The problem of the origin of the metazoan life cycle is analyzed on the base of various hypotheses on the origin of multicellular animals. According to the Gastraea theory and the Phagocytella theory, the ancestral metazoan life cycle was holopelagic. In the framework of the hypotheses of the primary sedentarity, the metazoan ancestor had a pelago-benthic life cycle with floating larvae, the synzoospores. In accordance with this hypothesis, Eumetazoa originated from the progenetic larvae of the sedentary ancestor. The primary life cycle of Eumetazoa (i.e., metazoans with a nervous system, musculature, mouth, and gut) was holopelagic. Particularly this life cycle is typical for recent Ctenophora, which is the earliest branch of the eumetazoans. Cnidaria and Bilateria are sister groups. Their last common ancestor acquired the pelago-benthic life cycle de novo. The pelagic part of the life cycle of cnidarians is comprised of the blastula and gastrula stages only. Some anthozoans still maintain the planktotrophic gastrula larvae in their life cycles. Planulae of Medusozoa are simplified lecithotrophic larvae that had lost the function of spread because of the appearance of the medusa stage in the life cycle. In triploblastic Bilateria, the prolongation of the pelagic stage of the life cycle occurred due to the appearance of the ciliated bilaterally symmetrical larvae, which are actually the juveniles raised into the water column. This phylogenetic modus can be designated by the special term “larvalization.” Thus, the ciliated pelagic larvae of all bilaterians have a common origin from the juvenile stages of the last common bilaterian ancestor. Their ciliated bands came from the modified ciliated tentacular apparatus of juvenile stages of the last common bilaterian ancestor. Homologous elements in the ciliated bands of trochozoan and deuterostomian larvae are traced.

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后生动物生命周期的进化和远洋幼虫的起源

后生动物生命周期的起源问题是在多细胞动物起源的各种假说的基础上进行分析的。根据Gastrea学说和吞噬细胞学说，祖先后生动物的生命周期是全息的。在初级定居假说的框架内，后生动物祖先具有浮游幼虫（合生孢子）的浮游底栖生命周期。根据这一假设，真真动物起源于久坐祖先的后代幼虫。Eumetazoa（即具有神经系统、肌肉组织、口腔和肠道的后生动物）的主要生命周期是全息的。特别是这种生命周期对于最近的栉水母是典型的，它是真后生动物的最早分支。Cnidaria 和 Bilateria 是姐妹群。它们的最后一个共同祖先从头获得了远洋底栖生命周期。刺胞动物生命周期的远洋部分仅由囊胚和原肠胚阶段组成。一些珊瑚虫在它们的生命周期中仍然保持着浮游营养原肠胚幼虫。水母的浮游生物是由于生命周期中水母阶段的出现而失去了传播功能的简化的卵磷脂幼虫。在三叶性双翅目中，生命周期的远洋阶段的延长是由于双侧对称的纤毛幼虫的出现，这些幼虫实际上是被养到水体中的幼虫。这种系统发育模式可以用特殊术语“幼虫化”来指定。因此，所有双边动物的纤毛远洋幼虫都起源于最后一个共同祖先的幼年阶段。它们的纤毛带来自最后一个共同的双侧动物祖先幼年阶段的改良纤毛触手装置。追踪了trochozoan 和deuterostomian 幼虫的纤毛带中的同源元素。