The protostomes are the largest group of bilaterians and include such successful and diverse groups as the arthropods and the mollusks.  As such, they represent the largest group of living things on earth.  Although they do have a few common features (see Table 1), the protostomes are quite variable in structure, developmental histories, and ecology.  

This great abundance and diversity in form has led to a plethora of taxonomic systems, many of which are incompatible and inconsistent with each other.  The Major Clades of the Animal Kingdom illustrates some of the incompatibilities in that Nielsen (2001), Raff (2001), Adouette et al. (2000), Brusca and Brusca (2003), and Tudge (2000) all divide the protostome phyla differently and even include different phyla within the protostomes.  In general, taxonomic systems based on structural characters (e.g. Nielsen 2001; and Brusca and Brusca 2003) separate the protostomes into groups that include the Cycloneuralia and Articulata plus a number of unassociated clades.  The phylogenetic systems based on molecular characters (e.g. Tudge 2000; Raff 2001), however, separate them into the Ecdysozoa and the Lophotrochozoa.  (Go to COMPARISONS OF PROTOSTOME PHYLOGENETIC TAXONOMIC SYSTEMS for further explanations and comparisons.)  

I have followed the system of Valentine (2004) in which he draws on molecular, structural, developmental, and fossil evidence to produce three monophyletic protostome clades: Ecdysozoa, Lophotrochozoa, and Paracoelomata.

This page is maintained by Jack R. Holt.  Last revised: 03/14/2008 .