Embryos and Larvae of a Lancelet,Branchiostoma floridae,from Hatching through Metamorphosis: Growth in the Laboratory and External Morphology

Florida lancelets were raised in laboratory cultures from the egg to the juvenile stage. At frequent intervals during development, elongation of the embryonic and larval body was measured at room temperature (22.5°C) and at the approximate temperature of the natural environment (30°C). Development was slower at the lower temperature, with metamorphosis commencing during the fifth week as compared to the third week at the higher temperature. Scanning electron microscopy (SEM) was used to describe a frequently sampled series of hatched embryos, pre-metamorphic larvae, metamorphic larvae, and juveniles. The advent (and sometimes subsequent disappearance) of the following structures was determined from the SEM data: general epidermal ciliation, peroral pit, mouth, primary gill slits, ciliary tuft, external opening of the club-shaped gland, sense cells, anus, metapleural folds, and preoral cirri. Our SEM did not substantiate the claims of van Wijhe for a transitory larval mouth near the anteriovental end of the larvae. The general epidermal cilation, which is uniformly distributed in the embryos, becomes somewhat reduced in the pre-metamorphic larvae and then disappears almost entirely during metamorphosis. The epidermis includes two distinct sense cell types (I and II) and possibly a third type (the ventral pit cells, to which an adhesive role has alternatively been attributed). The anus first opens on the right-hand side and only later migrates across the mid-ventral line to assume a position on the left-hand side of the larva; this is contrary to the established view that the anus of the larval lancelets opens on the left-hand side and remains there.