Sea urchins (regular echinoids), like asteroids, also have prominent madreporites, but their bodies are rigid.
The most concerted previous attempt to evaluate the role of the madreporite in sea urchins was that of Fechter (1965; see review of Nichols, 1966), who glued small capillary tubes to the madreporites of five urchins (Echinus esculentus) so that, by observing the movement of tiny air bubbles in the tubes, he could measure any influx or efflux of fluid through them.
purpuratus) with obstructed madreporites were repeatedly able to return to a near-normal body weight (and volume) within hours after removal of 1-3 ml of their PCF, clearly showing that the madreporite is not needed for acute large-scale volume changes.
In some cases, the madreporites of "test" animals were obstructed by first scraping the structure with a needle and blotting up the soft tissue, and then sealing the area with freshly mixed, finely ground hydraulic cement.
Only animals with intact madreporites were used for these observations; obstructing the madreporite would have destroyed the major area to be studied and would have precluded sectioning.
The madreporite is attached to a ciliated "stone canal" that connects through a series of passages to the animal's fluid-filled appendages, the tube feet.
specimens with madreporites destroyed and control animals with a comparable injury to another site
To test this possibility, animals with obliterated madreporites, and controls, were placed in dishes of seawater in which the osmotic levels had been elevated a modest amount (20 mosmoles/kg) with dextran (5300 M.
In contrast to the minimal madreporite of Ophioderma, the madreporites of asteroids have many pores and complex arrangements of ciliated gutters to keep them free of suspended foreign particles (Ferguson and Walker, 1991).
It is generally assumed, for example, that the madreporite admits seawater into the system, and that ciliary pumping by the stone canal connected to it forces the seawater taken in to flow out to the tube feet where it replenishes fluid losses due to their activity.
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.
Because the function of madreporite mechanism in different asteroids varies considerably, similar diversity should be expected in the other classes; indeed, differences in body form between the classes might relate to their individual approaches to solving the problems of body fluid maintenance.