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Reactions to Salinity

sea-water and water


Experiments to determine whether Tropiometra is sensitive to changes in the salinity of the water in which it lives gave very inter esting results. Two individuals were placed in an aquarium con taining 5 liters of ordinary sea-water and two specimens of similar size and appearance were placed in another aquarium containing 5 liters of 90 per cent sea-water (i. e., 4.5 liters of sea-water plus 500 c.c. of rain-water). After 12 hours all were normal and one could not tell from the responses to mechanical stimuli which were in the diluted sea-water. Similar experiments were tried with water only 80 per cent and 75 per cent sea-water. Such water extracted color from the comatulids and after 12 hours was distinctly yellow, yet the tropiometras survived and responded to the transfer to normal sea water by distinct arm-movements.

Experiments were then made to see how concentrated a sea-water could be survived. Two specimens, one dark-colored and one yellow

and purple, survived with no apparent injury 12 hours in only 2 liters of water 10 per cent more saline than normal. The brightly colored one was more active after the experiment than the other, which was not what I had expected in view of the greater evaporation in the natural habitat of the dark specimen. A similar experiment with water 20 per cent more saline than normal was tried. A brightly colored individual was still alive after 3 hours; it was then transferred to normal sea-water and lived over night. A dark-colored specimen was still alive after 44 hours, but although it was then transferred to normal sea-water, it failed to revive. These experiments show a sur prising indifference on the part of Tropiometra to the salinity of the water in which it lives.