SECTION OF NAKED-EYED MEDUSAE. 127 



manifest a power of penetration to which the 

 normal contractions of the tissues in which it occurs 

 bear so small a proportion ; why it is that the con- 

 tractile tissues should be so deficient in the power of 

 originating a spasm, even in response to the strongest 

 stimulation applied to themselves ; — these and other 

 questions at once suggest themselves as cpiestions of 

 interest. At present, however, I am wholly unable 

 to answer them ; though we may, I think, fairly 

 assume that it is the ganglionic element in the 

 margin, and probably also in the radial tubes, 

 which responds to direct stimulation by discharging 

 a peculiar impulse, which has the remarkable effect 

 in question. I'or the sake of rendering the matter 

 quite clear, let us employ a somewhat far-fetched 

 but convenient metaphor. We may compare the 

 general contractile tissues of this Medusa to a mass 

 of gun-cotton, which responds to ignition (direct 

 stimulation) by burning with a quiet flame, but to 

 detonation (marginal stimulation) with an explosion. 

 In the tissue, as in the cotton, every fibre appears 

 to be endowed with the capacity of liberating 

 energy in either of two very different ways ; and 

 whenever one part of the mass is made, by the ap- 

 propriate stimulus, to liberate its energy in one of 

 these two ways, all other parts of the mass do the 

 same, and this no matter how far through the 

 mass the liberating process may have to extend. 

 Now, employing this metaphor, what we find is 

 that, while the contractile fibres resemble the cotton 

 fibres in the respects just mentioned, the ganglion 

 cells resemble detonators, when themselves directly 



