Leeches are in Phylum Annelida (segmented worms, including earthworms). They have a sucker at each end. Leeches usually move by attaching alternate suckers and looping, although some species swim with an S-shaped movement (as in the photograph). Most species feed on vertebrates, but some are snail parasites. Many leeches secrete an anesthetic so that the host does not feel the bite, and an anti-coagulant to make the blood keep flowing. A leech can take in ten times its body weight of blood at one feeding, which allows it to feed infrequently.