Scanning and transmission electron microscopic observations on the larva of Halicryptus spinulosus (priapulida).

The larvae of Halicryptus spinulosus bear six rings of sensory/ locomotory appendages. Twelve pedicellate flosculi, 6 dorsal and 6 ventral, are associated with the neck-lorica junction. Newly described are two rows of 4-5 flosculi present on the dorsal and ventral plates, and 2 tubuli located on each dorsolateral and ventrolateral plate near their junction with the midlateral plate. The ultrastructure of all organ systems is described. The multilayered lorica differs significantly from that of other priapulids. The anterior fourth of the lorica is not underlain by epidermal cells. At the junction of the introvert and neck are a series of adhesive tubuli that have groups of secretory cells basally. A gland is described at the junction of the fore- and midgut.