SCOTT, STUDY OF CHANGES IN MUSTELUS CAXIS 3 



tion in the osmotic pressure of the body fluids of animals that inhabit 

 these various waters. For example, the blood of fresh water invertebrates 

 and fishes is much more concentrated than the water in which they live. 

 While the body fluids of these forms is maintained at a constant osmotic 

 pressure, the surrounding water is by no means isotonic with the blood 

 serum. The A of fresh water is 0.025°, while that of the blood of fresh 

 water fishes is about 0.60°. The blood of marine invertebrates and elas- 

 mobranch fishes has about the same molecular concentration as that of 

 the sea. The mean A of the waters of the Mediterranean Sea and the 

 mean A of the body fluids of the invertebrates and elasmobranch flshes 

 which inhabit it is 2.29°. The A of the water of the ocean is about 2.00° 

 and invertebrates inhabiting such waters have a similar A. In the case 

 of the elasmobranchs, it has been claimed that the surrounding medium 

 is isotonic with the blood. On the other hand, the A of the blood of ma- 

 rine teleosts is less than one-half that of the external medium. For 

 example, although the A of sea-water from the Baltic is 1.80°, according 

 to Dekhuysen ('04) the A of the blood of marine teleosts from these 

 waters is only 0.724°. The A of the blood of fresh water fishes is nearly 

 the same as that of the marine teleosts. The blood of other marine verte- 

 brates, such as chelonia and cetacea, and of the other fresh water and 

 land vertebrates is similar in its osmotic pressure to that of the marine 

 teleosts. 



In view of the fact that the elasmobranchs constitute the highest group 

 to possess blood and other body fluids with an osmotic pressure near to 

 that of sea-water, il appeared to the present writer that an extensive in- 

 vestigation should be made of the effects of changes in the molecular con- 

 centration of sea-water upon the blood and other tissues of the elasmo- 

 branchs. Since a dilution, rather than a concentration, of the sea-water 

 would be the modification of the external medium to which these fishes 

 might be subjected in a state of nature, more attention was given to the 

 effect of dilutions of the external medium. 



In physical experiments of an osmotic nature, two solutions are sepa- 

 rated by a membrane and the qualitative relations of the process by which 

 the fluids pass through the membrane is studied. In the present investi- 

 gation, the membrane with which we have to do is possibly one or any 

 combination of three living structures which separate the living substance 

 of the body from the sea-water. These three structures are : a, the skin 

 of the body ; h, the mucous membrane of the enteric canal ; c, the mem- 

 brane of the gills. In the following pages, these will be termed the limit- 

 ing membranes of the body. Outside of these membranes is the sea- 



