The Ecdysozoa is the second major clade within the Bilateria (Figures 1 and 6), and it includes a subset of the animal phyla generally considered part of the

Protostomia. The key synapomorphy uniting the ecdysozoans is the possession of a cuticle that is periodically molted (a process named ecdysis). The ecdysozoan phyla are the arthropods (e.g., insects, crustaceans, myriapods, and chelicerates), onycho-phorans (velvet worms), tardigrades (water bears), nematodes (roundworms), nematomorphs (horsehair worms), priapulids, loriciferans, and kinorhynchs (mud dragons). Probably concomitant with the evolution of a cuticle that covers the entire body surface, the ecdysozoans lost the motile epidermal cilia that are widespread in other animal phyla. Moreover, in contrast to many other invertebrates, the life cycle of ecdysozoans does not include a free-living ciliated larval stage. They are therefore referred to as exhibiting direct development, instead of indirect development via a larval stage, which characterizes the life cycles of many marine invertebrates.

The discovery of the clade Ecdysozoa has generated a large amount of debate, principally by refuting the widely accepted Articulata hypothesis (Giribet, 2003; Jenner and Scholtz, 2005). Traditionally, the annelids and arthropods are considered to be closely related as articulatans on the basis of the shared possession of body segmentation. The striking morphological and developmental similarities between segmentation in these two groups were widely regarded as being among the most reliable homologies that could be identified between different animal phyla. By placing the arthropods in the Ecdysozoa, and relegating the annelids to the third major clade of bilaterians, the Lophotrochozoa, it had to be assumed that either segmentation evolved independently in annelids and arthropods, or if in fact homologous, segmentation must have been lost in many other animal phyla.

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