J 092 



ZOOLOGICAI, SOCIF.TY BULLETIN 



THK. NKW UKUI.IS A(n'AKirM 



osmotic pressure of the blood of dog-fishes in 

 the New York Aquarium was distinctly lower 

 than tliat of dog-fishes from Buzzards Bay. 

 where the water is much more concentrated 

 than tile dilute harbor water in which the fish 

 live at the Aquarium. But tiie blood of these 

 Aquarium dog-fisiies would not have as low an 

 osmotic pressure as that of invertebrates living 

 in the same water. (This conclusion is drawn 

 from what we know of the behavior of the in- 

 vertebrates and not from actual observation.) 

 The point is that the sharks and shark-like 

 fishes are to a certain extent independent of the 

 osmotic pressure of the external medium, where- 

 as the invertebrates have little if any means of 

 protecting themselves. 



But what shall we say of the marine bony 

 fishes (cod, mackerel, etc.) which, in contrast 

 with the sharks and marine invertebrates, have 

 an osmotic pressure of the blood considerably 

 less than one-half of the sea water in which 

 they live? Important changes in the blood have 

 taken place, and in these fishes the gills have 

 become practically impermeable barriers to the 

 more highly concentrated outside medium. In 

 spite of tlie fact that the sea water has an os- 

 motic pressure of over twice that of the blood 

 of these fishes, the diffusion of its salts is pre- 

 vented. The parts of the body exposed to the 

 sea water of such high osmotic pressure are 

 constantly bombarded, so to speak, by the high 

 jiressure without. The scales covered with the 

 slimy membrane effectually prevent the changes 

 taking place through the skin, the wall of the 

 intestinal tract is equally eflieient, and the gill 

 membranes, througli which the gaseous ex- 

 changes must still take place between the blood 

 and the water, are so modified that they also 



act as barriers to the high salt pressures witli- 

 out. In our study of the osmotic pressures of 

 the blood we find no connecting links between 

 the conditions found in sharks and those found 

 in the bony fishes. 



It has been found that the osmotic pressure 

 of the blood of fresh w.iter bony fisiies is 

 slightly less than that of the salt water forms. 

 The fresh water streams and lakes, in all prob- 

 ability, became inhabited by forms which had 

 their original home in the sea. In this adapta- 

 tion the blood has become slightly modified 

 from the condition found in the marine bony 

 fishes. This would go to show that, whereas 

 the bony fishes are practically immune to the 

 ordinary high external pressures, yet extreme 

 changes in these would produce some effect. 

 Thus I found that the blood of the tautog living 

 in brackish water at the New York Aquarium 

 h.id an osmotic pressure slightly less than that 

 of the same fishes living in the more concen- 

 trated waters of Buzzards Bay. Furthermore, 

 if the bony fishes are alTected in no way by 

 the changes in the osmotic pressure of the ex- 

 ternal medium, then we would expect that the 

 blood of the anadromous bony fishes, which go 

 from the sea to fresh waters or back again, 

 would remain ))raetically stationary' under 

 these changed conditions. But this is not the 

 case, for, as Greene found in the case of the 

 Chinook salmon, the blood has a sliglitly lower 

 osmotic pressure in the fresii waters of the 

 spawning beds as compared witii tiiat in sea 

 water. Dakin in England found the blood of 

 the eel in fresh water slightly different from 

 that of the same species in the sea. I have 

 iu)ted a similar condition in the white perch. 

 The osmotic pressures of the blood of these 



