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The Annelida Major Characteristics • Animal Characteristics • Cylindrical, Bilateral, & Segmented • Triploblastic • Eucoelomate • Complete Digestive System • Closed Circulatory System Classification • Class Polychaeta (many hairs) • Class Oligochaeta (few hairs) • Class Hirudinae (Leeches) Body Plan • Annelids contain a true cavity or coelome that is located between layers of mesoderm • They are the first major phyla that showing segmentation (metamerism) which is advantageous to movement, safety, and tagmatization Movement • Each annelid segment contains its own longitudinal and circular • These muscle work antagonistically, such that when the longitudinal muscles contract the circular muscles relax and the segment become short and thick • In contrast, when the circular muscles contract, the longitudinal muscles relax and the segment becomes long and thin Feeding • Annelids have a complete digestive system and are protostomes (mouth forming first) • Their system contains mouth, pharynx, esophagus, crop, gizzard, intestine and anus • They use muscle to help in the mechanical digestion of food Reproduction • Oligochaetes are monoecious and during copulation they line up facing in the opposite direction from each other • The clitellum secretes a mucous that holds the worms in place • Later the clitellum acts as a cocoon where the fertilized egg develops