Rhythmical Pulsation in Scyphomedusce. 



F2I 



peeled away. Thus in figure 5, B, the area ruled with annular lines repre- 

 sents a part of the ring from which the epithelial layer with its nervous 

 network has been peeled away, leaving the muscles intact. Under these 

 conditions the contraction is at once destroyed as soon as it reaches the 

 border of the raw muscles and all movement ceases. 



If, on the other hand, we cut away both muscles and epithelium, and 

 allow the cut area to regenerate, the nervous network and epithelium will 

 regenerate before the muscles reappear. Thus in figure 5, A, the dotted 

 area represents a recently regenerated area, which contains no muscular 

 elements, but over which the epithelium and nervous network has regene- 

 rated. The contraction-stimulus passes readily through this region, although 

 it can produce no contractions in the dotted area where there are no muscles 

 to contract ; all other parts of the circuit wherein the muscles are found 

 contract as soon as the wave reaches them. Indeed, the pulsation-stimulus 

 is independent of the muscles, and passes through the nervous network 

 whether the muscles respond to it by contraction or remain inert. This 

 is illustrated by figure 6, where the dotted sector AD represents newly re- 

 generated epithelium, which contains the diffuse nervous network, but has 

 no muscular elements and therefore can not contract. The undotted part 

 of the sector ABC is normal tissue containing muscular elements, but it is 

 immersed beneath a ^m MgSO 4 solution, which renders the muscles in- 

 capable of contraction, although the pulsation-stimulus can still pass 

 through the sector. The sector CD 

 and the small undotted area around 

 s is normal subumbrella tissue. If, 

 now, a wave be started in this cir- 

 cuit, it will pass constantly around 

 the ring, and wherever it passes 

 through the sector CD or over the 

 area s these regions contract, for 

 they are normal tissue, but no con- 

 tractions or other visible signs of 

 the presence of the stimulus are 

 exhibited by the sectors which lack 

 muscular elements, or in which the 

 muscles are rendered incapable of 

 contraction through the effects of 



D 



FIG. 6. Showing how a pulsation-wave may 

 pass from normal tissue (plain area), 

 through tissue deprived of muscles (dotted 

 area), and over muscles which have been 

 rendered incapable of contracting through 

 the effects of magnesium (immersed area). 



magnesium. 



But to return to our subject, if we cut a ring from the medusa's disk 

 such as is shown in figure 7 and leave a long narrow strip (AB) attached 

 to it, and then start a contraction-wave traveling around the ring, every 

 time the wave passes the point A a side-tracked portion of the wave will 

 pass along the strip from A to B. When each side-tracked wave comes to 



