Microscopic Anatomy of the Tentacles of Thysanocardia nigra Ikeda, 1904 (Sipuncula) from the Sea of Japan

Abstract

Fine morphology of the tentacular apparatus of the sipunculan Thysanocardia nigra Ikeda, 1904 was conducted as a part of a larger study of microscopic anatomy in the species. The tentacular apparatus is composed of two rings of tentacles, the dorsal and oral crowns. The dorsal, or nuchal, crown consists of an arc of nuchal tentacles dorsally enclosing the heart -shaped nuchal organ. The oral disk carries numerous peripheral tentacles arranged in oral crown surrounding the mouth. Peripheral tentacles are arranged in paired rows, additional tentacles being developed posteriorly so as to form a radial series of U -shaped festoons. The parallel festoons extend down the oral disk and run alongside the spherical head. New pairs of festoons appear at the base of the nuchal crown. The tentacles are heart-shaped in cross section, the oral surface being widest. The nuchal tentacles face the oral disk by the oral surface while the peripheral, or oral, tentacles are twisted at the base at nearly right angles, so to face a ciliary groove of the corresponding festoon. The central groove of festoon is composed of a median ridge bordered by longitudinal lateral ridges with long cilia. The oral surface of tentacles is constructed of a multiciliated, pseudostratified, columnal epithelium with some intraepidermal mucous cells. There are three longitudinal tentacular canals lined by peritoneum. Hemerythrocytes flow through the lumen of the canals. Tentacles and grooves of festoons form a filtering system upon which food particles can be trapped. The filtering apparatus of Thysanocardia shows remarkable similarities with a filtering system of sabellid polychaetes.