Unlike roundworms, the segmented worms have an obvious head and tail. All segmented worms have one of two types of structures extending from their bodies that are used for locomotion. Most annelids have hair-like bristles known as setae and some have fleshy protrusions called parapodia . The earthworm belongs to this class and has two pair of setae on each segment. These marine animals have many setae projecting from parapodia.
Polychaetes also differ from other annelids in that they have antennae and specialized mouth parts.
Most leeches live in calm, fresh water and some live in moist vegetation on land, but none live in salt water. Many leeches are carnivores that feed on small invertebrates, but some species are parasites that suck blood from other animals. Arthropoda Three-fourths of all animal species, including spiders, crayfish, and insects , are arthropods. This phylum is composed of a very diverse group of animals that are bilaterally symmetrical. It is composed of three layers that are secreted by the epidermis.