EXPERIMENTAL OBSERVATIONS ON PENNATULIDS. 461 
body-wall, aud are also actively concerned in the extension 
and contraction of the colony in a vertical direction, and 
thereby assist in directing the circulating currents within 
the canals. 
The “transverse muscles” of the body -wall are more 
deeply seated than the longitudinal muscles, and usually lie 
on the inner side of the spongy tissue. In the stalk their 
general direction is usually at right angles to the longitudinal 
axis of the colony, and in this region the muscles are usually 
arranged in more or less complete rings. In the rachis the 
fibres are less definitely arranged and are also less numerous. 
These muscles materially assist in the support of the colony 
by strengtliening the central axis, but their main function is 
apparently that of instituting' and maintaining, by peristaltic 
action, a series of dilatations and contractions of the stalk 
and to a less degree of the rachis, thereby assisting in con- 
trolling the quantity of fluids contained in the CfTnals and also 
in directinof the circulating currents. 
Marshall believed the cii'culation of fluids within the colony 
to be largely dependent upon the muscular system of the 
body-wall. He wrote of Pennatula phosphorea: “The 
well developed muscular system (i. e. of the body-wall) may 
be supposed to have for its main function the maintaining, b}' 
compression of portions of the spongy mesh-work, of currents 
from one part of the pen to another.” As this system of 
muscles appears to be more feebly developed in this than in 
any of the other forms under discussion, Marshall’s statement 
would apply with greater force to Pennatula rubra, 
A n t h o p t i 1 i u m , V i r g u 1 a r i a, and with special emphasis to 
Pterceides. We have, however, evidence for believing 
that, with the exception of the “axis-less” form Eenilla 
(p. 402), the musculature of the body-wall is not the sole 
force in inducing, maintaining, and couti'olling the circulat- 
ing currents. 
Wilson observed certain creeping movements of living 
specimens of Renilla — a Peunatulid without a central cal- 
careous axis — which he atti'ibuted to the movement of fluids 
