ZOOLOGICAL SOCIETY BULLETIN 



1091 



variation as tar as the content of dissolved 

 salts is concerned, and tiierefore of their osmot- 

 ic pressures. What is the relation of all the 

 various groups of aquatic animals to the exter- 

 nal medium? As far as we can say the osmotic 

 pressure in the cells of an animal is jjrobably 

 the same as that of the blood, which can be 

 easily measured. Now, aquatic animals could 

 adapt themselves to the osmotic pressure of the 

 water in various ways. For example, they 

 could construct membranes at the surface of 

 the body in such a way that these membranes 

 would be absolutely impermeable to the external 

 medium. In the second place, all parts in con- 

 tact with the water could be freely permeable 

 so that the interior of the cells would be prac- 

 tically living matter or protoplasm permeated 

 with the water and with its salts in the same 

 proportion as tiiey exist in the water. Or, 

 thirdly, some of the parts in contact with the 

 water could be freelj' permeable and other 

 parts less so. 



Undoubtedly the first animals were formed 

 in the sea and the same salts were present in 

 this primeval living organism as are found in the 

 sea and in the same proportions. Even in the 

 jelly fishes, which are somewhat complex ani- 

 mals, this is true, and tliey are in perfect osmotic 

 equilibrium with the sea. Later arose animals 

 like the crustaceans with their hard external 

 parts. They have certain structures which pro- 

 tect them from the sea without, yet even here 

 also, on account of the permeability of other 

 parts, the blood is in osmotic equilibrium with 

 the sea water. While in higher forms, like the 

 mollusks and the crustaceans, the animal is not 

 as freely open to the sea as is the jellj' fish, 

 nevertheless the blood has the same osmotic 

 pressure as that of the sea water. For example, 

 if we studied these animals from the Mediter- 

 ranean, we would find that the osmotic pressure 

 of the blood is like that of the water from that 

 sea. In the more dilute waters of the Baltic, 

 the blood has the same concentration as that of 

 the water outside. Moreover it has been shown 

 experimentally that the osmotic pressure of the 

 blood changes according to the external medium. 



Now, of course, there are limits to which this 

 change can take place. We might say that liv- 

 ing matter is so constructed that its activities 

 can be manifested only within a certain range 

 of external conditions. The osmotic pressure of 

 the human blood is quite constant at about 

 seven atmospheres. The blood of the marine 

 invertebrates along our shores, as we have seen, 

 is equal to about 22.4 atmospheres, and is even 

 higher than this in the case of those forms 



found in the ^Mediterranean and much lower in 

 those found in the Baltic. The range of os- 

 motic pressures which the blood of these lowly 

 forms maj' take and j'et be compatible with 

 life is large. 



If, as we have said, the blood of these forms 

 becomes modified — i. e., more dilute as the ex- 

 ternal medium is made more dilute — water must 

 get into the blood of the animal or the salts of 

 the more concentrated blood of tlie animal must 

 get out. It is probable that both changes take 

 place. But through what part of the body? 

 Three possible structures have to be examined. 

 These are the skin on the surface of the bodj', 

 the wall of tlie intestinal trnct and tiie gill 

 membranes. Observation and experiment show 

 that it is probably through the gills that the 

 exchanges usually take place — for example, 

 when an oyster is "fattened" by being placed 

 in fresh water, water enters and salts leave the 

 oyster through the gills, with the result that 

 the oyster swells up. 



Oysters sometimes grow in very brackish or 

 even fresh water. This is explained bj' the fact 

 that marine invertebrates placed in a medium 

 diifering in density from that to which they are 

 accustomed, immediately respond by losing 

 salts and taking on water to the end that soon 

 their osmotic pressure is near or the same as 

 that of this external medium. Of course, if 

 they were placed at once in fresh water they 

 would die, but it is quite possible that, by sub- 

 jecting some of these invertebrates to success- 

 ive small reductions in the density of the wa- 

 ter, we could finally get them to live in fresh 

 water. 



Biologists tell us that the sharks and skates 

 are among the lowest of the fishes. In the os- 

 motic pressure of their blood they resemble the 

 marine invertebrates. That is, it is the same 

 as that of the external medium. The blood of 

 sharks from the Mediterranean is more concen- 

 trated than that from sharks of the more dilute 

 Atlantic ocean. However, extensive experi- 

 ments have shown that, while these animals re- 

 semble their invertebrate ancestors in this 

 respect, the change is not as great as that whicli 

 occurs in invertebrates under the same condi- 

 tions. In a certain series of experiments car- 

 ried on by the present author it was found that 

 the changes induced in the osmotic pressure of 

 the blood of dog-fishes were about one-fourth 

 of the magnitude of the changes made in the 

 external medium. A shark would, of course, 

 be killed in fresh water.* I found that the 



*It must not be forgotten that one species of shark 

 (Carcharins nicarayuensis) occurs in Lake Nicaragua 

 in absolutely fresh water. R. C O 



