Leeches: Hirudinea

Most leeches are flattened from top to bottom and measure
0.196 to 0.787 inches (5 to 20 millimeters) in length. The longest species may reach 17.72 inches (450 millimeters). Leeches have eyespots on the head that are able to detect movement from contrasting patterns of light and shadow. The mouth is located underneath the head and is surrounded by a sucker. The sucker helps draw blood and other bodily fluids into the mouth. Like earthworms, leeches have a clitellum (KLAI-teh-lum), a specialized collarlike band behind the head. The clitellum is filled with special tissues that secrete a protective covering, or case, for the eggs. Leeches are not covered with stiff bristles nor do they have paddlelike flaps. To gain traction, leeches have suckers at the front and rear of their bodies. The tail sucker is used for swimming, getting around, or remaining attached to a host. Blood-feeding species have special pouches that allow them to increase their intake of fluid. They can expand up to six times their normal weight.