The Trochozoa comprise four phyla of animals: the molluscs, the annelids, and the smaller phyla Sipuncula and Nemertea. These are united by the common presence of trochophore larvae, which have two bands of cilia around their middle, though these have been lost in some groups.
Originally, the arthropods were also considered to belong here, because although they lack trochophore larvae they share a segmented body plan with the annelids. More recently this has been called into question, and RNA studies remove them to the Ecdysozoa. They support the monophyly of the remaining Trochozoa, and place them within a larger group Lophotrochozoa.
Previously these were treated together as the Trochozoa, together with the arthropods, which do not produce trochophore larvae but were considered close relatives of the annelids because they are both segmented.
However, it appears that neither the lophophorates nor the Trochozoa are monophyletic groups by themselves, but are mixed together.
The flatworms and their allies form a clade, called the Platyzoa, which is closely related to the Lophotrochozoa and sometimes included within it.
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