488 G. H. PARKER 
of them may show rachidial waves and they may then be studied. 
But aside from this indirect way of inducing the formation of 
these waves, I know of no special method by which they may 
be excited. Ifa Renilla, in which rachidial peristalsis is in prog- 
ress, is slightly disturbed by being gently handled or even merely 
jarred, these movements like those of peduncular peristalsis are 
likely to cease for a time. Thus, although the excitation of 
rachidial waves was impossible for me to accomplish by external 
means, their cessation is in this manner easily brought about. 
The interruption of rachidial peristalsis is almost certain to occur 
if the stimulus is applied during the brief period that intervenes 
between waves; it is much less likely to occur if it is applied dur- 
ing the passage of the wave. This suggests another point of 
similarity between the rachidial wave and a heart beat, for just 
as the contraction of the cardiac muscle is followed by a refrac- 
tory period during which the muscle is not open to the reception 
of.a new stimulus, so the passage of a rachidial wave in Renilla 
prevents the reception of a stimulus which, had it been applied 
in the period between waves, would undoubtedly have been an 
effective agent. 
Rachidial peristalsis has been referred to by Eisen (’76, p. 18) 
as a means of locomotion. And it is true that when a Renilla is 
distended and in sea-water on the surface of the sand, rachidial 
peristalsis will bring about a movement from place to place. 
Thus in one example of this kind, a Renilla was observed as a 
result of ten rachidial waves carried out during twenty-five 
minutes to have shifted its position 4.2 cm. The movement was 
of a slow floundering kind and seemed to be the accidental result 
of the peristalsis rather than a direct and obvious effect of it as 
implied by Eisen. Although rachidial peristalsis may thus really 
result in locomotion, I am disinclined to regard it as a real means 
of locomotion in the same sense that I do peduncular peristalsis. 
The real significance of rachidial peristalsis, in my opinion, is 
not locomotion, but the emergence of the colony from the sand 
and its distention. After a colony has remained contracted for 
some time in a sand bank exposed by the falling tide, it is in a 
condition to reinflate itself on the return of the water. This it 
