CHAPTER XII 



OTHER COMPLEX RESPONSES AND THE 



NERVE-NET 



As the locomotor activities of most actuiians are ex- 

 tremely limited, the chief protective response of these 

 animals is general retraction whereby they are reduced 

 greatly in bulk, their more delicate parts are brought 

 under cover, and they shrink close to the substratum to 

 which they are attached. In many instances, in fact, re- 

 traction brings about a withdrawal of the body of the ac- 

 tinian into deep, rocky recesses and the like whereby very 

 efficient protection is secured. The reverse process, ex- 

 pansion, is one which involves an enlargement and pro- 

 trusion of the body as a whole and the opening of its 

 folded surfaces and apertures in such a way that the 

 operations of feeding, respiration, and so forth, may be 

 resumed. 



Retraction and expansion are relatively simple 

 processes. Retraction in its initial phases is chiefly the 

 result of the action of the mesenteric muscles, the longi- 

 tudinal muscles of the non-directive mesenteries depress- 

 ing the oral disc, those of the directives serving chiefly to 

 fold the siphonoglyphs, and the parietal muscles 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 digestive cavity of the actinian. 



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