1 05)0 



ZOOLOGICAL SOCIETY BILLKTIN 



THE RELATION OI" AQUATIC ANIMALS 



TO THE WATER IN WHICH 



THEY LIVE. 



By Prof. G. G. Scott, 



College of the City of New York. 



"I marvel liow tliu fislic-i live in the sea." — Shakesperire. 



DID you ever wonder why fishes do not get 

 drowned.'' Or how they can breathe in 

 the water, whicli, after all, is reallj- the 

 same question as the other? Or how it is that 

 they do not keep drinking w-ater all the time? 

 Or why a salt -water fish dies in fresh water 

 and vice versa ? I am not going to answer 

 these questions at all, but instead will discuss 

 a bit of dry pliysics and chemistry as a proper 

 setting for what ajiears to me to be a fascin- 

 ating story. 



W'lien two gases are confined in the same ves- 

 sel, they mix or diffuse into one another. Thus 

 light hydrogen gas will diffuse downward into 

 heavy carbon dioxide and the latter will diffuse 

 up into the ligiit iiydrogen. Tiie force is due 

 to the difference in the partial pressures of 

 the two gases. A similar behavior is observed 

 when two solutions of different substances are 

 brought into contact. Diffusion between the 

 two continues until the concentration through- 

 out the vessel is everywhere the same. The 

 driving force by which this result is obtained 

 in the case of solutions is called usmoilc pres- 

 sure and can be measured. 



One of the most characteristic features of 

 living things is that substances must pass to 

 and fro between the external world and the 

 living cell in order that that cell may obtain 

 energy for its work and get rid of its waste 

 substances. The cell wall is a membrane sep- 

 arating the living substance within from the 

 material outside, and tiirough this there must 

 be an incurrent and excurrent stream of m.i- 

 terials. 



In the lower aquatic .ininials the entire body 

 is surrounded by water and the substances 

 which the cell needs are in solution. Although 

 the simplest forms may take food particles di- 

 rectly into the cell, yet it soon occurs, in the 

 evolution of more complex forms, that the cell 

 can absorb its food in liquid form only: and if 

 the food at hand is not in liquid form tlic t-cll 

 secretes a solvent which changes tlie solid food 

 into the form of a solution. In liigher animals 

 with many organs composed of a multitude of 

 cells, the blood carries the food in soluble form 

 to all the various cells of the body. But these 

 substances had first to get into the blood, and 



in every step they must pass ttirough mem- 

 branes. So we observe that there is a constant 

 and repeated passage of materials in and out 

 through membranes formed of cell walls. 



We said above that when two solutions of 

 different concentration, or of different osmotic 

 pressure, were confined in tlie same vessel, the 

 molecules undergo a change of position until 

 finally a condition of equilibrium is attained. 

 If we stretch a permeable membrane across 

 this vessel, dividing it into two compartments, 

 and place a solution of low osmotic pressure 

 in one and a solution of great osmotic pressure 

 in the other, a condition will soon obtain in 

 which the osmotic pressure will be the same 

 on both sides of the membrane. This process 

 probably plays a great part in the life of the 

 cells of the body. The degree of permeability 

 of cells to various substances has been the sub- 

 ject of much investigation. 



The waters of the ocean contain solutions of 

 salts of various kinds, one of the principal 

 salts, as every one knows, being sodium chlor- 

 ide or common salt. Now the osmotic pressure 

 of ocean water is a little over 22.1- atmospheres, 

 the pressure of one atmosphere being 15 lbs. 

 to the square inch, so the driving force of the 

 molecules in such a solution is very large. The 

 Mediterranean Sea is saltier than the ocean, 

 hence the osmotic pressure, about 28 atmos- 

 ))heres. is greater even than that of the ocean. 

 The Black and Baltic Seas receive a great 

 deal of fresh water and hence are less salty 

 than the ocean, and so tiie osmotic pressure of 

 these waters is less than that of the open ocean. 

 The water of New York Harbor is very dilute, 

 due to the great volume of water flowing in 

 from the Hudson River, and the osmotic pres- 

 sure is only a little over one half that of ocean 

 water. Distilled or pure water of course con- 

 tains no salts in solution, hence its osmotic pres- 

 sure is zero. The osmotic pressure of spring 

 water or river water is very small, being les> 

 than half an atmosphere. When pure distilled 

 water is taken in great quantities into an empty 

 stomach, we have a solution of no osmotic pres- 

 sure bathing the walls of cells, whose osmotic 

 l>ressure, in the human being, is about 7 at- 

 mospheres. The great difference in osmotic 

 pressures causes injuries to the cells. The small 

 amounts of salts present in our drinking waters 

 are just sufficient to counteract this. The 

 harmful effect of eating snow and ice is rele- 

 v.int to this action, since snow and ice are prac- 

 tically frozen distilled waters. 



Now aquatic animals inhabit all the waters 

 of the earth, which present a great degree of 



