32 ANNALS NEW YORK ACADEMY OF SCIENCES 



full strength sea-water, i. e., having a A of 1.82°. These dog-fishes are 

 brought to the Aquarium from the fishing grounds near Sandy Hook. 

 The A of the blood of two specimens from Vineyard Sound, Mass., was as 

 follows : 



a — female 56 cm. long, Ar=l.81°. 



b— " 49 " " "=1.87°. Average = 1.84°. 



This is not far removed from that of Mustelus, which is 1.87°. A larger 

 number of determinations would probably average 1.87°. 



Evidently the normal osmotic pressure of the blood of Squalus under- 

 goes a reduction, when the fishes are kept in the harbor water. It is also 

 of interest to see that the blood does not become isotonic with the brack- 

 ish water in which the fishes are kept. There appears to be a new equi- 

 librium. If Ave assume that the blood of Squalus has normally the same 

 mean A as that of Mustelus, then we can conclude that the A of the blood 

 of Squalus has risen 0.18°, due to the immersion in brackish water. 

 Moreover this is the value to be expected, if the change in the osmotic 

 pressure of the blood bears a definite relation to the change in the osmotic 

 pressure of the surrounding medium. The following proportion shows 

 this : 1.795° : 0.82° : : 0.41° : X, in which 1.795° equals the difference be- 

 tween the A's of sea-water and fresh water, 0.82° equals the difference 

 between the A^s of sea-water and harbor water and 0.41° equals the maxi- 

 mum change in the freezing point of Mustelus after immersion in fresh 

 water. X, on the basis of the above theory, should equal the A of the 

 blood of the fish after immersion in harbor Avater. But X equals 0.187°, 

 whereas 0.18° Avas the observed change in A. It seems to me that the 

 chief point of interest, however, is that the organism maintains an os- 

 motic pressure of its blood greater than that of the water in Avhich it is 

 kept. From what is known of the marine invertebrates, this property of 

 the dog-fish is a distinct adA^ance. Many of the dog-fishes brought into 

 the Aquarium do not survive. It is interesting to speculate as to why any 

 survive. Is it because the limiting membranes of the body are more re- 

 sistant, or do these membranes become more resistant in response to the 

 change produced in the osmotic pressure of their blood ? On immersion 

 in fresh water, Squalis did not show as great a reduction in the freezing 

 point of its blood as was the case with Mustelus. This is shoAvn by the 

 following table, XIII : 



