COMPLEX RESPONSES 161 



of Metridium, especially of its oral disc. This form of 

 reaction has already been observed in Actinia by Pieron 

 (1906 b, 1908 d), who, however, points out that certain 

 forms of mechanical agitation also induce retraction 

 (Bohn, 1907 a). 



Since the expansion of the oral disc is dependent 

 chiefly upon the relaxation of the sphincter muscle, it 

 seems probable, as already pointed out (Parker, 1916 a), 

 that the mechanical stimulus of the moving water in one 

 way or another has a very specific effect on this muscle. 

 The condition of relaxation thus induced is apparently 

 exactly like that seen in such sponges as Stylotella, where 

 the oscular sphincter remains relaxed in running water 

 but contracts when the current ceases (Parker, 1910 a). 



The foregoing account shows quite clearly that the ex- 

 pansion and retraction of such a sea-anemone as Metrid- 

 ium is dependent upon a variety of factors. Light and 

 high temperature, especially when suddenly applied, pro- 

 duce retraction ; food and water currents, expansion ; the 

 oxygen supply, in Metridium at least, seems to have very 

 little, if any, direct influence on retraction and expansion. 



These operations in sea-anemones have been regarded 

 by some investigators, notably Bohn and Pieron, as oc- 

 curring in rhythmic fashion, and two types of rhythm 

 have been distinguished; a tidal rhythm and a daily or 

 nychthemeral rhythm. According to Bohn (1906 b, 1909, 

 1910 b) Actinia equina retracts when it is exposed to air 

 by the falling tide and expands when it is again covered 

 by water, and this rhythm may be retained for from 3 

 to 8 days in an aquarium though the animals under such 

 conditions are always under water. Pieron (1908 c), on 

 the contrary, questions the presence of a pronounced tidal 

 rhythm in Actinia equina. 

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