Trochozoan

Trochozoan, or Trochozoa, is a proposed clade within the larger animal superphylum Lophotrochozoa. The name derives from the trochophore, a free-swimming larval form that is characteristic of many trochozoans. Trochozoans are defined in part by developmental features that include trochophore- or trochophore-like larval stages, although some lineages exhibit direct development and have lost the larval stage.

The best-known trochozoans are the phyla Mollusca (such as snails, clams, and cephalopods) and Annelida (segmented worms). Many species in these groups display a trochophore larva early in development, which is typically ciliated and bears a band called the prototroch. However, not all trochozoans retain a trochophore stage, and some lineages have adopted direct development or alternative larval forms.

Other phyla have been variably classified as trochozoans. Sipuncula (peanut worms) and Echiura (spoon worms) are sometimes included in Trochozoa, while the placement of Nemertea (ribbon worms) and Phoronida in relation to Trochozoa varies among analyses. Because molecular data and morphological evidence can yield different results, the exact composition of Trochozoa remains a topic of scientific discussion.

In evolutionary terms, Trochozoa is a grouping within Lophotrochozoa, one of the major divisions of protostome animals. The concept emphasizes shared developmental features and larval biology, particularly the presence or historical presence of trochophore-like stages, linking certain mollusks, annelids, and related groups.