Development of the tentacular apparatus in sipunculans (Sipuncula): I. Thysanocardia nigra (Ikeda, 1904) and Themiste pyroides (Chamberlin, 1920).

The development and arrangement of the tentacular apparatus of Thysanocardia nigra (Ikeda, 1904) and Themiste pyroides (Chamberlin, 1920) are described and illustrated using scanning electron microscopy. In T. nigra, the tentacular apparatus is composed of two crowns: the nuchal arc enclosing the nuchal organ and a crown of numerous oral tentacles arranged in U-shaped festoons. In early juveniles, two dorsal horn-like protrusions develop into the first, or primary, pair of tentacles of the nuchal arc. The second pair of tentacles of the nuchal arc develops dorsolaterally on the bases of the primary tentacles. Two ventrolateral lobes of the oral disk grow and become subdivided by the longitudinal ciliary groove into anlages of one set of dorsal and one set of ventral tentacles, thus forming a first oral festoon. Later, a pair of dorsolateral lobes develop between the first festoons and the nuchal arc to form a second pair of oral festoons. The third and following pairs of oral festoons develop in the dorsolateral growth zones lateral to the borders of the nuchal arc, where they meet the oral crown. The growing festoons extend down the oral disk and run alongside the head. A new oral tentacle appears directly at/on the base of the previous tentacle, thus giving rise to a typical sympodium with an alternating arrangement of tentacles. In T. pyroides, a second pair of tentacles develops from two ciliary lobes that are ventrolateral outgrowths of the circumoral ciliary field around the terminal mouth opening. The third pair of tentacles appears from the dorsolateral lobes at the base of primary tentacles, between the first two pairs of tentacles. These six tentacles determine the position of six main stems of the tentacular apparatus designated the first tentacles in the corresponding stems. The second tentacle in every stem appears as a ventrolateral outgrowth at the base of the first tentacle. The third and following tentacles in the stem are developed between the two previous tentacles according to a sympodial pattern. In both species, the distinct sympodial pattern in the arrangement of tentacles in the tentacular apparatus is well evidenced by the outlines of the ciliary oral grooves. The branched stems of T. pyroides may be homologized structurally and functionally to the oral festoons of T. nigra. J. Morphol. (c) 2006 Wiley-Liss, Inc.