The pilidium larva is an idiosyncrasy defining the Pilidiophora. Its development is unique, and conserved even in derived pilidia; the juvenile is formed via a series of invaginations of the larval epidermis (imaginal discs), then bursts through the larval body while simultaneously consuming it in catastrophic metamorphosis. Pilidium nielseni is a lecithotrophic pilidium with two circumferential ciliary bands reminiscent of the “prototroch” and “telotroch” of a trochophore larva, the ancestral larval form of spiralians. However, pilidium nielseni represents a convergence on this larval form, not the resurgence of the ancestral larva, and typical pilidial development is conserved. In this thesis, I describe the development of pilidium nielseni, and determine it has converged on its body plan at least twice, independently.