204 G. H. PARKER 



recesses and the like whereby very efficient protection is secured. 

 The reverse process, expansion, is one which involves an enlarge- 

 ment and protrusion of the body as a whole and the opening of 

 its folded surfaces and apertures in such a way that the opera- 

 tions of feeding, respiration, and so forth may be resumed. 



The means by which retraction and expansion are carried out 

 have already been partly described (Parker, '16). Retraction 

 in its initial phases is chiefly the result of the action of the 

 mesenteric muscles, the longitudinal muscles of the non-direc- 

 tive mesenteries depressing the oral disc, those of the directives 

 serving chiefly to fold the siphonoglyphs, and the parietal mus- 

 cles acting on the column wall. After the depression of the 

 oral disc has proceeded somewhat, the contraction of the sphinc- 

 ter muscle completes the process by bringing the oral disc under 

 cover through the puckering effect of this muscle on the column 

 wall. Incidentally the process of general retraction involves the 

 expulsion of almost all the water contained in the gastrovascular 

 cavity of the actinian. The reverse operation, expansion, is 

 dependent first of all upon the relaxation of the sphincter and 

 of the mesenteric muscles; then follows the slow filling of the 

 gastrovascular spaces with sea-water through the ciliary cur- 

 rents in the siphonoglyphs; and probably as a last step the 

 circular muscles of the column contract on the fluid contents 

 of the body whereby the oral disc is forced well up above the 

 pedal attachment. The details involved in the processes of re- 

 traction and expansion allow retraction to be accomplished much 

 more quickly than expansion. This relation has all the appear- 

 ance of an adaptation, for the quickness of a withdrawal may 

 often be the essential part of the protection given by retraction, 

 whereas there is nothing about the economy of an actinian, such 

 as feeding, respiration, and so forth, that makes it vitally impor- 

 tant for the animal to expand quickly. 



The conditions under which a Metridium remains fully ex- 

 panded are by no means simple but include an aggregate of 

 factors. In the laboratory the fullest expansion was obtained 

 when the animals were in well-oxygenated, cool, running sea- 

 water in the dark. Under such circumstances this sea-anemone 



