|Image by Steve Corey|
|Image by "ratexla"/Josefine Stenudo|
|Image by Juncea|
|Image by Bárbol|
The top two pics (A+B) show an unattached tube foot.. But C through F? all show those attached to the bottom.
|Figure 1 from Hennebert, Santos and Flammang, 2012|
The authors indicate that suction may still play a secondary role, serving in conjunction with the adhesion/glue but for the most part it doesn't look like suction is a primary influence here.
2. Measuring the Attachment Strength of the tube feet
They tested the adhesion of the tube feet on glass relative to detachment force (how hard they pulled) and pulling angle (the direction). That is they tried to pull it off and at different angles on a smooth glass surface.
|fr. Fig. 3A in Hennebert et al. 2013|
B. Measure strength and tenacity on a porous bottom
Its possible of course that there are further refinements to how all of this works in sea cucumbers and crinoids but starfish and sea urchins have always been the "model organism" for studying tube feet in echinoderms.