3o6 PHYSIOLOGY OF THE JNVERTEBRATA. 



the exclusive seats of spontaneity, so far as tlie " primary 

 movements " are concerned. 



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

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

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

 such rate. 



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

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

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

 point below the original rate. The rate then remains 

 stationary at this point, but may again be made temporarily 

 to rise and permanently to fall by removing another portion 

 of the nectocalyx. " In these experiments the rhythm, 

 besides becoming permanently slowed, is also often rendered 

 permanently irregular. Again, paring down the contractile 

 tissues from around a single lithocyst* has the effect, when 

 the tissue is greatly reduced, of giving ri&e to enormously 

 long periods of inactivity. During such period, however, 

 stimulation may initiate a bout of rhythmical contractions, 

 to be folio .ved by another prolonged pause. These facts tend 

 to show that the apparently automatic action of the lithocysts 

 is really due to a constant stimulation supplied by other 

 parts of the organism." 



(4) " Temperature exerts a profound influence on the rate 

 of rhythm. This influence may be best observed within 

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

 spontaneity and even irritability, while water above 70° C. 

 permanently slows the rhythm after having temporarily 

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

 quickens the rhythm during the time that the MccIusk, which 

 have been removed from colder water, are exposed to its 

 influence. In very cold water the loss of spontaneity is a 

 gi-adual though rapid process, as is also its return in warmer 

 water. After having been frozen solid, Auniia will recover 



* The marginal bodies in the covered-eyed Medusa: occur in the form of 

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



