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Deuterostomia is a major subgroup of animals. It is comprised of two lineages, the Chordata and Ambulacraria (Edgecombe et al. 2011, Swalla & Smith 2008). Chordata consists of two exclusively marine groups, the fish-like lancelets (Cephalocordata) and the sea squirts, salps and relatives (Tunicata) as well as the vertebrates which include fishes, amphibians, reptiles, birds, and mammals. Ambulacraria contains the exclusively marine echinoderms (sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies) and hemichordates (soft-bodied benthic worm-like animals).
Ancestrally, all deuterostomes share a special form of embryonic development characterized by (1) indeterminate cleavage (early cells retain the capacity to develop into a complete embryo), (2) the initial opening of the developing embryo (blastopore) turning into the anus, and formation of the third body layer (mesoderm) from a pouch of the embryonic gut (archenteron). These basic developmental patterns have been modified in various ways during the evolution of deuterostome groups, making it impossible to recognize a deuterostome lineage based on ontogenetic criteria alone (Nielsen 1995).
In addition to chordates, echinoderms, and hemichordates, several other groups have been placed in the Deuterostomia. Lophophorates (phoronids, brachiopods, and bryozoans) and chaetognaths show a mixture of protostome and deuterostome traits. Molecular evidence indicates that they are more closely related to protostomes (Halanych et al. 1995, Helmkampf et al. 2008, Marletaz et al. 2006, Matus et al. 2006, Papillon et al. 2004, Passamaneck & Halanych 2006). Recently, the enigmatic Xenoturbellida have been aligned with deuterostomes (Bourlat et al. 2003, 2006, 2009; Perseke et al. 2007, Philippe et al. 2007, 2011), but there is also support for an alternative position of this group with the acoelomorphs near the base of the bilaterian tree (Edgecombe et al. 2011, Hejnol et al. 2009, Lundin 1998, 2001, Nielsen 2010, Pedersen and Pedersen 1986, 1988, Raikova et al. 2000).