474 G. H. PARKER 



can be excited to activity without being involved in a sequences 

 of changes making up the total act of contraction. 



If a number of specimens of Metridium attached normally 

 to stones, pieces of shell, and so forth, are allowed to stand for 

 a day or so in quiet seawater in which there is more or less de- 

 composing material such as dead Mytilus edulis, many of the 

 specimens will contract their sphincters even though their 

 columns remain elongated. Though the longitudinal and parietal 

 muscles of the mesenteries in these specimens may not be fully 

 relaxed, they are nearly so. Certainly the only muscle in these 

 animals which is in vigorous contraction is the sphincter, and 

 this remains firmly and tightly closed until the animals are 

 transferred to pure seawater. What it is in the foul water that 

 stimulates the sphincter to independent action and what part 

 of the body of the Metridium serves as a receptor for this stimu- 

 lus, I have not been able to find out, but of the essentially isolated 

 response of the sphincter under the circumstances mentioned 

 there can be not the least question. 



Wlien a Metridium is placed in the dark in a strong flow of 

 seawater, it usually expands to its fullest extent both in the 

 spread of its oral disc and in the lengthening of its column. If 

 now the flow of seawater is cut off, the animal is very likely to 

 cover the oral disc by a contraction of the sphincter without 

 shortening its column, at least to any great extent. Thus quiet 

 seawater following current action induces an independent con- 

 traction of the sphincter much as foul seawater does. 



The sphincter is opposed chiefly to the pressure of the fluids 

 within the actinian's body and it is this probably that restores 

 the sphincter on relaxation to its most expanded form. The 

 internal pressure that is effective in this respect is due in part 

 to the intake of water by ciliary means but particularly to the 

 action of certain muscles such as the circular muscle of the column. 



The longitudmal muscle of the acontium. When a Metridium 

 has drawn down its oral disc and covered this region by the 

 contraction of the sphincter, further stimulation is followed 

 usually by the discharge of numerous acontia through the mouth 

 and the cinclides. As many as seven of these thread-like bodies 



