Traditional morphological and modern molecular phylogenetic analysis have both recognized a major evolutionary transition from "non-<answer>bilaterian</answer>" animals, which are those lacking a bilaterally symmetric body plan (Porifera, Ctenophora, Cnidaria and Placozoa), to "<answer>bilaterian</answer>" animals (Bilateria) whose body plans display bilateral symmetry. The latter are further classified based on a major division between Deuterostomes and Protostomes. The relationships among non-<answer>bilaterian</answer> animals are disputed, but all <answer>bilaterian</answer> animals are thought to form a monophyletic group. Current understanding of the relationships among the major groups of animals is summarized by the following cladogram:
Animals in what group are thought to form a monophyletic group?
bilaterian