Adrianov A V, Malakhov V V
Institute of Marine Biology, Vladivostok 690041, Russia.
J Morphol. 2001 Feb;247(2):111-21. doi: 10.1002/1097-4687(200102)247:2<111::AID-JMOR1006>3.0.CO;2-C.
Larvae of priapulids are characterized by radial symmetry evident from both external and internal characters of the introvert and lorica. The bilaterality appears as a result of a combination of several radial symmetries: pentaradial 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 musculature and the circumpharyngeal nerve ring. Internal bilaterality is evident from the position of the ventral nerve cord and excretory elements. Externally, the bilaterality is determined by the position of the anal tubulus 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-dimensional environment of the benthic sediment.
鳃曳虫幼虫的特征是辐射对称，这从翻吻和兜甲的外部及内部特征都能明显看出。两侧对称是几种辐射对称组合的结果：齿的五辐对称、初级鳞片的八辐对称、鳞片的二十五辐对称、颈部的双辐对称以及躯干的双辐和十辐对称。肌肉组织和咽周神经环呈现出内部辐射性。腹侧神经索和排泄器官的位置明显体现出内部两侧对称。在外部，两侧对称由肛门小管的位置以及沿着腹侧神经索排列的两排缩短的腹侧中部鳞片来决定。兜甲元素定义了双辐射对称，而成体鳃曳虫中不存在这种对称。幼虫的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。
