INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION DEUTEROSTOMIA

PHYLUM ECHINODERMATA

CLASS CONCENTRICYCLOIDEA

This Class was established for a new genus and species, Xyloplax medusiformes, described in 1986. It was discovered on pieces of rotting wood dredged from depths exceeding 5,000 m. These are tiny creatures measuring up to 1 cm in diameter. Externally, they closely resemble sea stars of the genus Caymanostella, which were found on the same substrate. To date, another species, X. tumerae, has also been described.

Anatomy and Morphology. Sea daisies lack arms; their body is flattened, disc-shaped, and bordered by numerous marginal spines, giving them a flower-like appearance (hence the name). The aboral side of the body is covered with skeletal plates, among which five terminal plates stand out. On the oral side, three rows of plates (circumoral, adambulacral, and marginal) cover only the edges, while the central part is spanned by a thin membrane (Fig. 453).

Fig. 453. Sea daisy Xyloplax medusiformes (from Baranova):

a, b — aboral and oral views, respectively; c — diagram of the Water-Vascular System Structure (from Christensen):

1 — terminal plates; 2 — hydropore; 3 — marginal spines; 4 — marginal plates;

5, 6 — circumoral and adambulacral plates, respectively; 7 — gonad;

8 — tube feet; 9, 10 — outer and inner rings of the water-vascular system, respectively; 11 — Polian vesicles

The water-vascular system of Xyloplax differs in structure from that of other Echinoderms. It comprises two ring canals connected by short interradial canals. The inner ring canal corresponds to the ring canal of other echinoderms, whereas the outer one has no analogues. Radial canals are absent. Tube feet emerge outward from the outer ring canal through Pores in the adambulacral plates along the edge of the disc.

The Digestive System is partially or completely reduced. Xyloplax tumerae possesses a Mouth leading to a shallow pouch-like Stomach, while other PARTS OF THE digestive tract are absent. In X. medusiformes, the entire surface facing the substrate is covered by a thin membrane formed by the endodermal epithelium. Sea daisies feed by crawling with their mouth or ventral membrane over The surface of rotting wood and absorbing low-molecular-weight Organic compounds—the products of bacterial wood decomposition.

Reproductive System. Five pairs of Gonads are located in the interradiuses of the body cavity, containing embryos at various Selection/3.html">Stages of development, which indicates viviparity.

Sea daisies, together with Bacteria, participate in the degradation of sunken wood. Research on these animals is ongoing.