WOUND CLOSURE AND POLARITY IN METRIDIUM 455 



inward, so that the diameter of the opening was perceptibly 

 smaller than that of the tentacle. The inrolling must be caused 

 by an inequality of tension which is normally present in the cell 

 layers of the wall. 



As the expanding tentacle became filled with the body fluid, 

 the circular as well as the longitudinal muscles relaxed, except 

 for the space of one millimeter at the cut end. Consequently 

 the tentacle stump, except at its extreme tip, was larger in diam- 

 eter in the distal part of its length than before it was cut. The 

 closure had been effected, following the initial inrolling, by the 

 formation of a prominent sphincter, presumably due to the action 

 of the circular muscles of a narrow zone at the cut end. The 

 evidence of an existing sphincter began with the expansion of the 

 tentacle, and this sphincter gradually but quickly contracted as 

 the body fluid flowed into the tentacle. With the formation of 

 a nipple, the color of the region changed, the dark endoderm 

 becoming darker as an axial plug, and the ectoderm becoming 

 more opaque. The color of the nipple graded into the light brown 

 of the tentacle's rounded tip. 



That the nipple is muscular in character in Condylactis and 

 Aiptasia is shown, according to Rand ('09; p. 210), by these facts: 

 The form of the cut tentacle is that of an inflated structure closed 

 by a strong contraction at its tip . The color of the nipple is like 

 that of tentacle tissue when under strong muscular contraction. 

 The denser color of the closed end shades into the normal color 

 at the place where muscular relaxation is found. The quick- 

 ness of the sphincter-like action when the tentacle expands, 

 indicates that the closure cannot be due to growth of tissues or to 

 amoeboid migration of cells. The stump upon stimulation some- 

 times reacts by contracting, whereupon the nipple disappears for 

 a time but reappears as the stump expands. Further, the tentacle 

 end, which re-opens when the nipple disappears, shows an 

 orifice whose size varies inversely as the length of time after the 

 cutting of the tentacle. 



These conditions were found in Metridium also. The color, 

 form, and the quickness of the formation of the nipple have been 

 described. * The re-opening of a cut end upon contraction of the 



