RESEARCH SEMINAR. 239 



ment in securing orientation of the fishes experimented with is 

 an optical reflex of such a kind that the animal tends to retain 

 the same visual field. It is found however that blind fishes or 

 fishes in the dark are able to orient themselves. Investigation 

 of this phenomenon showed that it is primarily due to objects on 

 the bottom. Blind fishes do not orient themselves in a uniform 

 current unless they touch objects which are stationary. In more 

 violent streams of water where there are considerable differences 

 of velocity between closely proximated parts of the stream, orien- 

 tation may occur. Here apparently the same explanation holds, 

 /. e. , that the relatively stationary water constitutes the reference 

 points by which the animal establishes its direction in the stream. 

 It is believed that pressure in a gross, mechanical way cannot 

 explain rheotropism, but rather that it is always a response to 

 the relative motion between the fish being carried passively down 

 by the moving water, and more stationary parts of the environ- 

 ment. 



August 12. Osmotic Pressure of Sea Water and Marine Ani- 

 mals. By W. E. CARREY. 



1. The osmotic pressure of the sea water was determined by 

 testing the depression of the freezing point (J) with a Beckmann 

 apparatus. 



(a) Buzzards Bay water, J = - - 1.83 and - - 1.82. 



() Eel Pond water, J = - - 1.77. 



(V) Salt water tap of Marine Biological Laboratory showed 

 considerable variations; J= - - 1.86 in the early part of the 

 season and as low as - - 1.78 after heavy rains. Most of the 

 readings however varied between - - 1.84 and - - 1.82. 



(d] Basin of Fish Commission - - 1.84 and - - 1.81. 



These waters thus show a freezing point slightly less than that 

 of molecular cane sugar ( J = - - 1.85) and the osmotic pressure 

 is about 22 atmospheres calculated at o C. 



2. The body fluids of marine invertebrates have an osmotic 

 pressure very close to that of the sea water from which they are 

 taken. 



3. By immersion in diluted or concentrated sea water marine 

 invertebrates take up or lose water respectively and assume an 

 osmotic pressure approximating that of the external medium. 



