154 THE ELEMENTARY NERVOUS SYSTEM 



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 nature as a result of direct 

 exposure to sunlight. It occurs in situations where the 

 sea-anemones are subjected during a part of the day to 

 shadow and during the rest to full sunlight. Under the 

 latter circumstances they are almost invariably fully re- 

 tracted ; 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 or from 

 above by a 16-candlepower electric light at a distance of 

 half a meter the animal mil shorten considerably but, as 

 a rule, will not cover its oral disc. This is occasionally 

 induced by very strong artificial illumination, but it is a 

 reaction by no means easily called forth. It is, however, 

 often enough met with to warrant the conclusion that so 

 far as light is concerned Metridium mil undergo com- 

 plete 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 oval disc and 

 that its fullest expansion is called forth only in com- 

 plete darkness. 



The effect of temperature on actinian response has 

 been little studied. Specimens of Metridium upon which 

 observations were made were kept in an aquarium with 

 running seawater, the temperature of which was about 

 23 degrees centigrade. The temperature of the outside 

 water from which the supply for the aquarium was ob- 

 tained was about 21 degrees centigrade (August). At 

 such temperatures, as was to be expected, the animals re- 

 mained expanded when the other conditions were appro- 

 priate, and normally responsive. When the animals were 



