1877.] 



Locomotor System of Medusce, 



485 



§ 2. Nciked-eyed Medusce. — "With the naked-eyed Medusae the case is 

 more definite ; for the mere fact of Sarsia being able to follow a moving 

 beam of light is in itself sufficient to prove coordination on the part of 

 the locomotor centres *. Trom my previous observations on the physio- 

 logical harmony subsisting between the tentacles of Sarsia, I was led 

 to expect that the coordination of the locomotor ganglia is probably 

 effected by means of the same tissue tracts through which the interten- 

 tacular harmony is effected — viz. those situated in the margin of the 

 bell. Accordingly I introduced four short radial cuts, one midway 

 between each pair of adjacent marginal bodies. The coordination, how- 

 ever, was not perceptibly impaired. I therefore continued the radial 

 cuts, and found that when these reached to one half or two thirds of 

 the way up the sides of the inner bell, or contractile sheet, the coordina- 

 tion became visibly affected, and this for the first time. These experi- 

 ments, however, did not satisfy me that the coordination was not chiefly, or 

 exclusively, due to the marginal nerves ; since, even if the coordination were 

 destroyed by the short radial cuts, it might still appear to remain intact 

 for the following reasons. Supposing the four quadrants to have their 

 physiological harmony destroyed, and one of the quadrants to be slightly 

 prepotent over the others, every time this quadrant of the margin dis- 

 charged, the other quadrants would immediately do the same t ; and as 

 the bell of Sarsia is so small, and the passage of contractile waves in it so 

 rapid, the mere presence of physiological continuity might, in the case where 

 only very short radial cuts were introduced, give rise to the false appearance 

 of coordinated or harmonious action. Accordingly I tried the converse 

 experiment of leaving the margin intact, and making four radial incisions 

 from the apex towards the base of the cone. I found that these incisions 

 might be carried quite down to the marginal canal -^dthout the synchro- 

 nous action of the four quadrants being impaired. This proves that the 

 marginal connexions are alone sufficient to maintain the coordinated 

 action of the ganglia, and thus tends to substantiate the above view 

 concerning the results of the converse experiment. 



This view is still further confirmed by the results of the same experi- 

 ment in the case of the Discophorous species of naked-eyed Medusas. 

 In these species the passage of contractile waves is not nearly so rapid 

 as it is in the case of Sarsia ; and so in them it may be quite easily 

 observed that the four short radial cuts in the margin have the effect 

 of destroying the physiological harmony of the marginal ganglia +. Grreat 



^ EemoYing the mamibrium does not interfere witli this steering action ; but if any 

 considerable portion of the margin be excised, the animal seems no longer able to find 

 the beam of light. 



t Allusion is here made to the fact that when a contractile wave reaches a ganglion 

 it causes the latter to discharge. See "Croonian Lecture," Phil. Trans. 1876, p. 311. 



'I This operation, although so slight, has a very remarkable effect in enfeebling 

 the animal — a vigorous specimen being usually reduced by it, after a short time, 

 to absolute quiescence. 



