OSMOTIC PRESSURE OF SEA WATER. 265 



cyanin, of course do not diffuse owing to the enormous size of 

 the molecules with which they are incorporated. 



Selachians {Mustelus cams). - - Dog fish kept in fresh water for 

 one hour showed signs of asphyxia and were removed in a dying 

 condition. The defibrinated blood showed a considerable de- 

 crease in osmotic pressure, J being changed to - 1.45 C. 

 After three hours' immersion in sea water diluted with an equal 

 volume of distilled water the blood froze at 1.60 C. 



These experiments demonstrate the permeability of the mem- 

 branes to water. As has already been pointed out, the compo- 

 sition of the selachian blood is very different from sea water in its 

 salt content, but the osmotic pressure is maintained by the pres- 

 ence of large quantities of urea in the blood. It is evident then 

 that selachian membranes are semi-permeable. Little more than 

 this can be said, for this group has not been sufficiently investi- 

 gated. The same may be said of the cyclostomes. Since these 

 animals are found in both fresh and salt waters, and some species 

 migrate at certain seasons from salt into fresh water, the author 

 is making a more careful study of these groups. Mosso '" found 

 that selachian red-blood corpuscles were laked in a 2.5 per cent, 

 solution of sodium chloride, and that when a selachian (Scy/linui^ 

 was placed in fresh water death resulted in a few hours. He 

 describes a disintegration of the blood corpuscles with the forma- 

 tion from their debris of a sort of coagulum which plugs up the 

 branchial arteries with consequent death from suffocation. Death 

 resulted in one half hour if the tails had been cut off before im- 

 mersion in the fresh water. 



Telcosts. - - In nature we have experiments of the sort under 

 consideration, in the movements of such fish as the eels and sal- 

 mon, which go from salt into fresh water at the spawning season. 

 We have as yet no data relative to changes in osmic pressure of 

 the blood coincident with these migrations. The author experi- 

 mented with Anguilla chrysypa. This form lives equally well in 

 salt water or fresh-water aquaria and tolerate sudden transmission 

 from one medium to the other without apparent injury. In test- 

 ing the freezing point of the blood in different media variations 

 were found, but it was impossible to attribute them to the actual 

 changes in osmotic pressure, for similar variations were found in 



