EXPERIMENTAL OBSERVATIONS ON PENNATULIDS. 465 
is of great interest, for it confirms the supposition that these 
modifications of the stalk in this extremely large Pennatulid 
have arisen for hydrostatic purposes. The curiously folded, 
appearance of the tissue composing the thickened zone 
suggests the possibility during life of enormous dilatations 
of the stalk in this region, while the presence of the 
musculature on the inner surface of the laminae gives evidence 
of its function as an extremely powerful pumping apparatus. 
By its operations currents could be driven upwards with 
considerable force into the rachis and. thence distributed 
throughout the colony. 
In the extremely lai’ge and robust species of Penn at u la — 
P. naresi and P. borealis, a slight thickening and dis- 
tension of the body-wall was observed in the stalk, in the 
region corresponding to the folded and thickened zone in 
Anthoptilum (fig. 10). This was found, to be ‘due to a 
slightly increased development of the “spongy tissue” and 
“transverse musculature” of the body-wall in this region. 
It is evident that in the case of these enormous species 
greater force wouhl be required to induce and jnaintain the 
vertical currents from the stalk to the rachis than would be 
the case in more delicate and smaller species of Penn a tula, 
e. g. P. rubra and P. phosphorea. This modification has 
probably arisen in the species “ naresi ” and “borealis” 
in response to the increased, demands of the colony in this 
respect. In the presence of the slightly thickened zone in 
the stalk these species represent an intermediate condition 
between the comparatively small forms P. rubra and P. 
phosphorea and the more magnificent Anthoptilum 
g r a n d i fl o r u m . 
The Sphincter Muscle. 
In the genus Pteroeides the modification has proceeded 
still further. As in the forms just considered, the upper 
portion of the stalk in all species presents a pronounced, 
distension, due in this instance to the development of an 
