EFFECTOR SYSTEMS OF ACTINIANS 477 



teries and are the result of the contraction of their transverse 

 muscles which, are those concerned with the opening of the 

 oesophagus. 



If fragments of fish meat are put on the lips of a fully expanded 

 Metridium, they are carried into the animal by ciliary action 

 through an oesophagus which opens widely to receive them 

 and during this operation the column of the animal is marked 

 by the same vertical grooves that were seen in the experiment 

 with acidulated seawater. As the pieces of food pass into the 

 gastro vascular cavity the grooves fade out. It is clear, then, 

 that the transverse muscles of the complete mesenteries are 

 concerned with the expansion of the oesophagus for the recep- 

 tion of food. 



If a piece of fish meat is placed upon the tentacles of an ex- 

 panded Metridium, these organs become characteristically stimu- 

 lated and if the meat is removed before it is brought by the 

 tentacles to the animal's Hps, the oesophagus will still open, 

 accompanied by the formation of vertical grooves on the column. 

 This response could not be elicited by the application of weak 

 acid to the tentacles. Under such circumstances a withdrawal 

 of the oral disc took place. Thus it appears that not every form 

 of chemical stimulus that can be applied to the tentacles is 

 followed by an opening of the oesophagus. To appropriate 

 stimuli, however, the tentacles and lips may act as receptors 

 for the opening of this tube. I have found no other parts of 

 the body of Metridium from which I could elicit this oesophageal 

 response. 



The organ that would be regarded naturally as the opponent 

 of the transverse muscles of the mesenteries is the circular mus- 

 cle of the oesophagus. Its position is such that its contraction 

 would bring about a closure of this tube and probably such is 

 its action, though on this point I have no direct evidence. The 

 internal pressure of the seawater contained in the animal, slight 

 though it is, certainly aids materially in closing the oesophagus, 

 as can be seen in a Metridium that is feeding. After the piece 

 of food has passed down the oesophagus of such an animal, the 

 walls of this organ held in the beginning more or less apart, 



