The order Annelida is divided into 3 classes.
1. Polychaeta: Their body parts are covered with hairs. locomotion parapodia. Sensory organ is nucell organ. They are mostly marine. For example, Nereis caudata.
2. Oligochaeta: These are leech-like creatures. They live by digging holes. For example, Metaphire posthuma.
3. Hirudinia: They have clitellum. There is a substance called hirudin in the body. For example, Hirudinaria medicinalis.