478 G. H. PARKER 



approximate gradually as though under slight pressure from 

 the inside. 



Transverse muscles are found not only on complete mesenteries 

 but also on the incomplete ones, though in this situation they are 

 rather poorly developed. Such muscles in consequence of their 

 failure to reach the oesophagus can have nothing to do with its 

 expansion. When a Metridium is fed, however, one can often 

 see on its column beside the six or eight deep grooves marking 

 the positions of the complete mesenteries, a whole series of 

 minute grooves which, like the others, fade out as the food is 

 swallowed. These are probably due to the transverse muscles 

 of the incomplete mesenteries but their method of formation, 

 and the significance of the contraction that produced them, 

 if in fact it possesses any significance at all, have not been worked 

 out. 



The circular muscle of the column. This muscle is unquestion- 

 ably the chief antagonist of the longitudinal muscles of the 

 mesenteries and acts in conjunction with them in the contraction 

 and expansion of the animal as a whole, but it also has its own 

 activities. If a fully expanded Metridium is freely fed, it will 

 usually show upon its column ring-like constrictions which 

 form near the oral disc and proceed like a peristaltic wave over 

 the column to the pedal disc. A new constriction appears 

 every four or five minutes. These waves, which may have to 

 do with the movement of the food within, have been noted by 

 Gosse ('60, p. 253) in Halcampa, and very recently by Miilleg- 

 ger ('13, p. 487) in Metridium and Sagartia, in the latter of 

 which they have been photographed. According to Miillegger 

 they may run from the pedal to the oral pole as well as in the 

 reverse direction. They represent without question a specialized 

 and individual activity of the circular muscle, the receptor mecha- 

 nism of which has not been ascertained. 



Closely related to this peristalsis of the column are certain 

 responses that can be induced in animals that are partly con- 

 tracted but still well filled with seawater. If the column of such 

 an animal is stimulated by rubbing it lightly on a particular 

 spot with a blunt glass-rod, a constriction begins to form at the 



