Scanning and Transmission Electron Microscopic Analysis of the Introvert of Priapulopsis australis and Priapulopsis bicaudatus (Priapulida)

Abstract

The introvert of Priapulopsis australis and P. bicaudatus was investigated by scanning and transmission electron microscopy. The scalids, clustered normally in groups of 5 and arranged in 25 longitudinal rows, differ slightly between the two species. All scalids are sensilla containing three kinds of bipolar sensory cells with apical cilia. The cilia of one cell type reach the surface through cuticular tubules. The cilia of the other two receptor cell types are surrounded by 7 stereovilli and lie entirely beneath the cuticle. What previously was considered to be the first ring of pharyngeal teeth in P. australis may be seen by scanning electron microscopy to be a series of 10 conical sensilla. This condition may be derived by reduction from 5 pairs of teeth as found in P. bicaudatus. In turn, each pair of first-ring pharyngeal teeth in P. bicaudatus may have arisen by reduction of the median cusp of each of the 5 teeth in an ancestor, leaving two halves. Additional key words: ultrastructure, sensory receptors The phylum Priapulida is a small group of benthic marine worms comprising only 8 macrobenthic and 9 meiobenthic extant species (Higgins et al. 1993). However, it is of great scientific interest because these species are the last remnants of a fauna that was prominent in Cambrian Seas. The first species described from the Middle Cambrian was Ottoia prolifica Walcott 1911, which dominated the soft-bodied fauna of the Burgess Shale (British Columbia, Canada). Since 1911, several additional genera have been found in the Burgess Shale, in other parts of North America, and in China. The most common representative among the fossils of the Chengjiang fauna, which is found in Yunnan in southwestern China and dates back to the Lower Cambrian, is another priapulid, Maotianshania cylindrica Sun & Hou 1987. During the time-span between the Lower Cambrian (Chengjiang) and the Middle Cambrian (Burgess Shale), evolution seems to have produced little change (see Hou & Sun 1988; Chen et al. 1989, 1991). One biologically and taxonomically important character of the Priapulida is the introvert, the anterior pora Present address: Robert P. Higgins, 2 Pond Lane, Asheville, North Carolina 28804, USA. tion of the body which, when rhythmically everted from and withdrawn into the trunk, performs a locomotory function (Elder & Hunter 1980). The introvert is equipped with 20 or 25 longitudinal rows of scalids; priapulid genera differ in the number, morphology, and arrangement of scalids and in other cuticular structures. Scanning and transmission electron microscopic findings are available for the meiobenthic genera Tubiluchus (see Storch & Alberti 1985; Higgins & Storch 1989) and Meiopriapulus (see Storch et al. 1989), and the macrobenthic genera Halicryptus (see Storch et al. 1990; Storch & Higgins 1991) and Priapulus (see Higgins et al. 1993; Storch et al. 1994). These investigations have shown that in larval and adult Tubiluchus and Halicryptus, and in larval Priapulus, all scalids are sensilla. At the same time, however, these structures gain purchase on the substrate through which the animal is burrowing and thereby have both mechanical and sensory roles. By contrast, the majority of scalids on the introvert of Meiopriapulus fijiensis Morse 1981 (the so-called locomotory scalids) are merely cuticular specializations. In this species, only the two anterior rings of scalids and the posteriormost scalids, uniquely located on the trunk, perform a sensory function. In This content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 04:36:29 UTC All use subject to http://about.jstor.org/terms Introvert of Priapulopsis