On the Locomotor Si/stem 0/° Medusae. 251 



if the margin be now removed, the paralyzed bell will respond to 

 stimuli applied at any part, just as readily aud simultaneously 

 over its whole extent as it would do were there no system of in- 

 terdigitatiug cuts present. 



(h) If the margin of Sarsia be removed in a continuous strip, with 

 the exception of one end left attached in situ, and if the section be 

 then continued in the form of a spiral having two or more turns from 

 the base to the apex of the cone, the contractile waves originating 

 in the free portions of the severed margin pass into the spiral 

 upon reaching its point of origin, and then run round aiid round 

 the spiral from the base to the apex of the cone. If the mar- 

 ginal strip be now removed altogether, the paralyzed bell will 

 respond to stimuli applied to any part of the spiral. 



(c) If seven lithocysts be removed fi'om the gonocalyx of Avrelia 

 aurita, and if the remaining one be made the point of origin of a 

 spiral section, which is then carried round and round the disk- 

 shaped gonocalyx, the result is a long strip of contractile tissue, 

 the contractile waves always originating in the lithocyst at the end 

 of the strip, and then running along the strip until they deliver 

 themselves into the remainder of the gonocalyx, which thereupon 

 contracts. The length and width of such contractile strips are very 

 important factors in determining whether the waves pass all the 

 way along the strip or become blocked at some point in its length. 

 Nevertheless these are very far from being the only factors, there 

 being immense individual differences in the endurance of the con- 

 tractile tissue under this form of section. Sometimes the waA^es 

 will become blocked when the strip is only an inch or less in 

 length ; while at other times the waves will pass freely from end 

 to end of a contractile strip which is only an inch wide and a yard 

 long. 



§ 2. How are the impulses transmitted from the locomotor cen- 

 tres of MedusiB to the contractile tissues of their swimming-organs? 

 Have we any evidence of more or less definite lines of discharge 

 being present ? or must we conclude that the contractile tissues are, 

 throughout their extent, of a functionally homogeneous nature ? 



(a) The fact that the contractile tissues endure such severe 

 section without losing their physiologic-al continuity, appears to settle 

 this question in favour of the last-mentioned alternative. Never- 

 theless there is a weighty body of evidence to be adduced on the 

 other side. In the first place, the extreme variations in their 

 tolerance of spiral section which are manifested by different in- 

 dividuals of the species Aurelia aurita appear to be irreconcilable 

 with the hypothesis of the tissue concerned being functionally 

 homogeneous. But the following invariable fact is still more dif- 

 ficult to I'econcile with this hypothesis, viz. at whatever point in a con- 

 tractile strip that is being progressively elongated hg section the con- 

 tractile ivave becomes bloched, the blocking is sure to take place com- 

 2>letely and exclusively at that point. In view of these facts, there- 

 fore, at the present stage of my inquiries, I provisionally accept the 

 hvpothesis of more or less definite lines of discharge being pre- 



17* 



