Microscopic anatomy and ultrastructure of the introvert of Priapulus caudatus and P. tuberculatospinosus (Priapulida)

Introverts of Priapulus caudatus and P. tuberculatospinosus bear 25 rows of scalids, as well as 8 spines and scattered papillae in the region the circumoral lip. These, as well as the first ring of pharyngeal teeth in P. tuberculatospinosus , are sensory organs. Although superficially they differ between species, they are all characterized by apical and/or subapical openings which are located on tiny cuticular tubules. All sensory organs contain cilia bearing bipolar receptor cells. The 8 sensory spines situated between the circumoral area and the beginning of the scalids are ultrastructurally similar to the scalids. The introvert and pharyngeal body walls, and associated muscles are described. © 1994 Wiley‐Liss, Inc.