No. 599] OSMOTIC PRESSURE OF THE BLOOD 



(55:5 



A of 0.571°, similar to the fresh-water fishes. Others of 

 this lot were placed in sea water for two days, when the A 

 of their blood was 0.766°. Others taken directly from the 

 Eel Pond (sea water) showed a A of 0.735°. The result 

 is similar to Dakin's. On the whole the conclusion seems 

 justified that anadromous fishes are able to adapt them- 

 selves to a degree to the great changes in the osmotic 

 pressures of the external medium, which they meet in 

 passing from salt to fresh water or vice versa by a slight 

 corresponding change in the osmotic pressure of the 

 blood. 



It is commonly known that sturgeons are anadromous. 

 For some reason the elasmobranch has been shut out of 

 fresh water. There is but one elasmobranch known to 

 inhabit fresh water, Carcharias nicaraguensis of certain 

 lakes in Nicaragua. Although the integument of the 

 shark is impermeable, yet I have found the gills to be still 

 permeable to salts (Scott & Denis, '13). The ganoids de- 

 rived from elasmobranchs ventured up fresh-water 

 streams. They returned to the sea. Kodier ('99) states 

 the A of the blood of Acipenser sturio to be 0.76°, which 

 places it in the same group as the marine teleosts. What 

 the A is in fresh water is not known. The modern 

 sturgeon is a long way from the modern shark. Never- 

 theless it is conceivable that the ancestral ganoids tried 

 fresh-water conditions. Is it not possible that these con- 

 ditions, fresh water and food found in fresh water had 

 some influence on the change in structure. During all 

 subsequent periods when evolutionary changes were tak- 

 ing place some forms went back and forth from sea to 

 fresh water. Some forms remained in fresh water. Dur- 

 ing this period of experimentation, impermeable mem- 

 branes were built up. In the meantime the blood had 

 become modified, due to the temporary sojourn m fresh 

 water. The osmotic pressure was reduced; the mem- 

 branes once made practically impermeable remained so 

 and when those forms returned to the sea and remained 

 there they retained almost unmodified the osmotic pres- 



