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.

Ability to regenerate is found in many groups of metazoans but the majority of research is focused on adults from just a few taxa, such as planarians and hydra (Agata and Inoue, 2012; Bely et al., 2014). Increasing the diversity of study organisms and life stages can reveal new and interesting aspects of regeneration mechanisms. This study focuses on regeneration of the nemertean pilidium larva. The planktotrophic pilidium of Maculaura alaskensis provides a unique model in which to observe several components of the regeneration process. Here I have documented a timeline for regeneration and have begun to evaluate the cells responsible for regenerative success. This study has revealed the interplay between regeneration and degeneration, a tradeoff between larval and juvenile structures, as well as the important relationship between global versus local signaling in proliferation and differentiation responses.

This study doubles the known diversity of nemertean species in one region along the northeast Pacific coast by utilizing the often over-looked larval life-history stage. Prior to this work, the nemertean fauna in this region was believed to be well described; however, previous assessments were based on adult life-history stages only and significantly underestimated the real diversity. With this dissertation, we update what is known about nemertean diversity and expand upon this “life-history” approach to describe new species, identify and describe larval forms, and speculate on the phylogenetic relevance of nemertean larvae. A considerable amount of new diversity takes the form of cryptic species complexes, where existing descriptions include characteristics of several species. Micrura alaskensis, a common intertidal nemertean and an emerging model system for developmental studies, existed as a species complex consisting of five species. In this dissertation we designate a new genus, re-describe M. alaskensis, and describe four new species in this complex. In doing so we make accurate identification possible for future comparative research. The complete development of few nemertean species was known before this project began, thus few species could be identified as larvae. We have identified over 30 nemertean larvae using both embryological and DNA barcoding approaches in this work. Intriguingly, many wild-caught larvae could not be matched to species previously reported from this region and instead contribute to previously unknown diversity. This new diversity includes species previously reported only from distant geographic regions as well as species new to science. The first record of a hubrechtid on the west coast of North America and the identification of two new species in the currently monotypic genus Riserius were revealed in larval assessments. Aside from increasing known species-level diversity, we revealed novel larval types. Barcoding larvae allowed us to place larval morphotypes into a phylogenetic context and identify potentially useful larval synapomorphies for nemertean phylogenies. Our results emphasize the importance of a life-history approach to biodiversity assessments for all species with biphasic life-cycles. This dissertation includes published and unpublished co-authored material.