EFFECTOR SYSTEMS OF ACTINIANS 479 



spot Stimulated and extends slowly around the column, an 

 operation which is completed in about half a minute. If several 

 spots at different heights on the column are similarly stimulated, 

 as many as three such rings can be formed at once on the same 

 individual. From the position and method of formation of 

 these bands of constriction it is quite clear that they are due 

 to the contraction of bundles of fibers in the circular muscle. 



If in a completely contracted Metridium a spot on the ex- 

 posed portion of the column is mechanically stimulated, a ring 

 of constriction passes round the animal encircling the pore which 

 marks the location of the mouth. If a radial cut is made from 

 near the pore out to the periphery of such an animal and care 

 is taken that this cut goes no deeper than through the column 

 wall, the constriction resulting from a local stimulus on reach- 

 ing this cut fails to pass across it, showing that this activity 

 is entirely resident in the column wall. If now a fully expanded 

 Metridium which will contract its tentacles when a given spot 

 on the column wall is mechanically stimulated, has this spot 

 fully anaesthetized by dropping on it crystals of magnesium 

 sulphate, a ring of constriction will still form around its colunrn 

 when this spot is repeatedly touched by a blunt glass-rod. As 

 the nervous mechanism in this portion of the animal has been 

 rendered inoperative through the magnesium sulphate, it follows 

 that the formation of the groove must be a purely muscular 

 operation thus demonstrating independent action on the part 

 of the circular muscle. 



The radial and circular muscles of the oral disc as well as the 

 basilar and circular muscles of the pedal disc form natural 

 pairs of muscles that undoubtedly act in conjunction with each 

 other in the movements of their respective parts. I have been 

 unable, however, to find any means by which these muscles 

 can be brought into action individually, though judging from the 

 results obtained in other instances it would not be surprising 

 if such means were sooner or later discovered. 



This examination of the musculature of Metridium brings 

 out very clearly the fact that its anatomical differentiation is 

 not without physiological significance. Jordan ('08) has already 



