Observers often assume that seawater is drawn in through the madreporite by the stone canal cilia to hydraulically extend the tube feet.
But in addition to this osmotic uptake, studies with fluorescent microbeads demonstrate that seawater does flow into the madreporite pores of asteroids (Ferguson, 1990b) and, to a lesser extent, of ophiuroids (Ferguson, 1995), and that it is distributed to peripheral parts of the water vascular system.
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.
Their study led Ellers and Telford (1992) to question Fechter's argument that the madreporite is primarily involved in acute pressure equalization.
Fechter's contention is, furthermore, not consistent with the anatomy of the madreporite and its associated stone canal.
Thus, the functions of the prominent madreporite system of sea urchins remain unknown, and the normal osmotic differences that might exist between their various body fluids and the media have not been accurately measured.
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.
Its madreporite cannot be seen externally, and the structure of its water vascular system must be inferred from old descriptions of other ophiuroid species (cf.
To locate the site of the madreporite and examine the organization of the entire system, two small specimens (6-7 mm disk diameter) were fixed in Helly's fixative and decalcified for one week in changes of cold (4 [degrees] C) 5% disodium EDTA.
The extent of seawater entry through the madreporite was evaluated in two sets of experiments.