specific Gravity and Luminescence in Noctiluca, etc. 239 



salts of sea-water — i. e., impermeability to the salts and permea- 

 bility to water. Out in the open bay, although most of the ani- 

 mals are at the surface on a calm night, many can be seen through a 

 glass-bottom bucket well below the surface. Moreover, on windy 

 days, they are not found at the surface, indicating that they have the 

 power of increasing their specific gravity and sinking, owing to a tem- 

 porary increase in permeability which allows the salts of sea-water 

 to pass in or water to pass out. When noctilucas die they immediately 

 sink, the permeabihty having been increased, causing an increase in 

 specific gravity. 



That these animals can also decrease their specific gravity is shown 

 by the following facts. In all dilutions of sea-water with fresh-water 

 down to 6 sea-water to 4 fresh-water, they rise immediately to the top, 

 becoming and remaining somewhat swollen for more than 10 days. 

 These dilutions are still of greater density than the animals. In a 

 mixture of 6 sea-water to 4 fresh-water they remain distributed through 

 the water for about 15 minutes, and then rise to the top; this mixture 

 is of approximately the same density as the noctilucas. In a mixture 

 of half sea-water and half fresh-water and also in a mixture of 4 sea- 

 water to 6 fresh-water, the animals sink at first, their salt-content being 

 now greater than that of the surrounding medium, but during the next 

 hour they gradually swell and rise to the top. The process is wholly 

 independent of movement of the tentacle. If water were merely 

 absorbed by the animals until their concentration was the same as 

 that of the surrounding medium, the animals would remain suspended 

 through the Uquid; but since they eventually float, they must keep on 

 absorbing pure water until their salt-content and hence their specific 

 gravity is again less than that of the new medium (5 sea-water to 5 

 fresh-water or 4 sea-water to 6 fresh- water) , thus reestablishing their 

 normal relation to their surrounding medium. This must involve the 

 absorption of water against the osmotic pressure of the salts of sea-water, 

 a condition contrary to physical laws. This regulatory mechanism 

 is characteristic of living cells only and is not destroyed under any 

 experimental conditions. Anesthetics, acids and alkalies, KCN, and 

 pure salts of sea-water do not affect the regulation, except when they 

 cause irreversible changes and death of the cells. It is quite probable 

 that this peculiar type of osmo-regulation is not characteristic of noc- 

 tilucas alone, but may be possessed by other marine plankton forms 

 without gas-chambers which float and sink as occasion demands. 



The water absorbed by noctilucas in mixtures of 5 sea-water to 

 5 fresh-water and 4 sea-water to 6 fresh-water accumulates in large 

 vacuoles formed by strands of protoplasm pulling away from the cell- 

 wall and forming membranes around the accumulated Uquid. I have 

 observed these vacuoles bursting or being expelled bodily from the cell. 

 In these same concentrations of sea-water an interesting protoplasmic 



