On Changes in the Sea and Their Relation to Organisms. 243 



remain constant for 5 minutes after each short step of stretching or 

 relaxation, the rate will approximate a mean value. 



This behavior of the ring may be regarded as a form of hysteresis, 

 since the number of revolutions per second of the contraction-wave 

 tend to remain constant immediately after stretching or relaxation. 

 I do not see how this can be explained on the assumption that the 

 stretching of the neuraxon is the only factor, and it is significant to note 

 that Carlson records no such hysteresis in the stretched nerve of the 

 slug, where synapses are less numerous or entirely absent. One explana- 

 tion of the increased rate immediately after stretching might be the 

 thinning of the plasma membrane of the neuraxon, the regeneration 

 in thickness taking appreciable time. Another suggestion is made 

 that the immediate effect of stretching is increase in length of the neu- 

 raxon, but that this may be proportionately less than the increase in 

 length of the strip of tissue, since the nerve-paths may be straightened, 

 and that the apparent hysteresis is due to the possibility that after the 

 nerve is stretched synapses gradually open, due to the tension, until 

 the nerve-paths are proportionately as zigzagged as at the start. 

 This supposition may similarly be applied to the fact that by prodding 

 a cassiopea (having a trapped wave) with a stick, the number of revolu- 

 tions of the wave per second is reduced, although no apparent increase 

 in length of the conducting-path remains. The local stretching of 

 the subumbrella with the stick might break some of the synapses. 



Such complicated suppositions are not very useful, however, since 

 they are not easily tested. A more probable hypothesis is given below 

 in connection with the amplitude of the contraction-wave. The sig- 

 nificant fact is that (ignoring the period of readjustment) the ring may 

 be stretched until the circumference is increased 72 per cent with 

 practically no change in rate (millimeters per second), although in 

 order to accomplish this the number of revolutions per second or pass- 

 ages of the wave through the same tissue may be reduced 46 per cent. 

 This is analogous to the effect of stretching a metallic wire on the 

 passage of an electric current through it, mth the difference that the 

 process is completely reversible in Cassiopea after an increase of 84 

 per cent in length due to stretching. The results on two rings are 

 shown in table 18, the rate being given in millimeters per second. 



If the rate of wave propagation is the same, we would expect that 

 in umbrellas of cassiopeas of different sizes the number of revolutions 

 of the wave per second would be inversely proportional to the diam- 

 eter. The diameter is measured before the wave is started and there 



