FISHES AND THEIR SURROUNDING MEDIUM. 303 



some cases to yield less chlorine than passed from the body of a 

 living fish in the course of a few hours. Likewise those fishes 

 whose bodies were analyzed gave approximately the same per- 

 centages of chlorine whether or not the alimentary canal was 

 included in the analysis. It must be remembered also that the 

 fishes employed in these experiments had, in all cases, been kept 

 unfed for some days previously. 



5. In certain fishes, at least, it was found that the membranes 

 cJiicfly concerned in such exchanges were those of the gills. 



In the case of certain specimens, salt water was passed through 

 the gills by means of a rubber tube placed in the mouth, the body 

 being bathed with fresh water ; while in others the arrangement 

 was reversed, the gills receiving fresh water and the general in- 

 tegument salt. In six experiments with the carp it was found 

 that a considerable loss of weight occurred in all of those cases 

 in which the former conditions obtained, while the weight re- 

 mained practically stationary in those cases in which the condi- 

 tions were reversed. 



A complete historical review of previous researches in this field 

 of physiology would be beyond the scope of the present paper. 

 The investigations of Fredericq, 1 Bottazzi, 2 Rodier, 3 Garrey 4 and 

 Greene 5 agree in showing that the blood of marine teleosts is far 

 from being isotonic with the surrounding sea-water, but that it has 

 an osmotic pressure which is roughly about one half that of the 

 latter. But it has likewise been shown (Fredericq, Greene, op. 

 cit.} that the osmotic pressure of the blood of salt-water teleosts 

 is considerably higher than that of fresh-water ones, though this 

 fact has been almost lost sight of in the zeal to prove that the 

 internal medium is not isotonic with the external, and that its 

 osmotic pressure is relatively constant. Indeed it does not seem 

 to have been generally appreciated that there is a certain corre- 

 lation between the inner and outer fluids, both as regards osmotic 

 pressure and salt content ; and certain authors have been free to 

 state that the membranes of teleost fishes form an effective 

 barrier against osmotic changes. Fredericq makes this assertion 



1 Archives de Biologic, 1904. 3 Cited by Fredericq, 1904. 



2 Archives italiennes de Biologic, 1897. 4 BIOLOGICAL BULLETIN, 1905. 

 5 Bulletin U. S. Bureau of Fisheries, 1905. 



