Characteristics Of Annelids

The phylum Annelida consists of about 15,000 species of bilaterally symmetrical, segmented worms. Segmentation is the most distinctive feature of annelids. Like mollusks, annelids have a true coelom, but the coelom in annelids is divided into separate compartments by partitions. Division of the coelom represents an evolutionary advance over the earliest wormlike coelomates. In an undivided coelom, the force of muscle contraction in one part of the body is transmitted to other parts by the fluid in the coelom. A segmented coelom enables different parts of the body to contract or expand independently. In addition, duplication of some of the organ systems in each segment provides a form of insurance against injury. If one segment becomes disabled, the others can still function.

Most annelids have external bristles called setae (SEET-ee) (singular, seta), and some have fleshy protrusions called parapodia (PAR-uh-POH-dee-uh) (singular, parapodium). Both of these structures are visible in Figure 35-10. The number of setae and the presence or absence of parapodia provide the basis for dividing annelids into three classes: Oligochaeta (AHL-uh-goh-KEET-uh), Polychaeta (PAHL-i-KEE-tuh), and Hirudinea (HlR-yoo-DIN-ee-uh). All organ systems are well developed in most members of each class.

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