Polychaetes are segmented worms. The number of segments vary with the individual species. A typical polychaete segment has two foot-like attachments called parapodia, which give the animal locomotive powers and in some species function as gills. A number of bristles, or setae, are attached to each parapodium. These hair-like bristles gave the polychaete its name. Polychaete literally means "many hairs." Most polychaetes live in the sea. Many species prefer the sandy bottom, but some float or swim about in the midst of the water. Some live close to shore, others at great depth. Some species dwell in tubes from which they extend fanlike projections to capture food. In the species Sabella pavonia, this fanlike projection is so colorful that this polychaete has earned the common name "peacock worm." Sabella pavonia is unusually large for a polychaete. It has as many as 600 segments and grows up to 30 centimeters long. In contrast, the species Filograna implexa grows to only a few millimeters in length.
Leeches also have segmented bodies, but their segmentation is not as prominent as that of the polychaetes. Only a small minority of their species live in sea water. Some marine leeches attach themselves to turtles. They spend their entire lives on their host, even when mating and depositing their cocoons. Other species parasitize fish, usually attaching themselves in the neighborhood of the gills. In contrast to the turtle parasites, some fish parasites leave their host after feeding while others leave their host only when it is necessary to find a place to deposit their cocoons. Leeches have both male and female organs, so all eventually leave to deposit their cocoons.
Some marine flatworms are parasitic flukes, but most are free-living species that swim around in the ocean and prey upon small creatures. Flatworms have no segments and no internal body cavity. They have no gills or blood vessels, and their bodies are so flat that they can squeeze between the shells of a tightly-closed clam and begin to eat. They do not have a complete alimentary canal, and one opening serves for both ingestion and elimination. Some species are toxic, and their bright coloration warns predators not to eat them.
Most species of ribbon worms are marine animals. They are flat, but unlike the flatworms, they have blood vessels and a complete alimentary canal. Some species have a sharp beak called a stylet. For this reason, these animals are also called "proboscis worms."
Roundworms have unsegmented bodies. They typically have cylindrical bodies that taper to a point at both ends. Some roundworms are marine parasites. As the roundworm species Anisakis simplex and Phocanema decipiens mature, they successively parasitize a crustacean, a fish and finally a marine mammal. However, there are also many free-living roundworms in sea water. The genus Sabatieria even survives in the frigid waters near Antarctica.