SECTION OF COVERED- EYED MEDUSAE. 103 



consideraLle functional power, but yet not of power 

 enough to originate contraction-waves unless re- 

 enforced by some stimulating iniiuence, which 

 reaches them from the lithocyst through the 

 nervous plexus. 



Regeneration of Tissues, 



The only facts which remain to be stated in the 

 present chapter have reference to the astonishing 

 rapidity with which the excitable tissues of the 

 Medusae regenerate themselves after injury. In 

 this connection I have mainly experimented on 

 Aurelia aurita, and shall, therefore, confine my 

 remarks to this one species. 



If with a sharp scalpel an incision be made 

 through the tenuous contractile sheet' of the sub- 

 umbrella of Aurelia, in a marvellously short time 

 the injury is repaired. Thus, for instance, if such 

 an incision be carried across the whole diameter of 

 the sub-umbrella, so as entirely to divide the excit- 

 able tissues into two parts while the gelatinous 

 tissues are left intact, the result of course is that 

 physiological continuity is destroyed between the 

 one half of the animal and the other, while the form 

 of the whole animal remains unchanged — the much 

 greater thickness of the uninjured gelatinous tissues 

 serving to preserve the shape of the umbrella. But 

 although the contractile sheet which lines the 

 umbrella is thus completely severed throughout its 

 whole diameter, it again reunites, or heals up, in 

 from four to eight hours after the operation. 



