The relationships of chordates used to be rather problematic, but intensive analyses of morphological and molecular data have shown a clearer picture (Eernisse and Peterson, in press; Smith et al., in press). The Phylum Chordata is part of a larger clade, the Deuterosto-mia, which in turn is part of a yet larger clade of all the bilaterally symmetrical animals, the Bilateria (see p. 2). But what exactly diagnoses the Deuterostomia? The clue comes from embryology, the study of the early phases of development in, and just out of, the egg.
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