Many of the lower metazoans reach adulthood via complex developmental stages. Each of these stages is described with a specialized terminology, which is different for every major group of organisms. Biologists with a proper training in biodiversity may have learned about these complex issues during their undergraduate days, but many of us do no longer encounter the intricacies of lower metazoan development in later stages of our careers. We may be familiar with the ontogenetic stages in the group on which we specialize, but only a few will have practical knowledge and an overview of the larval stages in such groups as crustaceans, echinoderms, polychaetes, sipunculans, molluscs, flatworms, bryozans, and nemertines. Donald Williamson certainly belongs to the last-mentioned category of biologists. In this book, Williamson addresses the situation that marine invertebrates from highly distinct and not closely related phylogenetic lines may develop from very similar larvae. There are many examples of distinct lower metazoan lineages sharing highly similar early developmental stages.