NEUROMUSCULAR MECHANISM IN METRIDIUM 441 



region of its transition to the oral disc. The fibers in their 

 circular course pass the Hnes of attachment for the mesenteries 

 at right angles but are not to any great extent interrupted at 

 these lines. The circular muscle is a well developed sheet 

 (Hertwig, 79-80, p. 502). 



12. The sphincter (fig. 2, spht.) near the oral end of Metridium 

 is a firm circular band of muscle embedded in the supporting 

 lamella of the column at the level where the thick, colored por- 

 tion of this structure passes into the thin, more translucent 

 part. Although the bundles of fibers that make up the sphincter 

 are fully embedded in the supporting lamella, their distribution 

 shows at once that they have come from the entodermic side 

 of the body wall and that the sphincter is to be regarded merely 

 as a differentiated part of the circular muscle of the column 

 (Hertwig, 79-80, p. 505; Carlgren, '93, p. 105). 



In small specimens the sphincter contracts in such a way 

 that the oral aperture of the actinian is reduced to a minute 

 pore. In large specimens this aperture under full contraction 

 is more frequently an elongated slit. The length of the sphinc- 

 ter under the conditions of relaxation and contraction is very 

 different; in a large specimen the relaxed sphincter measured 

 37.5 cm. in length and contracted 4.2 cm., or about one-ninth 

 its former length. 



13. The longitudinal muscle of the acontium in Metridium 

 is best demonstrated in a transverse section of this thread-like 

 organ. In such a section the axial region of the acontium is 

 seen to be occupied by a mass of supporting lamella roughly 

 T-shaped in outline and so placed that the shaft of the T points 

 away from the center of the acontium. The longitudinal mus- 

 cle consists of two bands of delicate fibers one on each side of 

 the shaft of the T and away from the face of the acontium on 

 which the nematocysts are situated. 



These observations agree with the statement made by Carl- 

 gren ('93, p. 135) for the acontia of Metridium dianthus, but are 

 opposed to those made by the Hertwigs ('79-80, p. 564) for the 

 acontia of the closely allied genus Sagartia. According to the 

 Hertwigs the longitudinal muscle band is to be seen on the outer 



