Symmetry of priapulids (Priapulida). 1. Symmetry of adults

Priapulids possess a radial symmetry that is remarkably reflected in both e xternal morphology and internal anatomy. It results in the appearance of 25 -radial (a number divisible by five) symmetry summarized as a combination o f nonaradial, octaradial, and octaradial (9+8+8) symmetries of scalids. The radial symmetry is a secondary appearance considered as an evolutionary ad aptation to a lifestyle within the three-dimensional environment of bottom sediment. The eight anteriormost, or primary, scalids retain their particul ar position because their innervation directly from the circumpharyngeal br ain. As a result of a combination of the octaradial symmetry of primary sca lids, pentaradial symmetry of teeth, and the 25-radial symmetry of scalids, the initial bilateral symmetry remains characterized by the single sagitta l plane. (C) 2001 Wiley-Liss, Inc.