The lancelet (amphioxus) performs metamorphosis and produces minute and ciliate pelagic larvae commonly found in other metamorphic marine invertebrates. During larval life and metamorphosis, however, the animal displays interesting combination of features not found in other animals such as long coexistence of ciliate and muscular locomotion and no change in feeding behavior. The uniqueness of lancelet metamorphosis can provide important data to understand the evolutionary history of this animal as well as the metamorphosis broadly appeared in metazoans. Although lancelet metamorphosis has been studied, all previous studies depended on cross-sectional observations. To get serial data on metamorphic events, we performed longitudinal observations on the Japanese lancelet under the culture condition and confirmed the following: (1) there were individual variations of the duration of metamorphosis from 15 to 27 days; (2) growth was arrested for a month and the maximum reduction of the body length (2.2%–3.2%) occurred when gill slits became paired; (3) during rather long duration of metamorphosis, the oral transformation and the division of the gill pores by tongue bar were completed within two to four days. Our observations suggest that the duration and mode of lancelet metamorphosis depend mainly on intrinsic requirements rather than on extrinsic selective pressures. 1. Introduction Profound change of body plan under the same genomic background during life cycle is common in metazoans. It is called metamorphosis and found in at least some species of most extant phyla [1]. Metamorphosis of lancelets (amphioxus) has been noticed from the first embryological study of Kowalevsky [2] and interested the following scientists such as Lankester and Willey [3], Willey [4], and van Wijhe [5]. Like other marine invertebrates, lancelets change their life mode from pelagic to benthic passing through metamorphosis. However, their diet and feeding method are not changed principally. Lancelet ontogeny uniquely displays a long-term coexistence of cilia- and muscle-driving locomotion before metamorphosis [6, 7]. The metamorphosis takes place after pelagic larval life, the duration of which depends on the environmental condition and species [8–10], and animals after metamorphosis settle into sand substratum. Metamorphic changes mainly occur in the pharyngeal region in lancelets. Metapleural folds appear first on the ventral trunk on both sides and fused ventrally covering the original body surface in the pharyngeal region and abdominal region anterior to the iliocolonic ring
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