238 



Papers from the Department of Marine Biology. 

 CHANGE OF OSMOTIC PRESSURE. 



Noctiluca can stand a considerable decrease in osmotic pressure with- 

 out having its luminous response affected, as indicated in table 1. We 

 may dilute the sea- water with fresh- water in anj^ amount down to a pro- 

 portion of half and half, and the animals still give a normal response — 

 i. e., they flash on stimulation. The response is not quite so bright 

 as in the control in a half-and-half concentration nor in a mixture of 

 4 sea-water to 6 fresh-water, but the animals continue giving a normal 

 response for 7 days. In the latter concentration the animals also give 

 a steady glow for the first few minutes, owing doubtless to the dying 

 condition of certain of them. If the amount of fresh water is still 

 further increased, the anunals do not flash on stimulation, but give a 

 steady glow, lasting for 8 minutes in a mixture of 3 sea- water to 7 



Table 1 . — Effect of diluting sea-water with fresh-water. 



^The constant glow at the start is without doubt due to the dying condition of some of the 

 animals. 



fresh-water and of 2 sea-water to 8 fresh- water; and for 5 minutes in 1 

 sea-water to 9 fresh-water, and for about 3 minutes in pure fresh-water. 

 When the osmotic pressure is increased by concentrating the sea-water 

 to half its volume, a steady glow is given for more than 20 minutes. 



Especially interesting results have been obtained in this series of 

 experiments with regard to specific gravity. Normally, noctilucas are 

 less dense than sea-water, so that when placed in an aquarium jar they 

 soon rise to the top, where they form a layer somewhat pinkish in 

 color. Unlike the floating siphonophores, there are no air-bubbles, 

 and unlike certain pelagic eggs, there are no large oil-drops in the 

 animals, so that their lower specific gravity must be due to the fact that 

 their salt-content is less than that of sea-water. When placed in sea- 

 water concentrated to half its volume they shrink, and when the 

 sea-water is diluted with fresh-water they swell. The plasma mem- 

 brane, therefore, shows the usual semipermeability to the balanced 



