Neural Development in Invertebrates

Invertebrates comprise the vast majority of animal species on Earth and exhibit a diverse range of body forms which arise through different developmental programs. This chapter focuses on evolutionary relationships between different animal groups. It organizes into a scheme which groups the simplest animals, the diploblasts, and divides the more complex bilaterian invertebrates among three clades: Lophotrochzoa, Ecdysozoa, and Deuterostomia. Together with their roughly radial symmetry, the defining characteristic of the diplobastic phyla, Cnidaria and Ctenophora, is their possession of only two germ layers, an outer ectoderm and inner endoderm. The layers are then generally separated by a loose extracellular matrix forming the mesoglea. The Platyzoa include the platyhelminthes, gastrotrichs, rotifers and assorted minor phyla. The Ecdysozoa comprise only eight phyla but contain far more species than the Lophotrochozoa and Deuterostomes combined. The deuterostomes consist of only four extant phyla: Echinodermata, Hemichordata, Chordata and Xenoturbellida.