EFFP;CTS UPON FISHEt^ OF CHANGES IN SALINITY OF WATER. 105 



im^ to tlic fish. These conditions seem impossible to state in advance for a given 

 case. In general we may say that: 



(1) Measurable changes in weight result only from considerable changes in the 

 density of the surrounding water, but — 



(2) Not all such changes of density sutHcc to produce changes of weight, even 

 when the fish is transferred to a medium which is known to be strongly hypertonic 

 or hypotonic to its own body fluids. 



(3) Changes in the salinity of the water may or may not result in changes in the 

 salt content of the body. 



(4.) Changes in the body salt content may or may not be accompanied l)y 

 changes in weight. 



(a) Neither the changes in weight nor in salt content are at all pi-oportional to 

 the changes in the density of the external medium. 



It would appear that there is normally a tendency on the part of the fish to 

 resist osmotic changes and to maintain the fiuLds of the body at a definite degi'ee of 

 concentration. Under various conditions, however, this resistance is overcome and 

 ■ a certain degree of permeability is established. This is generally a differential per- 

 meability, resulting in osmosis and consequent changes of weight. In such cases, 

 however, the membranes are not strictly semipermeable, but transmit salts in some 

 measure. Indeed, it would seem that at times the permeabilit}' is indiscriminate, in 

 which case the salts may diffuse freely, but no changes in weight occur. These vari- 

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

 normal resisting power of the fish reasserts itself and no further alteration occurs 

 so long as the medium is constant. Complete osmotic equilibrium between the fish 

 and the water is probaf)ly never attained except in waters having roughly a medium 

 degree of salinity. The osmotic pressure of the "internal medium" fiuctuates 

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



The foregoing conclusions are intended to apply onl}' to normal fishes. It seems 

 certain that the enfeebleraent 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 can not 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 efl'ectof these changes. Death is accompanied (perhaps in some 

 cases caused) by a giving way in the power to resist an abnormal degree of osmotic 

 exchange. The bod\- becomes water-soaked (if in fresh water), or dehydrated (if in 

 salt). The difference between the more hardv and the more delicate species in this 

 regard seems to lie partly in the resisting power of the limiting membranes (chiefly 

 those of the gills); partly, also, in internal difl'erences, such as composition of blood, 

 etc., which determine whether a given InHux or efflux of water or salts shall prove 

 fatal. 



The actual cause of death following a change in the salinity <if tlie water seexns 

 to differ in difi'erent cases. With those fishes which succumb rapidly with but a 

 slight change of weight (e. g., scup, experiments 69-71), it is unlikely that any 

 appreciable alteration occurs in the tissues at large. Such changes are probably con- 

 fined to the blood, perhaps, as Bert held, to that in the gill capillaries. In those 

 cases, on the contrary, where the fatal ett'ects are not manifested for some days, it 



