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 liquid. I have observed these vacuoles bursting or being expelled bodily from the cell. In these same concentrations of sea-water an interesting protoplasmic 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 Paramxcium, Ameba, 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 NadiLuca is put under conditions where it must rid itself of a certain amount of fresh-water, similar though lees 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 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).