4 ANNALS NEW YORK ACADEMY OF SCIENCES 



water. Within the body, the rapidly circulating blood comes into such 

 relation with sea-water as to insure the exchange of gases. It is a com- 

 mon belief that, by virtue of these structures alone, the organism is main- 

 tained in osmotic equilibrium with the surrounding medium. How far is 

 this position tenable ? 



The relation of each of these structures to the solution within and 

 without the organism may be that of a freely permeable membrane, a 

 semi-permeable membrane or an impermeable membrane. The limiting 

 membranes of the marine invertebrate body have been shown to be quite 

 freely permeable. Sea-water is isotonic with its blood; but if the sea- 

 water is diluted, salts leave the body by way of these structures, and 

 water from without enters into the body with the result that the blood 

 soon attains the same molecular concentration as the outside medium. 

 Similar adjustments are said to take place in the case of the elasmo- 

 branchs. In this case, however, it is claimed that the resulting equality 

 is attained not by the loss of salts from the body but by changes in the 

 relative amount of water in the blood. Further investigation of this 

 matter seems necessary. In addition the following questions call for in- 

 vestigation. What are the lethal limits of departure from the normal 

 osmotic pressure of the blood of elasmobranchs ? Is the modification in 

 the osmotic pressure of this blood dependent upon the time of immersion 

 in the changed external medium ? Is the change dependent upon the de- 

 gree of change in the osmotic pressure of the external medium ? Does a 

 lethal change in the osmotic pressure of the blood affect the corpuscles ? 

 If so, in what manner and to what degree? What is the effect of the 

 modified blood upon blood pressure, heart beat and respiration ? Is there 

 any evidence of a mechanism for the maintenance of the normal osmotic 

 pressure of the blood ? Is there any evidence that a new and permanent 

 normal osmotic pressure of the blood is established under conditions in 

 which the concentration of the external medium is permanently modi- 

 fied? Does the blood of the elasmobranch under such conditions remain 

 of the same molecular concentration as that of the modified external 

 medium ? Evidence on these and related problems is offered in the pres- 

 ent paper. 



The experiments described below were carried on at the Biological 

 Laboratory of the United States Bureau of Fisheries at Woods Hole, 

 Massachusetts, and at the N"ew York Aquarium. I wish to thank the 

 Commissioner of Fisheries, the Hon. George M. Bowers, Dr. Francis B. 

 Sumner and Mr. T. E. B. Pope for the many facilities extended at Woods 

 Hole, and to extend thanks likewise to Dr. Charles H. Townsend, the 



