1877.] 



Locomotor System of Medusa. 



465 



confirm my original statement to the effect that these contractions 

 *' usually last for several days." In most cases, indeed, they continue 

 even after decomposition of some parts of the tissues has set in and pro- 

 ceeded to the extent of giving the latter a certain ragged appearance, 

 which is due to certain portions disintegrating into shreds. 



II. Ehtthm. 



§ 1. Relation between Size and Bhytlim. — I am able to endorse Dr. 

 Elmer's statement that in Aia^elia aurita the rate of the natural rhythm 

 has a tendency to bear an inverse proportion to the size of the individual. 

 Size, however, is far from being the only factor in determining the differ- 

 ence between the rate of rhythm of different specimens, though what the 

 other factors are I am unable to suggest. 



§ 2. Relation between tJie periods of Activity and the periods of Rejpose. — 

 Dr. Eimer affirms that the duration of the natural pauses, which in Au- 

 relia alternate with bouts of swimming, bears a direct proportion to the 

 number and strength of the contractions that occurred in the previous 

 bout of swimming. In my postscript I pointed out that Sarsice are much 

 better adapted than Aurelice for determining whether any such precise 

 relation obtains, and observed that here I was quite sure no such pre- 

 cise relation did obtain, although in a very general way it was true, as 

 might be expected, that unusually prolonged bouts of swimming were 

 sometimes followed by pauses of unusual duration. These statements 

 have now been amply confirmed. 



§ 3. Effects of Artificial Segmentation on the Rhythm. — I have also 

 this year investigated Dr. Elmer's rule with regard to the half, quarter, 

 and eighth-part segments of Aurelia manifesting respectively a rhythm 

 one half, one quarter, and one eighth part the rate of the rhythm which 

 had previously been manifested by the unmutilated animal. I am glad 

 to observe that Dr. Eimer himself seems to regard this rule as a somewhat 

 uncertain one, for I cannot say that my experiments have tended to 

 confirm it. As stated in my previous paper, there is a general tendency 

 for the smaller segments of an Aurelia divided in this way to contract 

 less frequently than the larger segments ; and more careful observations 

 this year have not resulted in establishing any more precise relationship. 

 On the other hand, these observations have resulted in confirming my 

 previous opinion, that the potency of the lithocysts which happen to be 

 left in any given segment has more to do with the rate of the rhythm 

 than has the size of the segment ; for it is not difficult to obtain an eighth- 

 part segment with a more rapid rhythm than a segment four times that 

 size, provided that the Aurelia is divided so as to leave a decidedly pre- 

 potent lithocyst to animate the smaller segment. In such cases, how- 

 ever, the endurance of the smaller segment will be less than that of the 

 larger, notwithstanding the greater activity it displays during the first 

 day or two after the operation. I can only attribute this fact to the more 



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