240 Papers from the Department of Marine Biology. 



fragmentation also takes place, secondary smaller masses of protoplasm 

 being formed, sometimes isolated and sometimes connected with the 

 main mass by strands of protoplasm. The vacuoles formed by 

 Noctiluca in these mixtures of fresh-water with salt-water are perhaps 

 comparable with the contractile vacuoles of Paramoecium, Amoeba, 

 and other fresh-water forms, where its function is likewise osmo- 

 regulatory. It is true that in general in salt-water protozoa no 

 contractile vacuoles are present, since the osmotic pressure of sea- 

 water balances that of the animal and no osmoregulatory mechanism 

 is needed. When Noctiluca is put under conditions where it must rid 

 itself of a certain amount of fresh-water, similar though less perfect 

 vacuoles can be formed. 



In still more dilute mixtures of sea-water and fresh-water, the 

 animals absorb water until they burst and then shrink ; they are killed 

 and sink to the bottom. That this is an osmotic effect and not due to 

 the mere dilution of salts is shown by diluting the sea-water with iso- 



TABLE 2. Effect of m cane-sugar -\-sea-watcr. 



m cane-sugar to 



Luminous response, 

 sea-water. 



m sugar 



9 to 1 



8 to 2 



7 to 3-1 to 9 . . 



Normal response 2 hours; constant glow during last hour. 



Normal response 2 days. 



Normal response 4 days. 



Normal response 9 days (experiment discontinued). 



tonic cane-sugar, thus diminishing the salt-content but not the osmotic 

 pressure. After testing various concentrations of cane-sugar to de- 

 termine which was isotonic, it was found that the animals responded 

 normally longest in m cane-sugar, so that this was used to dilute the 

 sea-water. In m cane-sugar in distilled water the animals give 

 a normal response for 2 hours; this is accompanied during the last hour 

 by a steady glow; but when sea-water is added to the m sugar in 

 various proportions, the effect on the luminescence is only slight 

 (table 2). When 3 parts sea- water are added to 7 parts m sugar or 

 any greater proportion of sea-water is used, the animals give a normal 

 response for more than 9 days. In mixtures of 2 sea- water to 8 sugar 

 a normal response is given for 4 days, and in 1 sea-water to 9 sugar 

 a normal response is given for 2 days. The noctilucas, therefore, 

 like muscle and other cells, must be bathed by a certain minimal 

 salt-content as well as surrounded by a fluid with a certain osmotic 

 pressure. They behave in this respect like the luminous bacteria 

 investigated by E. N. Harvey (1914). 



