SCOTT, STUDY OF CHANGES IN MUSTELUS CANIS 63 



Discussion 



The changes in the osmotic pressure of the blood of Mustelus canis 

 after immersion in diluted and concentrated solutions of sea-water have 

 been shown in the preceding pages. It has been shown that this dog-fish 

 differs from the marine invertebrates in that the osmotic pressure of its 

 blood does not become equal to that of the surrounding medium, when 

 this differs in its concentration from the sea-water. Moreover, when a 

 considerable change has been produced in the osmotic pressure of the 

 blood of Mustehis by immersion in a modified solution of sea- water, the 

 normal osmotic pressure of the blood is not regained on the return to 

 sea-water. In this respect, also, the elasmobranch differs from the marine 

 invertebrate. 



The elasmobranchs differ also from the marine teleosts, the osmotic 

 pressure of whose blood is between one-third and one-half that of the ex- 

 ternal medium, and whose blood maintains a constant osmotic pressure 

 despite marked changes in the osmotic pressure of the external medium. 

 The elasmobranchs cannot in truth be termed either poikilosmotic or 

 homoiosmotic animals. It has been shown that the freezing point of the 

 blood rises about 0.40° C. on immersion in fresh water until near death, 

 and 0.18° C. on immersion in sea-water diluted with an equal volume of 

 fresh water, having a freezing point of — 1.00°. In both cases the 

 change in the osmotic pressure of the blood is about one-fourth of the 

 change in the external medium. In concentrated sea-water having a 

 freezing point of about — 2.60°, or about 0.80° below that of sea-water, 

 the osmotic pressure of the blood increases about one-fourth as much as 

 the change in the external medium. In these three cases, the change in 

 the osmotic pressure of the blood though not equal to the change in the 

 osmotic pressure of the external medium, yet bears a rather constant 

 ratio to the external change. 



This appears to be the index of a certain degree of independence on 

 the part of the animal of the osmotic pressure of the external medium. 

 From this point of view, it would be correct to consider the elasmo- 

 branchs as occupying a position midway between the marine teleosts and 

 the marine invertebrates as to the relations of the osmotic pressure of 

 the internal fluids of the body to the external fluids in which these forms 

 live. 



The problem, however, calls for further analysis. In the first place, it 

 is necessary to know what parts of the elasmobranch body are concerned 

 in the osmotic changes which cause the death of the animal in the modi- 

 fied external medium. Evidence has been presented showing that there 



