eflFect any filtration whatever. We 

 conceive that the renal-portal system, derived from the 

 protovertebrate, has always been valuable to the fresh- 

 water fishes, since it is preserved in all recent forms, as 

 well as in the Amphibia; but its value to the marine 

 teleosts is beyond question. Without it, it is doubtful 

 if there would be any fishes (apart from the elasmo- 

 branchs) living in the sea today. 



The renal tubule has also undergone extensive changes 

 in the marine teleosts. We have spoken of the tubule in 

 fresh-water forms as consisting typically of two seg- 

 ments, a proximal and distal segment. In such marine 

 fishes as have been studied in this respect, only the proxi- 

 mal tubule is present— the distal tubule has disappeared 

 entirely. It is believed that among the fimctions attribut- 

 able to the distal tubule are the excretion of a dilute urine 

 (water diinresis) and the regulation of the acidity of the 

 urine. Typically, however, the marine fish never goes 

 into fresh water and has no excess water to excrete. And 

 the acidity of the urine is fixed on the extreme acid side: 

 were the urine to become neutral or alkaline, magne- 

 sium, of which substantial quantities are absorbed from 

 sea water in the gastrointestinal tract, would precipitate 

 as magnesium oxide or magnesium phosphate and clog 

 the tubules and collecting ducts. It is a matter of in- 

 ference that the disappearance of the distal tubule is re- 

 lated to one or the other, or both, of these circumscrip- 

 tions, which render this segment useless to a fish hving 

 in salt water. 



In the glomerular nephron of both the fresh- and salt- 



