Nielsen Claus
Zoological Museum (University of Copenhagen), Universitetsparken 15, DK-2100 Copenhagen, Denmark.
J Exp Zool B Mol Dev Evol. 2004 Jan 15;302(1):35-68. doi: 10.1002/jez.b.20001.
The trochophora concept and the literature on cleavage patterns and differentiation of ectodermal structures in annelids ("polychaetes") and molluscs are reviewed. The early development shows some variation within both phyla, and the cephalopods have a highly modified development. Nevertheless, there are conspicuous similarities between the early development of the two phyla, related to the highly conserved spiral cleavage pattern. Apical and cerebral ganglia have almost identical origin in the two phyla, and the cell-lineage of the prototroch is identical, except for minor variations between species. The cell-lineage of the metatrochs is almost unknown, but the telotroch of annelids and the "telotroch" of the gastropod Patella originate from the 2d-cell, as does the gastrotroch in the few species which have been studied. The segmented annelid body, i.e. the region behind the peristome, develops through addition of new ectoderm from a ring of 2d-cells just in front of the telotroch. This whole region is thus derived from 2d-cells. Conversely, the mollusc body is covered by descendants of cells from both the C and D quadrants and a growth zone is not apparent. This supports the notion that the molluscs are not segmented like the annelids, and that the repeated structures seen in polyplacophorans and monoplacophorans do not represent a segmentation homologous to that of the annelids.
本文综述了担轮幼虫概念以及有关环节动物（“多毛纲动物”）和软体动物中卵裂模式与外胚层结构分化的文献。早期发育在这两个门内都表现出一些差异，头足类动物具有高度特化的发育过程。然而，这两个门的早期发育之间存在明显的相似性，这与高度保守的螺旋卵裂模式有关。顶神经节和脑神经节在这两个门中的起源几乎相同，除了物种间的细微差异外，原担轮的细胞谱系是相同的。中担轮的细胞谱系几乎未知，但环节动物的端担轮和腹足纲动物笠贝的“端担轮”都起源于2d细胞，已研究的少数物种中的胃担轮也是如此。分节的环节动物身体，即口后区域，是通过在端担轮前方的一圈2d细胞添加新的外胚层而发育形成的。因此，整个这一区域都源自2d细胞。相反，软体动物的身体由C和D象限细胞的后代覆盖，且未观察到明显的生长区。这支持了软体动物不像环节动物那样分节的观点，以及在多板纲动物和单板纲动物中所见的重复结构并不代表与环节动物同源的分节这一观点。
