PHYSIOLOGY OF THE INVERTEBRATA. 307 



on being thawed out, but the original rate of rhythm was 

 not observed fully to return." 



(5) Oxygen accelerates the rhythm, while carbonic anhy- 

 dride retards it, and in strong doses destroys both sponta- 

 neity and irritability. Deficient aeration of the water in 

 which the Medusae are living, causes irregularity of their 

 rhythm, as well as the occurrence of pauses ; until at last 

 spontaneity altogether ceases; but on now removing the 

 animals to fresh sea-water, their recovery is surprisingly 

 sudden. 



(6) As regards stimulation, Romanes has shown that a 

 few drops of hot water allowed to run over the excitable 

 tissues of these animals cause a responsive contraction. 

 Single mechanical or chemical stimuli applied to paralyse the 

 nectocalyces of covered-eyed Medusce frequently produces in 

 response a small series of rhythmical contractions. 



(7) Light acts as a powerful stimulus to some species of 

 Medusce; and it may be stated that the stimulus has been 

 proved to be light per se, and not the sudden transition from 

 darkness to light. 



(8) The period of latent stimulation in the case of Aurelia 

 aurita is greatly modified by certain conditions. Of these, 

 temperature exerts the greatest influence, but the most im- 

 portant influence, from a physiological point of view, is that 

 of the summation of stimuli. At the bottom of a " stair- 

 case" the latent period is | of a second, while at the top 

 of a " staircase " it is only f of a second. Summation of 

 stimuli also greatly increases the amplitude of the contrac- 

 tions ; so that it both develops in the tissue a state of ex- 

 pectancy and arouses it into a state of increased activity. 



(9) The excitable tissues of Aurelia may be thrown into 

 tetanus by means of strong faradaic stimulation ; and Romanes 

 has proved that the the tetanus is due to the summation of 

 contractions. 



(10) Reflex action occurs in various species of Medusce. 

 In Sarsia definite nervous connections of constant occurrence 



