Introduction to the symposium.

Forty-five years ago a symposium on the ‘‘Lower Metazoa’’ was held in Pacific Grove, California. The resulting volume (Dougherty et al., 1963) sampled much of the then current study and thought about simple animals—sponges, cnidarians, ctenophores, flatworms, pseudocoelomates, and more. Extensive discussions of homology and phylogeny formed the basis for testing a variety of hypotheses concerning the origin of and relationships among various metazoan taxa. A number of well-developed theories (e.g., the ciliateacoel and the planula-acoel hypotheses) were widely discussed. As is clear from the reference lists, these theories have a long history. Some of the speculative ideas advanced seem perhaps less far-fetched today, e.g., Dougherty (1963, p. 3): ‘‘. . . is novel genetic information, once acquired, too precious to be thrown away; may it perhaps be shifted about and put to seemingly different use, but, like the genie in the bottle, remain to emerge again in its earlier form?’’ For example, consider in this context the possibility of medusa loss and reappearance in cnidarian lineages (Boero et al., 1996; Marques and Migotto, 2001). Throughout the Dougherty et al. volume, the premise that is clear above all else is that data from the basal metazoans will be decisive in testing various hypotheses and theories of the origin of animals in general and bilaterians in particular. A number of symposia since 1960 (often sponsored by the American Society of Zoologists and its successor, the Society for Integrative and Comparative Biology) reaffirm and update these themes (see Garey, 2002). The basic assumption remains intact as well: without detailed knowledge of the basal metazoans, it is impossible to provide an effective comparative framework for the study of animal evolution. Recent views of animal phylogeny (e.g., Halanych, 2004) favor somewhat altered hypotheses for the early-diverging taxa compared to those current when the Dougherty et al. (1963) volume was compiled. Most investigations of some well-studied simple animals including the nematodes and most or all of the flatworms no longer find these to be basal bilaterians. The position of the ctenophores is now much debated as well—are they sister to all other tissue-grade metazoans or are