SCOTT, STUDY OF CHAyGES IN MVSTELU8 CAN IS 29 



+0.50°, +0.31°, +0.45°, +0.48°, the average being +0.41°. In the 

 experiments shown in Table X the maximal rises were: +0.48°, +455°, 

 +0.31°, +0.34°, +0.45°, the average being +0.407°. There is no 

 marked difference in the changes in the two groups. 



After about thirty minutes' immersion of the entire body in fresh 

 water, the average maximum rise in the freezing point of the blood of 

 three dog-fishes was +0.23°. The rise in two other specimens whose gills 

 alone were bathed with fresh water for about the same time was +.247°, 

 an unimportant difference. After the treatment with fresh water, all the 

 specimens were transferred back to sea-water. The freezing point of the 

 blood fell in each case. Moreover, the reverse change in the case of those 

 fishes whose gills alone were exposed to the outside medium was 0.118°, 

 while for those entirely immersed in fresh water the fall was 0.12°. 



The average fall in the A of the blood of three specimens of Mustelus 

 which were entirely immersed for forty minutes in a concentrated solu- 

 tion of sea-water having a A of 2.60° w^as 0.19°. The fall in A of one 

 specimen with ligated oesophagus, the body surface out of water and the 

 gills bathed with a hypertonic solution having a A of 3.15° for seventy- 

 five minutes was 0.23°. The striking fact here is that the fall was no less 

 in this specimen than in the others, in which all three structures were 

 exposed to the experimental medium. The greater changes in the second 

 case was due to the greater density of the external medium and the longer 

 time of immersion. 



In all the experiments described here, it will be noted that as great a 

 change takes place in the osmotic pressure of the blood wiien the gills are 

 the principal structures exposed to the experimental medium as when the 

 gills, body surface and intestinal tract together are exposed. It is ac- 

 knowledged that, with the gills, the outside membranes of the head and 

 the lining membranes of the buccal cavity were exposed to the media. 

 The surface membranes of the head are, however, excluded, inasmuch as 

 the non-immersion of a much greater portion of the body surface made 

 no difference in the results. It is extremely improbable that the lining 

 membrane of the buccal cavity takes any part in the above changes, be- 

 cause of its histological structure and blood supply in comparison with 

 the gill membranes. There is but one conclusion to be drawn. Osmotic 

 changes which take place in the blood of Mustelus canis when the organ- 

 ism is surrounded by solutions more dilute or more concentrated than 

 sea-water take place through the gill membranes. 



