EXPERIMENTAL OBSERVATIONS ON PENNATULIDS. 471 
stalk4 Through the iunumerable apertures in the walls of 
the canals curz-ents could be directed into the distensible 
spongy tissue (figs. 13, 14, Ap.). By this means the lower 
portion of the stalk would become considerably dilated. 
The concerted retraction of these znuscles would be accom- 
panied by a simultaneous shortening of the stalk, and the 
hooking up of the axis in its muscular sheath in the doi’sal 
canal would give an upward impetus with a piston-like move- 
ment to the fluids contained in it; simultaneously the i-etrac- 
tion of the oblique muscles embedded in the vei’tical septum 
would shorten the ventral and latei’al canals, which would 
increase the basal impetus ah-eady mentioned of the fluid in 
an upward vei-tical direction. This upward cui’rent would be 
maintained by the peristaltic action of the musculature of the 
body-wall of the stalk and to a less degi'ee in the rachis. 
In accordance with the needs of the colony the dii-ection 
of the cuiment may be reversed, as in the observatioizs by 
Wilson (1893) in the case of llenilla. 
In an experiment on Pterceides spinosum (fig. 3) an 
inhaleut cui-rent was observed to enter the basal poi’e of the 
venti’al canal, while several exhzilent currents were obsei-ved 
to issue simultaneously from the pores of the dorsal and 
other canals in the neighboui-hood of the base of the stalk. 
In this instance, however, the coloity appeared to be in a 
perfectly quiescent condition, and during the expei-iment 
exhibited zione of the muscular movements observed in 
otlier forms. 
In the case of small and comparatively delicate species of 
Penn atu lids (e. g. Penn at nl a rubra and P. phosphor ea) 
the concerted action of the apical musculatui-e of the I’achis, 
the oblique musculatuz-e of the stalk, and the longitudinal and 
'transverse musculature of the body-wall in stalk and rachis 
ai’e appzii'ently sufficient to induce and maintain a complete 
cii’culation of fluids thi’oughout the colony. 
* In the case of Anthoptilum the canals of the stalk would be 
I’apidly filled by inhalent cuzTents through the siphonozooids specially 
developed in that region. 
