Jefferies R P
Natural History Museum, Department of Palaeontology, London, UK.
Ciba Found Symp. 1991;162:94-120; discussion 121-7. doi: 10.1002/9780470514160.ch7.
The chordate sagittal plane is perpendicular to the sagittal plane primitive for the bilaterally symmetrical metazoans (Bilateria). The earliest metazoans, when symmetrical at all, were probably radial in symmetry. The axis of symmetry was vertical and the mouth, when present, opened either upward or downward. The Bilateria evolved from the primitive metazoan condition by acquiring bilateral symmetry, mesoderm, a brain at the anterior end and protonephridia. Perhaps in the stem lineage of the Bilateria a hydroid-like or medusoid-like ancestor fell over on one side onto a substrate (pleurothetism). If so, the anteroposterior axis of Bilateria would be homologous with the vertical axis of radial symmetry in coelenterates. The bilaterian plane of symmetry arose to include the anteroposterior axis. The Deuterostomia (the Hemichordata, Echinodermata and Chordata) evolved within the Bilateria by producing the mouth as a secondary perforation. Within the deuterostomes the echinoderms and chordates constitute a monophyletic group named Dexiothetica. Hemichordates retain the primitive bilaterian sagittal plane. The Dexiothetica derive from an ancestor like the present-day hemichordate Cephalodiscus which had lain down on the primitive right side (dexiothetism) and acquired a calcite skeleton. The echinoderms evolved from this ancestor by losing the ancestral locomotory tail and gill slit, becoming static, moving the mouth to the centre of the new upper surface and developing radial pentameral symmetry. The chordates evolved from the same ancestor by developing a notochord in the tail, losing the water vascular system, evolving a filter-feeding pharynx and developing a new vertical plane of bilateral symmetry perpendicular to the old bilaterian plane. Evidence derived from certain bizarre Palaeozoic marine fossils (calcichordates) gives a detailed history of the early evolution of echinoderms and chordates and shows how the new bilateral symmetry was gradually acquired in chordates. This symmetry began in the tail (which contained the notochord and was also the leading end in locomotion) and advanced forward into the head.
脊索动物的矢状面与两侧对称后生动物（两侧对称动物）的原始矢状面垂直。最早的后生动物，即便存在对称性，可能也是辐射对称的。对称轴是垂直的，口若存在，则向上或向下开口。两侧对称动物从原始后生动物状态演化而来，通过获得两侧对称、中胚层、前端的脑和原肾管。也许在两侧对称动物的主干谱系中，类似水螅或水母状的祖先倒向一侧并附着在基质上（侧卧式）。如果是这样，两侧对称动物的前后轴将与腔肠动物辐射对称的垂直轴同源。两侧对称平面的出现包括了前后轴。后口动物（半索动物、棘皮动物和脊索动物）在两侧对称动物内部演化，通过产生次生穿孔作为口。在后口动物中，棘皮动物和脊索动物构成一个单系群，称为右旋动物。半索动物保留了原始两侧对称动物的矢状面。右旋动物起源于一个类似于现代半索动物头盘虫的祖先，它侧卧在原始右侧（右旋式）并获得了方解石骨骼。棘皮动物从这个祖先演化而来，通过失去祖先的运动尾和鳃裂，变得静止，将口移到新上表面的中心并发展出辐射五辐射对称。脊索动物从同一个祖先演化而来，通过在尾部发育脊索，失去水管系统，演化出滤食性咽并发展出一个与旧的两侧对称平面垂直的新的垂直两侧对称平面。来自某些奇异的古生代海洋化石（钙脊索动物）的证据给出了棘皮动物和脊索动物早期演化的详细历史，并展示了脊索动物如何逐渐获得新的两侧对称。这种对称始于尾部（包含脊索且也是运动的前端）并向前延伸至头部。
