Аннотация:The problem of origin of the metazoan life cycle is analyzed based on various hypotheses of the origin of multicellular animals. Accordingly to Gastrea theory and Phagocytella theory, the ancestral metazoan life cycle was holopelagic. In the framework of the hypotheses of the primary sedentarity, the metazoan ancestor had bentho-pelagic life cycle with dispersal larvae, the synzoospores. In accordance with this hypothesis, Eumetazoa came from the progenetic larvae of the sedentary ancestor. The primary life cycle of Eumetazoa (i.e., metazoans with nervous system, musculature, mouth, and gut) was holopelagic. Exactly this life cycle is typical for recent Ctenophora, which is the earliest branched stem of the eumetazoans. Cnidaria and Bilateria are the sister groups. Their last common ancestor acquired the bentho-pelagic life cycle de novo. The pelagic part of cnidarians life cycle is comprised of blastula and gastrula stages only. Some anthozoans still maintain the planktotrophic gastrula larvae in their life cycles. Planulae of Medusozoa are simplified lecithotrophic larvae, which had lost the function of spread because of appearance of medusa stage in the life cycle. In Bilateria, the prolongation of the pelagic part 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 designed by the special term “larvalization”. Thus, the ciliated pelagic larvae of all bilaterians have the 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.