Three ways by which various polychaetes obtain their food are:
• The clamworm Nereis is predatory and equipped with jaws or teeth. They have an eversible, muscular pharynx armed with teeth that can be thrust out with surprising speed to capture prey.
• Most sedentary tube and burrow dwellers are particle feeders, using cilia or mucus to obtain food like plankton and detritus. Some deposit feeders, like Amphitrite , protrude their heads above the mud and extend long tentacles over the surface to find food. Cilia and mucus on the tentacles entrap particles found on the sea bottom and move them toward the mouth.
• Fan worms, or "featherduster" worms, emerge from their secreted tubes and unfurl their tentacular crowns to feed. The food which is drawn to the feathery arms, or radioles, by ciliary action is trapped in mucus and carried down ciliated food grooves to the mouth.
The evolutionary significance of segmentation to its earliest possessors is that the efficiency of burrowing is made possible by change in the shape of the organism in individual compartments [segments] of the hydrostatic skeleton.
The evolutionary significance of coelom is that it acted as a hydrostatic skeleton in burrowing animals. The contraction of the muscles in one part of the animal could act antagonistically on muscles in another part of the body by transmitting the force of contraction through the coelom.
Leeches are popularly parasitic, but many are predaceous.
Most freshwater leeches are active predators or scavengers which are equipped with a proboscis that can be extended to ingest small invertebrates or to draw blood from cold-blooded vertebrates.
Some can force their pharynx or proboscis into soft tissues like gill of fish. Some terrestrial leeches feed on insect larvae, earthworms, and slugs, to which they hold by an oral sucker while using a strong sucking pharynx to ingest food. Other terrestrial forms climb bushes or trees to reach warm-blooded vertebrates like mammals or birds.
Most leeches are fluid feeders. They prefer to feed on tissue fluids and blood pumped from open wounds. Some freshwater leeches are true bloodsuckers, preying on cattle, horses, humans, and other mammals.
True bloodsuckers, like Hirudo medicinalis , have cutting plates, or chitinous "jaws", for cutting through tough skin.