ULTRASTRUCTURE OF AXIAL VASCULAR AND CELOMIC ORGANS IN COMASTERID FEATHERSTARS (ECHINODERMATA, CRINOIDEA)

The spongy body of Davidaster rubiginosa, D. discoidea, and Comactinia meridionalis, is an axial haemal plexus consisting of two structurall y similar, but positionally distinct, regions: an oral circumesophagea l part and an aboral part which lies lateral to the axial organ. The a xial organ is a large axial blood vessel which is infiltrated by hollo w cellular tubes lined with monociliated epithelial cells. The spongy body plexus is a tangle of small blood vessels overlain by podocytes a nd myocytes. The spongy body and the axial organ are situated in the a xial coelom, which is confluent with the perivisceral coelom, the wate r vascular system, and the parietal canals. The parietal canals open t o the exterior via ciliated tegmenal ducts and surface pores. The crin oid spongy body is morphologically similar to the axial gland of aster oids, ophiuroids, and echinoids (AOE). Although the axial glands of th ese three classes of echinoderms are mutually homologous structures, t he homology of the crinoid spongy body and the AOE axial gland is ques tionable because of differences in organization and developmental orig in. Alternatively, the crinoid spongy body may be homologous to astero id gastric haemal tufts, which are podocyte-covered blood vessels susp ended in the perivisceral coelom. The functional organization of the s pongy body suggests a filtration nephridium and predicts an excretory function. An alternative hypothesis is that the spongy body is a site of nutrient transfer from the blood vascular system to the periviscera l coelom.