SECTION OF COVERED-EYED MEDUS. 89 
waves is retarded by narrowing the area through 
which they pass :— 
Time from end to end of a 
circular strip... ms 
Time after width has bee 
reduced to one-half 
Time after width has been 
reduced to one-quarter... 
Time after width has been 
reduced to one-eighth ... 
Fig. 13. 
In such experiments it generally happens, as here 
represented, that reducing the width of a circular 
strip by one-half produces no effect, or only a slight 
effect, on the rate, while further narrowing to the 
degree mentioned produces a conspicuous effect. 
I may also state that if, as occasionally happens, the 
immediate effect of narrowing a circular strip to one- 
half is to temporarily block the contraction-waves, 
when the latter again force their passage, their rate 
is slower than it was before. It seems as if the 
more pervious tissue tracts having been destroyed 
by the section, the less pervious ones, though still 
able to convey the contraction-wave, are not able 
to convey it so rapidly as were the more pervious 
tracts. 
In order to ascertain whether certain zones of the 
circular contractile sheet in all individuals habitually 
convey more of the contractile influence than do 
other zones, I tried a number of experiments in the 
following form of section. Having made a circular 
strip, I removed all the lithocysts save one, and 
then cut the strip as represented in Fig. 14. On 
