ACTIVITIES OF COLONIAL ANIMALS 479 
moving very slowly by what was clearly peduncular peristalsis 
and had left behind it in the sand a trail 6 cm. in length. Subse- 
quently two other instances of a like kind were observed; in one 
the trail was 21 cm. in the other 24cm. long. Peduncular peri- 
stalsis, therefore, is not only a means of anchoring and even 
burying the Renilla colony, it is a significant means of locomotion. 
RACHIDIAL PERISTALSIS 
Rachidial peristalsis differs fundamentally from peduncular 
peristalsis in the direction its waves take. These begin in the 
peduncle and spread through the rachis to disappear on the mar- 
gin of that structure opposite the attachment of the peduncle, 
a region which, since it corresponds to the apex of the ordinary 
sea-pen, may be called the apical margin in Renilla (pl. 1, figs. 
5 to 8). Thus the direction of rachidial peristalsis is away from 
the tip of the peduncle, not toward it as in peduncular peristalsis. 
It is thus easy to distinguish rachidial from peduncular peri- 
stalsis, and it is certain that the former was recorded as early 
as 1864 by Miller in R. Edwardsii and R. reniformis. 
Rachidial peristalsis begins in the distal half of the peduncle 
and spreads over that structure into the rachis, where it appears 
as a pronounced transverse constriction represented by a right 
and a left indentation on the corresponding rachidial margins. 
These indentations proceed slowly around the edges of the rachis 
till they meet and obliterate each other on its apical margin. 
The following records taken from Renillas in sea-water at 23°C. 
will give a more detailed view of rachidial peristalsis. In one 
specimen (table 1, A) ten individual waves appeared at intervals 
varying from 132 seconds to 160 seconds and averaging 146.2 
seconds. The period taken by the wave in its passage over the 
colony varied from 125 seconds to 135 seconds with an average 
of 130.2 seconds. The resting periods between waves were from 
5 to 35 seconds with an average of 16.0 seconds. In a second 
specimen (table 1, B) the average duration of the wave was 110.5 
seconds and of the rest period 21.5 seconds and in a third (table 
1, C) 102.5 and 8.5 seconds, respectively. The general averages 
from these observations show that rachidial waves start about 
