3o6 PHYSIOLOGY OF THE JNVERTEBRATA. 



the exclusive seats of spontaneity, so far as the " p 

 movements " are concerned. 



(2) The rate of the natural rhythm has a tendency 1 

 an inverse ratio to the size of the individual, though, 

 be remarked, that size is not the only factor in deterj 

 such rate. 



(3) The cutting off the manubrium (polyprite) or a \ 

 of the nectocalyx (swimming-bell), causes, first acceh 

 of the rhythm, and then a progressive decline to a ( 

 point below the original rate. The rate then r( 

 stationary at this point, but may again be made temp 

 to rise and permanently to fall by removing another ] 

 of the nectocalyx. " In these experiments the rl 

 besides becoming permanently slowed, is also often re: 

 permanently irregular. Again, paring down the cont 

 tissues from around a single lithocyst* has the effect, 

 the tissue is greatly reduced, of giving rise to enon 

 long periods of inactivity. During such period, ho 

 stimulation may initiate a bout of rhythmical contra 

 to be followed by another prolonged pause. These fad 

 to show that the apparently automatic action of the litl 

 is really due to a constant stimulation supplied by 

 parts of the organism." 



(4) " Temperature exerts a profound influence on tl 

 of rhythm. This influence may be best observed 

 moderate limits of variation ; for water below 20° C. su; 

 spontaneity and even irritability, while water above 

 permanently slows the rhythm after having temp 

 quickened it. But water between 50° and 60° C. perma 

 quickens the rhythm during the time that the Medusce, 

 have been removed from colder water, are exposed 

 influence. In very cold water the loss of spontaneil 

 gradual though rapid process, as is also its return in ti 

 water. After having been frozen solid, Aurelia will 1 



* The marginal bodies in the covered-eyed Medusce occur in the 

 little bags of crystals ; hence they have been termed lithocysts. 



