The Morphology, Development and Taxonomic Status of Xyloplax Baker, Rowe and Clark (1986) (Echinodermata: Concentricycloidea), with the Description of a New Species

The recently described new class of echinoderms, the Concentricycloidea Baker, Rowe and Clark (1986), is characterized by its medusoidal shape, concentrically arranged skeletal ossicles and the double-ring form of the water vascular system, with interradial connections between the two rings and peripheral tube-feet. We have studied the morphology of the type-genus, Xyloplax Baker et al. (1986), now represented by two species, and we give here a detailed account of our observations and conclusions. Besides the ossicles already described, other skeletal structures include a uniquely shaped primary interradial plate produced into a curved spur, an odontophore that straddles the interradial line between the ring ossicles, and a reticulate skeletal form of the ring ossicles. A shallow, sac-like stomach is described for X. turnerae sp.nov. but for X. medusiformis a vestigial gut is seen only in prenatal, developing juveniles. The oral surface of postnatal X. medusiformis supports a thin velum derived from the coelomic epithelium of the stomach. Both species are dioecious and sexually dimorphic. Fertilization is internal. The ovaries of X. turnerae contain large vitellogenic oocytes, further developmental stages are unknown but there is no evidence of viviparity. X. medusiformis is an intra-ovarian, non-placental, viviparous species. It is concluded that the Concentricycloidea are derived from the class Asteroidea. It is proposed that one of the extant families, Asterinidae, Korethrasteridae or Caymanostellidae, may share some precursor of valvatid-like type. The status of the class Concentricycloidea is maintained on the basis of unique morphological features. Recent classifications of the Echinodermata are discussed.