Fine structure of the podia in three species of paxillosid asteroids of the genus Luidia (Echinodemiata).

Individuals of the paxillosid asteroid genus Luidia use their podia in locomotion and burrowing. Each podium in the three considered species consists of a stem with a pointed knob at its tip. The knob consists of four tissue layers that are, from the inside to the outside, a mesothelium, a connective tissue layer, a nerve plexus, and an epidermis. The latter is made up of four cell categories : secretory cells, neurosecretory cells, non-secretory ciliated cells, and support cells. The epidermal cells of the podial knob are presumably functioning as a duo-gland adhesive system in which adhesive secretions would be produced by secretory cells while de-adhesion, on the other hand, would be due to neurosecretory cell secretions. Although the podia of the three considered species of Luidia share numerous similarities, there are nevertheless several important morphological differences between, on the one hand, the podia of L. ciliaris and L. maculata, and, on the other hand, the podia of L. penangensis. These differences stress that carefulness is required when generalizations, drawn from the morphology of a single species, are applied to related species having the same life style; but also that the genus Luidia needs to be re-examined from a taxonomic point of view.