304 PHYSIOLOGY OF THE INVERTEBRATA. 



This effect may be produced a great number of times in 

 succession. 



(5) " Exhaustion of the excitable tissue of the nectocalyx 

 may be easily shown by the ordinary methods. Exhausted 

 tissue is much less sensitive to stimulation than is fresh 

 tissue. Moreover, so far as the eye can judge, the contrac- 

 tion is slower, and the period of latent stimulation prolonged. 



(6) "The tetanus produced by faradaic electricity is not 

 of the nature of an apparently single prolonged contraction 

 (except, of course, such among the naked-eyed Medusce, as 

 respond to all kinds of stimuli in this way), but that of a 

 number of contractions rapidly succeeding one another — as 

 in the heart under similar excitation." 



Eomanes, in his important papers {hoc. cii.), has shown the 

 amount of section which the neuro-muscular tissues of the 

 Medusce will endure without suffering loss of their physiolo- 

 gical continuity ; and this is in the highest degree astonishing. 

 He has also investigated the rate of transmission of stimuli ; 

 as well as the regeneration of excitable tissues in these 

 animals (i.e., after injury). It may be remarked that if the 

 contractile sheet, which lines the nectocalyx is completely 

 severed throughout its whole diameter, it again reunites, or 

 heals up, in from four to eight hours after the operation. 



The nervous system of the naked-eyed Medusce is more 

 highly developed than it is in the covered-eyed Medusce ; and 

 Eomanes has demonstrated the occurrence of reflex action 

 in the Medusce. This reflex action occurs " only between the 

 marginal ganglia (in Sarsia) and the point of the bell from 

 which the manubrium is suspended — it being only the pull 

 which is exerted upon this point when the manubrium con- 

 tracts and acts as a stimulus to the marginal ganglia." 



Eomanes has brought much physiological evidence to bear 

 on the distribution of nerves in Sarsia and it may be stated 

 that his researches prove " that nervous connections unite the 

 tentacles with one another and also with the manubrium ; or 

 perhaps more precisely, that each marginal body acts as a 



