ACTINIAN BEHAVIOR 207 



contracted Metridium a beam of reflected sunlight is thrown, the 

 animal will after a minute or so almost invariably shorten its 

 column completely and contract its oral disc, thus assuming the 

 condition of complete retraction. This state is commonly met 

 with in natm-e as a result of direct exposure to sunKght. It 

 occurs in situations where the sea-anemones are subjected dur- 

 ing a part of the day to shadow and during the rest to full sun- 

 light. Under the latter circumstances they are almost invariably 

 fully retracted; under the former they are more or less expanded. 



When a fully expanded Metridium in running water in the 

 dark is illuminated either from the side and Irom above by a 

 16 candlepower electric light at a distance of half a meter the 

 animal will shorten considerably but, as a rule, not cover the 

 oral disc. This was occasionally induced by very strong arti- 

 ficial illumination, but it is a reaction by no means easily called 

 forth. It was however often enough met with to warrant the 

 conclusion that so far as light is concerned Metridium will 

 undergo complete contraction of both column and oral disc only 

 in the very brightest illumination ; that in weaker light it shortens 

 the column but does not cover the oral disc and that its fullest 

 expansion is called forth only in complete darkness. 



The effect of the temperature on actinian response has been 

 little studied. The specimens of Metridium upon which my 

 observations were made were kept in an aquarimn with running 

 seawater, the temperature of which was about 23°C. The temper- 

 ature of the outside water from which the supply for the aquarium 

 was obtained was about 21°C. (August). At such temperatures, 

 as was to be expected, the animals remained expanded when the 

 other conditions were appropriate, and normally responsive. 

 Wlien the animals were supplied with running seawater that 

 had been artificially cooled to about 8°C, they remained fully 

 expanded in the dark and would shorten in the light. They 

 responded to a mechanical stimulus by contraction, and in other 

 respects they reacted as they did under more usual temperatures. 



If sea-anemones in seawater at 23°C. are -flooded with water 

 at 35°C. even though they are kept in the dark, they invariably 

 contract completely. This response is in agreement with what 



