The third major clade within the Bilateria is Lophotrochozoa (Figures 1 and 8). This clade includes the largest number of traditionally recognized animal phyla, and it embodies the greatest amount of disparity among body plans found within the Metazoa. Lophotrochozoans span an enormous range of body architectures, life cycles, developmental modes, habitats, behaviors, and include both the smallest and the biggest living invertebrates, ranging from the tiny loriciferans that measure about a tenth of a millimeter, to the gigantic deep-sea squids with body lengths measuring in multiple meters. Consequently, without question the Lophotrochozoa represents the phy-logenetically most challenging metazoan clade.

The name Lophotrochozoa derives from the fact that many members of this clade either possess ciliated feeding tentacles, called a lophophore, such as the brachiopods and phoronids, or they include a ciliated trochophore larva in their life cycles, such as the mollusks, annelids, echiurans (spoon worms), sipunculans (peanut worms), and the entoprocts. However, the Lophotrochozoa also includes the ectoprocts (moss animals), platyhel-minths (flatworms), nemerteans (ribbon worms), gnathostomulids, rotifers, acanthocephalans, cycliophorans, micrognathozoans, and possibly the gastrotrichs and chaetognaths.

Traditionally, all nondeuterostome animals were united in a clade Protostomia, and indeed in several recent phylogenetic analyses the clades Lophotrochozoa and Ecdysozoa together form a clade Protostomia as sister group to the deuteros-tomes. However, molecular phylogenetic analyses have not yielded strong support for the monophyly of Protostomia. Moreover, the value of the morphological characters in support of a clade Protostomia is currently being reinterpreted, and it might turn out that the protostomes are a paraphyletic taxon, perhaps giving rise to the deuterostomes.

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