COMPLEX BESPONSES 151 



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 digestive 

 spaces with seawater through the ciliary currents 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 retraction and expansion allow retraction to be accom- 

 plished much more quickly than expansion. This rela- 

 tion has all the appearance 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 important 

 for the animal to expand quickly. 



The conditions under which a Metridium remains fully 

 expanded are by no means simple, but include an aggre- 

 gate of factors. In the laboratory the fullest expansion 

 was obtained when the animals were in well-oxygenated, 

 cool, running seawater in the dark. Under such circum- 

 stances this sea-aneinone will extend itself to as much as 

 six times the diameter of its column, and hold its oral 

 disc fully opened. In nature a greater degree of expan- 

 sion than that seen in the laboratory under the circum- 

 stances just stated apparently does not occur. This max- 

 imum degree of expansion under natural circumstances 

 has often been observed in sea-anemones in pools during 

 the night or even during the day in dark situations such as 

 under bridges and so forth. The elements that contrib- 

 ute to this extreme expansion are certainly diverse. Of 

 these light, temperature, food, oxygen supply, and water 

 currents are of significance. 



