Larval Development and Metamorphosis in Sipuncula

In a brief review of development of the phylum Sipuncula, four patterns of development are recognized: (1) direct with no pelagic stage; (2) one larval stage, a lecithotrophic trochophore; (3) two larval stages, a lecithotrophic trochophore and a lecithotrophic pelagosphera; (4) two larval stages, a lecithotrophic trochophore and a planktotrophic pelagosphera. Larval types and their metamorphoses are described, with special attention to the development and morphology of the larval cuticle. In the majority of species studied, the egg envelope is transformed into the larval cuticle at metamorphosis of the trochophore. The cuticle of many planktotrophic pelagosphera larvae is characterized by surface papillae of diverse form and pattern. The underlying cuticle in some species is composed of layers of fibers at right angles to one another.