The function of the echinoderm madreporite.

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The function of the echinoderm madreporite was studied with a focus on the mechanism of the bidirectional transport by the ciliated epithelium of the pore canal and the possible sorting mechanism. DIC microscopy of the movement of microbeads injected near the openings of pore tubes isolated from the madreporite by collagenase treatment revealed that the beads larger than 0.5 mum in diameter were always transported oralwards whereas the smaller beads (<0.2mum) often moved in the opposite direction, indicating selection by size. Bidirectional transport was also observed on the flat surface of ciliated epithelium obtained by cutting open an isolated pore tube. This showed that the bidirectionality did not arise from the tubular geometry of the tissue but an inherent characteristic of the cilia. Observation of the movement of individual cilia indicated, however, that their beat form was similar to that of typical cilia.