SECTION OF COVERED-EYED MEDUSA. 103 
considerable functional power, but yet not of power 
enough to originate contraction-waves unless re- 
enforced by some stimulating influence, 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 
Medusze 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. 
