1879.] 



the Locomotor System of Medusce. 



267 



discharge, but also in large measure to an alternate process of ex- 

 haustion and restoration of excitability on the part of the responding 

 tissues — the ganglionic period coinciding with that during which the 

 process of restoration lasts, and the ganglionic discharge being thus 

 always thrown in at the moment when the excitability of the respond- 

 ing tissues is at its climax. 



Light has been found to stimulate the lithocysts of covered-eyed 

 Medusae into increased activity, thus proving that these organs, like 

 the marginal bodies of the naked-eyed Medusae, are rudimentary 

 organs of vision. 



The polypite of Aurelia aurita has been proved to execute move- 

 ments of localization of stimuli, somewhat similar to those which the 

 author has already described as being performed by the polypite of 

 Tiaropsis indicans. 



Alternating the direction of the constant current in the muscular 

 tissues of the Medusae has the effect of maintaining the make and 

 break stimulations at their maximum value ; but the value of these 

 stimulations rapidly declines if they are successively repeated with the 

 current passing in the same direction. 



In the sub-umbrella of the Medusae waves of nervous excitation are 

 sometimes able to pass when waves of muscular contraction have 

 become blocked by the severity of overlapping sections. 



Exhaustion of the sub- umbrella tissues — especially in narrow con- 

 necting isthmuses of tissue — may have the effect of blocking the 

 passage of contractile waves. 



Lithocysts have been proved sometimes to exert their ganglionic 

 influence at comparatively great distances from their own seats — con- 

 tractile waves, originating at points in the sub-umbrella tissue 

 remote from a lithocyst, and ceasing to originate at that point when 

 the lithocyst is removed. A nervous connexion of this kind may be 

 maintained between a lithocyst and the point at which the waves of 

 contraction originate even after severe forms of section have been 

 interposed between the lithocyst and that point. 



When the sub-umbrella tissue of Aurelia is cut throughout its 

 whole diameter, the incision will again heal up, sufficiently to restore 

 physiological continuity, in from four to eight hours. 



January 23, 1879. 



W. SPOTTISWOODE, M.A., D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



u2 



