Cytology and function of the madreporite systems of the starfish Henricia Sanguinolenta and Asterias Vulgaris

The madreporite regions of two species of starfish were examined by high‐resolution light microscopy and transmission electron microscopy. In both, the madreporite gutter epithelium is predominantly composed of ciliated cells, each bearing a single elongate cilium and numerous microvilli that support and are embedded in cuticle. Fine, contractile pores (10–12 μm) open from the gutters into canals that descend mainly to the underlying ampulla and stone canal. Cells at the pore openings lack cuticle, have rounded, sometimes protruding apices, and bear intermeshing cilia that extend outward through the pores. Below them, the pore canal is lined with irregular, granule‐containing ciliated cells possessing retractable apical head pieces and secretory cells containing clusters of unstainable vesicles. Above the basal lamina here and elsewhere is a well‐developed nerve plexus. Where the pore canals open into the ampulla and connecting axial sinus, the ciliated cells are more cuboidal, vesiculated secretory cells are absent, and free coelomocytes are numerous. The stone canal is lined with tall, densely packed ciliated cells with numerous microvilli and long iuminal extensions. Cells on its central ridge tend to form choanocyte‐like collars. The stone canal appears to be a strong ciliary pump that draws fluid from the axial sinus and madreporite pores. It is likely that the complex form of the madreporite gutters and pore canals provides multiple levels of coordinated defense against the entry of undesirable materials, and probably satisfaction of much of the nutritional needs of the parts from incoming and recirculated fluid.