302 PHYSIOLOGY OF THE INVERTEBRATA. 



swam hither and thither without paying it any regard." 

 Sarsia tuhulosa and Tiaropsis polydiademata are probably the 

 only two naked-eyed Medusm sensitive to light. But the 

 action of light on Sarsia and Tiaropsis differs considerably. 

 In the case of the latter, sunlight causes it to go into a kind 

 of tonic spasm — the whole of the nectocalyx being drawn to- 

 gether. - The period of latency * in Sarsia is instantaneous 

 with all stimulations (mechanical, electrical, luminous, &c.) ; 

 but in Tiaropsis the period of latency is not instantaneous 

 with luminous stimulation, for a little more than a second 

 elapses after the first occurrence of the stimulus. With all 

 other stimulations, in Tiaropsis, the period of latenqy is in- 

 stantaneous. Romanes has shown "that the enormously long 

 period of latent excitation in response to luminous stimuli 

 was not, properly speaking, a period of latent excitation at 

 all ; but that it represented the time during which a certain 

 summation of stimulating influence was taking place in the 

 ganglia, which required somewhat more than a second to 

 accumulate, and which then caused the ganglia to originate 

 an abnormally powerful discharge." The ganglionic matter 

 of Tiaropsis represents, according to Romanes, the most rudi- 

 mentary type of visual organ. 



All the excitable parts of the Medusce are highly sensitive 

 to electrical stimulation, but the most sensitive parts are those 

 which correspond with the distribution of the main nerve- 

 trunks. The external or convex surface of the nectocalyx, 

 and the whole of the " gelatinous substance to which 

 the neuro-muscular sheet is attached," a.re insensitive to 

 stimulation. 



The extreme sensitiveness of the tissues to electrical stimula- 

 tion suggested to Romanes the idea of ascertaining whether 

 there is any localization of definite excitable tracts in these 

 animals. In the case of Sarsia, " the apex of the swimming- 

 bell is much the least excitable portion of the animal ; and 



* The time which elapses between the application of a stimulus and the 

 response to that stimulus. 



