COMPLEX RESPONSES 153 



Sagartia (G-osse, 1860; Fleure and Walton, 1907; Hargitt, 

 1907; Pieron, 1908 <?), Paractis (Jourdan, 1879), Cladac- 

 tis (Hertwig, 1879-1880), Aiptasia (Jennings, 1905), 

 Tealia (Fleure and Walton, 1907), Eloactis (Hargitt, 

 1907), Ceractis (Schmid, 1911), and Bunodes (Hess, 

 1913). Although closure in the presence of light is the 

 ordinary form of response for most actinians there seems 

 to be good evidence that a few react in the opposite way. 

 Actinia equina, according to Bohn (1908 a), is expanded 

 in the daytime and retracted at night, and the same is 

 true of Cribrina zanthogrammica as observed by Gee 

 (1913), 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 

 tissues, 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 which may 

 be given off by the symbiotic algae 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, Metridiwn 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 con- 

 tinuously. The shortened state produced in Metridium 

 by general illumination represents the ordinary condi- 

 tion in which many of these sea-anemones are found in 

 nature during the daytime. If on such a partly con- 

 tracted Metridium a beam of reflected sunlight is thrown, 



