206 G. H. PARKER 



bright illumination has also been recorded for a number of 

 actinians among which are the following: Edwardsia^ (de Quatre- 

 fages, '42, p. 76; Fischer, '88, p. 23), Cerianthus^ (Haime, '54, 

 p. 348; Nagel, '94, p. 545; Hess, '13, p. 438), Phillia (Gosse, '60, 

 p. 350), various species of Sagartia (Gosse, '60, pp. 81, 111; 

 Fleure and Walton, '07, p. 217; Hargitt, '07, p. 275; Pieron, 

 ('08 c, p. 1021), Paractis (Jourdan, '79, p. 28), Cladactis (Hert- 

 wig, '79-80, p. 56), Aiptasia (Jennings, '05, p. 459), Tealia (Fleure 

 and Walton, '07, p. 217), Eloactis (Hargitt, '07, p. 275), Ceractis 

 (Schmid, '11, p. 538), and Bunodes (Hess, '13, p. 438). Although 

 closure in the presence of light is the ordinary form of response 

 for most actinians, there seems to be good evidence that a lew 

 react in the opposite way. Actinia equina according to Bohn 

 ('08 a) is expanded in the daytime and retracted at night and 

 the same is true of Cribrina zanthogrammica as observed by 

 Gee ('13, p. 309), who also adds that a closed Cribrina in the 

 dark will expand under the influence of a 32 candlepower light. 

 Both Actinia and Cribrina contain symbiotic algae in their tis- 

 sues and it is easy to imagine that their expansion in daylight 

 may be an advantage so far as photosynthesis is concerned, 

 but whether this expansion is a reversal of the usual form of 

 actinian response to light or is due to the effects of some such 

 substance as oxygen w^hich may be given off by symbiotic alga 

 in the light is not known. It thus appears that aside from a 

 few indifferent actinians and a few that open in the light, the 

 majority respond to the stimulus by retraction. In this respect, 

 as already intimated, Metridium is not exceptional. 



If a fully expanded Metridium in the dark is suddenly exposed 

 to diffuse daylight, it will shorten its column to one-third or 

 one-fourth its former length and with its oral disc fully expanded 

 remain in this state more or less continuously. The shortened 

 state produced in Metridium by general illumination represents 

 the ordinary condition in which many of these sea-anemones 

 are found in nature during the daytime. If on such a partly 



* These instances, Edvvardsia and Cerianthus, are often attributed to Bronn 

 ('60, p. 23) who apparently simply repeated the statements made by de Quatre- 

 fages and by Haime without giving references. 



