ACTIVITIES OF COLONIAL ANIMALS 489 
does primarily by the currents of water generated by the lateral 
siphonozodéids, but the process of distention and the elevation of 
the colony as a whole is greatly aided by rachidial peristalsis. 
Thus a contracted Renilla that had more or less buried itself 
in the sand of the aquarium was seen to begin distending itself 
by taking in water. In a short time one of its autozodids had 
expanded and was projecting through the thin layer of sand that 
covered the rachis. In half an hour five zodids had expanded, 
and in an hour almost all were expanded, whereupon rachidial 
peristalsis set in and in a short time the whole rachis was full 
and plump and lifted well above the level of the sand. Thus 
rachidial peristalsis is apparently a very effective supplement 
in the process of expansion and may in fact be essential to its 
completion. At least I have never seen a Renilla reach full and 
complete distention without exhibiting vigorous rachidial peri- 
stalsis toward the close of the operation. 
Although neither peduncular nor rachidial peristalsis is essen- 
tial to inflation, both these processes seem to aid this operation 
greatly, for in their presence it goes on more rapidly and to 
greater completion than otherwise. They are both doubtless 
the means of moving the sea-water contained within the colonial 
spaces, and thus they may be regarded as important distributors 
of the fluid supplied by the lateral siphonozoéid. This view of 
the significance of the peristaltic movements was long ago 
advocated by Marshall (Musgrave, ’09, p. 461). 
Peduncular peristalsis takes its origin apparently somewhere 
in the proximal half of the peduncle. Rachidial peristalsis is 
initiated in the distal half of that part. Hence the middle of the 
peduncle is a region that may at one time be occupied by pedun- 
cular waves running distally and at another time by rachidial 
waves running proximally. It is, therefore, not surprising that 
under normal conditions peduncular and rachidial peristalsis 
never occur at the same time. In the hundreds of examples of 
these movements that I have observed in living normal Renillas, 
I have never seen a single instance in which these two forms of 
peristalsis occurred on the same individual at once. This rela- 
tion is a natural one, for, if peduncular peristalsis has to do with 
