§2] EFFECT ON STRUCTURE AND FUNCTIONS 79 



undergoes periodic enlargement and discharge as in the case of 

 the contractile vacuole. KoFOiD ('95, p. 104) found that 

 when eggs of Physa and Amnicola were placed in a 0.19% or 

 0.10% NaCl solution, the contents of the central cavity, once 

 extruded, were not so quickly restored as in the control eggs, 

 and that the maximum volume attained by the vacuole in the 

 salt solution was less than that attained in fresh water. For 

 example, " the cavity of the control eggs attained a diameter of 

 5 to 7 units, while that of the eggs in the salt solution was 

 only 3 to 4 at the time of elimination. There were, however, 

 a very few cases in which the cavity reached a diameter of 5 

 units." Of interest is the additional fact that marine Gastro- 

 poda do not seem to have a " cleavage " cavity, but that this is 

 confined to eggs developing in fresh water or moist situations. 



The effect of density upon the higher animals is very com- 

 plex, according to the observations of Bert ('71) upon the 

 gold-fish. Plunged into sea water it shows violent, unco- 

 ordinated movements ; then it becomes immobile, and rises to 

 the surface by virtue of its relatively lower specific gravity. 



The effect of fresh water upon marine organisms is equally 

 striking, as GoGOEZA* ('91) has shown. They go immediately 

 to the bottom and move with difficulty. Swimming animals 

 swim badly if at all, and small fishes have to make much exer- 

 tion to rise to the surface. The sensibility also undergoes 

 great changes. Many animals soon become lethargic. Echiuo- 

 derms and molluscs act as if anaesthetized, since they do not 

 respond as quickly as usual to external stimuli, and, finally, 

 pass into complete paralysis. The action is slower upon 

 Crustacea and fish; but here, too, fresh water acts as an an- 

 sesthetic. The respiratory movements become deep and rapid, 

 bivalves extend their branchiae, and Crustacea beat the water 

 rapidly with their appendages in order to renew the supply. 

 Animals ordinarily transparent, like medusae, become opaque ; 

 first externally, then internally. The cornea of fish becomes 

 opaque and the external slime coagulates. The tissues become 

 swollen, so that soft-bodied animals are visibly deformed — in 



* For an abstract of the work of Gogoeza, I am indebted to the kindness of 

 Mr. F. C. Waite. 



