72 ANNALS NEW YORK ACADEMY OF SCIENCES 



When Mustelus is immersed in hypertonic solntions of sea-water, not 

 only does the osmotic pressure of the blood increase but also its chlorine 

 content. 



The specific gravity of the blood decreases on immersion of the fish in 

 fresh water. 



When the fish is immersed in fresh water, a certain amount of decrease 

 in the osmotic pressure of the blood can be ascribed to dilution of the 

 blood caused by the absorption of water through the gill membranes. In 

 addition, a further change is due to diifusion of salts outward through 

 the gill membranes, as is shown by the presence of considerable quantities 

 of chlorine in the water in which the fish is immersed. 



The tissues of the body tend to maintain the osmotic pressure of the 

 blood by absorbing water from the hypotonic blood and this tends to raise 

 the pressure. 



By secreting rapidly a diluted urine, the kidneys also tend to maintain 

 the normal osmotic pressure of the blood. By this process, the urea and 

 a certain amount of the salts of the blood are conserved. 



The changes in blood pressure due to immersion in fresh water are 

 slight as compared with the effects upon respiratory and cardiac activity. 



On immersion in fresh water, there is a gradual failure of respiration : 

 this is marked by irregularly repeated spasmodic respiratory movements 

 which increase in intensity for a period and then decline. 



When the sea-water in which the fish is immersed is gradually changed 

 to fresh water, the heart beat increases in amplitude and decreases in 

 rate. The contractions gradually diminish in force, although the heart 

 continues to beat faintly after respiration has ceased. 



Coincident with and similar in character to the spasmodic movements 

 of respiration, spasmodic contractions of the heart occur. 



The normal osmotic pressure of the blood of Mustelus is maintained 

 only by the organism remaining in sea-water. It is probably provided 

 with a sensory apparatus by which it is able to avoid great modifications 

 of the external medium. In slightly brackish waters, the osmotic pres- 

 sure of the blood is diminished by the influx of water through the gill 

 membranes; but because of the regulative activity of the kidneys and 

 other bodily tissues, the changes are less than the changes in the external 

 medium, and are still within the range of pressures compatible with life. 

 With greater changes in the molecular concentration of the external 

 medium the organism succumbs. 



The gill membranes are probably not greatly injured by this absorption 

 of water, for the animal continues to live indefinitely, as is shown by the 

 elasmobranchs in the New York Aquarium. It may be concluded that 



