SCANNING AND TRANSMISSION ELECTRON-MICROSCOPIC ANALYSIS OF THE INTROVERT OF PRIAPULOPSIS-AUSTRALIS AND PRIAPULOPSIS-BICAUDATUS (PRIAPULIDA)

The introvert of Priapulopsis australis and P. bicaudatus was investig ated by scanning and transmission electron microscopy. The scalids, cl ustered normally in groups of 5 and arranged in 25 longitudinal rows, differ slightly between the two species. All scalids are sensilla cont aining three kinds of bipolar sensory cells with apical cilia. The cil ia of one cell type reach the surface through cuticular tubules. The c ilia of the other two receptor cell types are surrounded by 7 stereovi lli and lie entirely beneath the cuticle. What previously was consider ed to be the first ring of pharyngeal teeth in P. australis may be see n by scanning electron microscopy to be a series of 10 conical sensill a. 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 t eeth in P. bicaudatus may have arisen by reduction of the median cusp of each of the 5 teeth in an ancestor, leaving two halves.