Many ecdysozoans are wormlike in form, although others—the arthropods, onychophorans, and tardigrades—have limbs. In this section we will look at the two clades of wormlike ecdysozoans: the priapulids, kinorhynchs, and loriciferans in one clade and the nematodes and horsehair worms in the other. Key Concept 31.4 will be devoted to the most diverse ecdysozoans—the arthropods and their relatives—and the many forms their appendages take.

As we noted in Key Concept 31.1, ecdysozoans can be identified by the presence of a body covering known as a cuticle, which provides these animals with both protection and support. The name of the group, Ecdysozoa, is derived from the Greek ecdysis ( “to get out of”) plus zoa (“animals”). Periodic shedding of the cuticle is necessary so that ecdysozoans can grow. In many ecdysozoans that have wormlike bodies, the cuticle is relatively thin and flexible; it offers the animal some protection but provides only modest body support. A thin cuticle allows the exchange of gases, minerals, and water across the body surface, but it restricts the animal to moist habitats. Many species of ecdysozoans with thin cuticles live in marine sediments from which they obtain food, either by ingesting sediments and extracting organic material from them or by capturing larger prey using a toothed pharynx (a muscular organ at the anterior end of the digestive tract). Some freshwater species absorb nutrients directly through their thin cuticles, as do parasitic species that live within their hosts (endoparasites). Many wormlike ecdysozoans are predators, eating protists and small animals.