Lophotrochozoans derive their name from two different ciliated features: a feeding structure known as a lophophore and a free-living larval form known as a trochophore. Neither the lophophore nor the trochophore is universal to all lophotrochozoans, however.

Several distantly related groups of lophotrochozoans (including bryozoans, entoprocts, brachiopods, and phoronids) have a lophophore, a circular or U-shaped ring of ciliated, hollow tentacles around the mouth (Figure 31.2). This complex organ is used for both food collection and gas exchange. Biologists once grouped taxa that have lophophores together as “lophophorates,” but it is now clear that they are not one another’s closest relatives. The lophophore appears to have evolved independently at least twice, or else it is an ancestral feature of lophotrochozoans and has been lost in many groups. Nearly all animals with a lophophore are sessile as adults. They use the tentacles and cilia of the lophophore to capture small floating organisms from the water. Other sessile lophotrochozoans have less well developed tentacles that they use for the same purpose.

660

Some lophotrochozoans, especially in their larval form, use cilia for locomotion. The larval form known as a trochophore moves by beating a band of cilia (see Figure 30.11A). This movement of cilia also brings plankton closer to the larva, where it can capture and ingest them (its cilia are therefore similar in function to the cilia of the lophophore). Trochophore larvae are found among many of the major groups of lophotrochozoans, including the mollusks, annelids, ribbon worms, entoprocts, and bryozoans. This larval form was probably present in the common ancestor of lophotrochozoans but has been subsequently lost in several lineages.

Some lophotrochozoans (including flatworms, ribbon worms, annelids, and mollusks) exhibit spiral *cleavage in early development, in which layers of new cells spiral obliquely around the axis of the animal–vegetal pole of the early embyro. Some biologists group these taxa together as “spiralians,” although phylogenetic analyses of gene sequences do not support monophyly of taxa with spiralian cleavage. Nonetheless, spiral cleavage may have been present in the lophotrochozoan ancestor and subsequently lost in several descendant lineages.

Many lineages of lophotrochozoans have a wormlike body form, which means that they are bilaterally symmetrical, legless, soft-bodied, and at least several times longer than they are wide. A wormlike body form enables animals to burrow efficiently through marine sediment or soil. However, as you will see in Key Concept 31.2, the mollusks—the most familiar of the lophotrochozoans to many people—have a very different body organization.