SCANNING AND TRANSMISSION ELECTRON-MICROSCOPIC OBSERVATIONS ON THE LARVAE OF PRIAPULUS-CAUDATUS (PRIAPULIDA)

Scanning electron microscopical studies revealed four distinct morphological larva-types (not instars) of Priapulus caudatus whose lorica-length measured 82-860 mum. The smallest of the larvae are round to oval, have 20 longitudinal ridges, a series of transverse ridges, and have two pairs of laterally situated tubuli near the posterior limits. The second larva-type is dorsoventrally compressed, has a single dorsal and single unsculptured ventral plate each with a prominent midridge near the anterior limits, three infolded lateral plates with a pair of tubuli near the posterior limits. The third and fourth larva-types remain dorsoventrally flattened; the third larva-type has less pronounced sculpturing than the fourth larva-type and is smaller; two pairs of tubuli are situated as in the previous stage. The number and arrangement of scalids on the introvert and pharyngeal teeth differ according to each of the four larva-types and are described. The ultrastructure of all organ systems is described. All scalids are sensilla equipped with ciliated receptor cells. The cuticle is similar to the cuticle in the adult and differs fundamentally from the cuticle of larval Halicryptus. The trunk is richly supplied with sensory structures. Trunk tubuli contain secretory cells. Posteriorly, a gland complex was found which presumably is the precursor of the equivalent gland in postlarvae and adults. The implications of the first larva-type's structural similarity to the larva of Tubiluchus are discussed.