Section 2

Annelida

• Annelids have a true coelom and a body that is divided into many segments. Most annelids have external bristles called setae, and some have fleshy protrusions called parapodia.

• The number of setae and the presence or absence of parapodia provide the basis for dividing annelids into three classes: Oligochaeta, Polychaeta, and Hirudinea.

• Segmentation enables the different parts of the body to carry out various functions independently. It also protects the organism in case of injury because uninjured segments can continue to function.

• Members of the class Oligochaeta generally live in the soil or in fresh water. They have no parapodia and relatively few setae.

• The most familiar member of the class Oligochaeta is the earthworm, which feeds on organic matter as it burrows through the soil. Earthworms have a closed circulatory system. They exchange gases through their skin and eliminate cellular wastes and excess water through excretory tubules called nephridia.

• Polychaetes have numerous setae that project from parapodia. They also have antennae and specialized mouthparts, and they pass through a trochophore stage during their development. Most polychaetes live in the ocean.

• Members of the class Hirudinea, leeches live in fresh water or on land. They have no setae or parapodia. Many leeches are carnivores that prey on small invertebrates,

Vocabulary seta (p. 713) parapodium (p. 713) crop (p. 714)

gizzard (p. 714) typhlosole (p. 714) aortic arch (p. 715)

nephridium (p. 715) clitellum (p. 715) seminal receptacle (p. 715)

chitin (p. 715)

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