104 HULLETIN OF THE BUREAU OF FISHERIES. 



erally held (Fredericq, Atwater", Quinton, Garrey) that the bounding membranes, 

 or some of them, are permeable both to water and salts. Botazzi and Enriques, on 

 the other hand, from experiments upon the excised gat of Aplysla, conclude that, in 

 a normal condition, this and presumably the other limiting surfaces of the body are 

 only semipermeable. This condition, of course, would be sufficient to insure an 

 osmotic equilibrium between the organism and its environment. Equivalence in the 

 proportions of the various saline ingredients is maintained, according to these writers, 

 on the one hand through the process of (nutritive) absorption, occurring chiefly in 

 the ducts of the digestive gland; on the other through the organs of excretion. 



For the elasmobranchs, a permeability to water seems to follow from th(> facts 

 above stated. An unlimited permeability to salts must, on the other hand, l)e 

 excluded, if, as seems proved, the salt content of the blood is so far below that of sea 

 water. Whether the (gill?) membranes are in any degree permeable to salts has not 

 been determined experimentally. 



In the case of teleosts, it does not seem to have been generally appreciated that 

 there is a certain correlation between the inner and outer fluids, both as regards 

 osmotic pressure and salt content; and certain authors have been free to state that 

 the membranes of teleost fishes form an efl^ective barrier against osmotic changes. 

 Fredericq (as quoted above, p. 100) makes this a.ssertion broadly; while Garrey says 

 oi I^undidm JieterocUtim: "The integument and gills are therefore impermeable." 

 Garrey is cautious enough, however, not to postulate an absolute impermealniity 

 either for Fundulux or for teleosts in general. 



SUMMARY AND CONCLUSIONS. 



The more important results of the foregoing experiments may be very briefly 

 summarized as follows: 



(1) Certain brackish and salt-water tishes were unable to survive even a gradual 

 transfer to pure fresh water, though enduring an abrupt transfer to water of a very 

 low degree of salinit}*. Thus fresh water, as such, proved fatal to these tishes, the 

 degree. of abruptness of the change being of .secondary importance. 



(2) Considerable changes of weight were found to result, in many cases, from 

 changes in the salinity (hence the osmotic pressure) of the surrounding medium. 



(3) Considerable changes in the .salt (chlorine) content of the body were likewise 

 found to result, in many cases, from changes in the salinity of the water. 



(4) ('areful control experiments excluded the possibility that the water or salts 

 entered or passed from the body through the alimentary canal, leaving as the only 

 probable alternative an osmotic exchange through one or more of the limitini: 

 membranes. 



(5) In certain tishes, at least, it was found that the membranes chiefly concerned 

 in such exchanges were those of the gills. 



Accordingly, we can not conclude from the absence of osmotic equilibrium 

 between the fish and its environment that no osmotic interchanges normalh- occur. 

 On the contrary, abundant experiments seem to prove that both water and salts may 

 under certain conditions be transmitted in either direction without any harm result- 



<i For oysters, op. cit., p. 814. 



