Symmetry of priapulids (Priapulida). 2. Symmetry of larvae

Larvae of priapulids are characterized by radial symmetry evident from both external and internal characters of the introvert and lorica. The bilatera lity appears as a result of a combination of several radial symmetries: pen taradial symmetry of the teeth, octaradial symmetry of the primary scalids, 25-radial symmetry of scalids, biradial symmetry of the neck, and biradial and decaradial symmetry of the trunk. Internal radiality is exhibited by m usculature and the circumpharyngeal nerve ring. Internal bilaterality is ev ident from the position of the ventral nerve cord and excretory elements. E xternally, the bilaterality is determined by the position of the anal tubul us and two shortened midventral rows of scalids bordering the ventral nerve cord. The lorical elements define the biradial symmetry that is missing in adult priapulids. The radial symmetry of larvae is a secondary appearance considered an evolutionary adaptation to a lifestyle within the three-dimen sional environment of the benthic sediment. (C) 2001 Wiley-Liss, Inc.