EFFECTS rPON FISHES OF CHANGES IN SALINITY OF WATER. \> i 



of t(>le()st tislies is prot'ouiully iiitcrcstinu- in v'u-w of the great physiological impor- 

 tance to certain species of ev(>n sucii a small iimportion of salt in the water. It is 

 interesting to recall in this connection that the •• l.Oni" water did not appear to lie 

 quite .saline enough to support the life of /•'. ///<'/.///.v indetiniteh-. (Experiment 

 31 bis.) 



The records of other investigators (to be discussed later) show that the osmotic 

 pressure of the blood of salt-water teleosts is somewhat higher than that of fresh- 

 water ones, though this fact has been almost lost sight of in the zeal to prove that 

 the internal medium is not isotonic with the external and that its osmotic pressure is 

 relatively constant. 



It may be objected once more at this point that all this discussion of the relative 

 salt content of fishes inhabiting fresh and salt water, together with the resulting 

 difl'erences in osmotic pressure, is quite irrelevant, since the difterences found may 

 depend upon the degree of salinity of the food eaten, and not directly upon the 

 salinity of the water which bathes the body. This objection can only be met b}' 

 reference to the changes which certain species were found to undergo in the course 

 of a single day. no food being taken. It is theoretically possible, even in these 

 cases, on the one hand that salt water was swallowed and the salts absorbed, on the 

 the other that salts should have left the l)ody l)y way of excretion. The former 

 possibility I regard as sufficii^ntly met by the analyses of the contents of the alimen- 

 tary canals (indeed, of the entire viscera) described at)ove. The second possibility 

 is not entirely excluded, though it seems unlikely a priori that the decrease in 

 salinity which occurs in fresh ^^•nter should lie due to a cause quite distinct from that 

 resiwnsililc for the increase in salt water. 



THE PART PLAYED BY THE GILLS IN OSMOTIC EXCHANGES. 



The thin membranes eovering the gill tilanumts are especially adapted to facilittiting 

 exchange between the gases conbiined in the water and those contained in tiie blood. 

 It would therefore be natural to look here for one jiath of diflusion for water and 

 salts as well. The greater part of the body .surface, on the contrary, is in most tele- 

 osts covered with a layer of scales, which would seem to present a barrier to any 

 great amount of osmotic exchange between the tissues and the water which bathes 

 the body. The lining of the alimentary canal is of cour.se readily permeable to f.uids 

 and to various substances in solution, but this, it is needless to >ay, is not freely 

 exposed to the surrounding medium. Water or .salts, in order to be thus absorbed, 

 must either be swallowed or force their waj' in through the anus. Such an entry of 

 fluids would not. however, be consistent with the decrease in the weight of a fish 

 which fre(jaently follows its transfer to a stronger salt solution. Yet it seems to be 

 proved by some of the foregoing experiments that salts are in some way taken into 

 the l)ody after such a transfer. 



The part played l)y the gills in this process I have demonstrated by a series of coni- 

 parati\ely simple experiments. A piec-e of apparatus was devised, by the aid of 

 which it was possible to pass .salt water through the gills, while the remainder of the 

 body w as 1)athed in fresh water, or vice versa. A wooden frame was made (fig. 1), 

 consisting of a rectangular piece of planed board, near one end of which a shelf was 



