FISHES AND THEIR SURROUNDING MEDIUM. 305 



changes continue until a new level of stability is established, after 

 ivhich the normal resisting power of the fish reasserts itself and no 

 further alteration occurs as long as the medium is constant. Complete 

 osmotic equilibrium between the fish and the rvater is probably never 

 attained except in waters having roughly a medium degree of sa- 

 linity. The osmotic pressure of the " internal medium" fluctuates 

 within a much narrower range than that of tJie " external medium.'" 



The foregoing conclusions are intended to apply only to nor- 

 mal fishes. It seems certain that the enfeeblement of the fish 

 may result in an increased permeability of the membranes, 

 which, in turn, would doubtless result in a further enfeeblement 

 of the fish. The death of those fishes which cannot withstand 

 transfer to a medium very different from that to which they are 

 accustomed is thus probably in part a cause and in part an effect 

 of these changes. Death is accompanied (perhaps in some cases 

 caused) by a giving way in the power to resist an abnormal de- 

 gree of osmotic exchange. The body becomes water-soaked (if 

 in fresh water) or dehydrated (if in salt). The difference be- 

 tween the more hardy and the more delicate species in this re- 

 gard seems to lie partly in the resisting power of the limiting 

 membranes (chiefly those of the gills) ; partly, also, in internal 

 differences, such as composition of blood, etc., which determine 

 whether or not a given influx or efflux of water or salts shall 

 prove fatal. 



The actual cause of death following a change in the salinity of 

 the water seems to differ in different cases. With those fishes 

 which succumb rapidly with but a slight change of weight (e. g. t 

 scup), it is unlikely that any appreciable alteration occurs in the 

 tissues at large. Such changes are probably confined to the 

 blood, perhaps, as Bert ! held, to that in the gill capillaries, in 

 which case death may result from asphyxiation (Bert, 2 Mosso 3 ). 

 In those cases, on the contrary, where the fatal effects are not 

 manifested for some days, it seems likely that the manner of 

 death is different. In the case of P. heteroclitns, it was found in 

 most instances that the eudosmotic flow of water had ceased, 

 and that a secondary decrease in weight had ensued, within one 



1 Comptes Rendus de V Academie des Sciences, 1871, 1883. 2 Of. cit. 



3 Biologisches Centralblatt, 1890. 



