Adrianov A V, Malakhov V V
Institute of Marine Biology, Vladivostok 690041, Russia.
J Morphol. 2001 Feb;247(2):99-110. doi: 10.1002/1097-4687(200102)247:2<99::AID-JMOR1005>3.0.CO;2-0.
Priapulids possess a radial symmetry that is remarkably reflected in both external morphology and internal anatomy. It results in the appearance of 25-radial (a number divisible by five) symmetry summarized as a combination of nonaradial, octaradial, and octaradial (9+8+8) symmetries of scalids. The radial symmetry is a secondary appearance considered as an evolutionary adaptation to a lifestyle within the three-dimensional environment of bottom sediment. The eight anteriormost, or primary, scalids retain their particular position because of their innervation directly from the circumpharyngeal brain. As a result of a combination of the octaradial symmetry of primary scalids, pentaradial symmetry of teeth, and the 25-radial symmetry of scalids, the initial bilateral symmetry remains characterized by the single sagittal plane.
鳃曳动物具有辐射对称，这在其外部形态和内部解剖结构中都有显著体现。这导致出现了25辐射对称（一个能被5整除的数字），概括为鳞片的非辐射、八辐射和八辐射（9 + 8 + 8）对称的组合。辐射对称是一种次生现象，被认为是对底栖沉积物三维环境中生活方式的一种进化适应。最前面的八个，即主要的鳞片，由于它们直接从围咽神经节获得神经支配，所以保持着它们特定的位置。由于主要鳞片的八辐射对称、牙齿的五辐射对称和鳞片的25辐射对称的组合，最初的两侧对称仍然以单一矢状面为特征。
