If the number of tube feet scales isometrically with increasing body size, the greater mass that would have to be supported per tube foot
might cause the arm margins to be in contact with the substratum for a greater proportion of the stride cycle, and the increased friction would tend to reduce crawling speed.
Table 3 Contents of muscle and connective tissue in wall of tubular organs of Actinopyga mauritiana estimated from histological sections (% crosssectional area of tube wall) [dagger] Body component Muscle Connective tissue Tube foot
In one specimen, a couple of skeletogenic cells were observed in a tube foot (Fig.
Cells producing a disk of the primary tube foot adopted a ring-shaped arrangement in advance of spicule formation.
The tube foot had been excised from a live Pycnopodia with fine scissors and was held with a pair of self-closing forceps.
In fact, a very brief (<1 s) touch with an individual Pycnopodia tube foot was usually sufficient to elicit a swim.
1-10 nM), suggesting that nitric oxide produced by activation of type 1 nicotinic acetylcholine receptors regulates tube foot relaxation.
We hypothesize, further, that outer layer epithelial cells produce nitric oxide in response to acetylcholine released from the nerve plexus, and that nitric oxide, a lipid soluble gas, diffuses through the connective matrix to mediate relaxation of tube foot muscle.
Solute secretion by the tube foot
epithelium in the starfish Asterias forbesi.
Despite intensive searching with TEM, no SCB were ever found lying in the subcuticular space of either species, although the bacteria were always found in tube foot
squash preparations with epifluorescent microscopy.
Although crossed-fiber helical arrays of connective tissue fibers in the walls of the tube feet of ophiuroids have been described and analyzed (Woodley, 1967, 1980), previous studies of the tube feet of asteroids do not describe crossed-fiber helical reinforcement of the tube foot walls.
Many of the previous studies cited above have emphasized the interdependence of the tube foot and ampulla in movement; i.
More recently, my own work with fluorescent molecular tracers and microbeads, and other methods, has shown that seawater does routinely enter the madreporite of all starfishes tested; but this uptake is not specifically directed at tube foot inflation.
When the tube foot retracts, it pulls up into a surrounding sheath that is then closed over by two or three flattened spines.
Ultrastructure of the tube foot
sensory-secretory complex in Ophiocomina nigra (Echinodermata, Ophiuroidea).