27-3 Annelids

Septum

• Internal wall between the segments of an annelid’s body

Seta

• Bristle attached to the segments of many annelids

crop

• In earthworms, part of the digestive system in which food can be stored; in birds, structure at the lower end of the esophagus in which food is stored and moistened.

gizzard

• In earthworms, part of the digestive system in which food is ground into smaller pieces; in birds, a muscular organ that helps in the mechanical breakdown of food

Closed circular system

• System in which blood is contained within a network of blood vessels.

Gill

• Filamentous organ in aquatic animals specialized for the exchange of gases with water.

nephridium

• Excretory organ of an annelid that filters fluid in the coelom

Clitellum

• Band of thickened, specialized segments in annelids that secretes a mucous ring into which eggs and sperm, are released.

Key Concept

• Annelids are worms with segmented bodies. They have a true coelom that is lined with tissue derived from mesoderm.

Key Concept

• Oligochaetes are annelids that typically have stream-lined bodies and relatively few setae compared to polychaetes. Most oligochaetes live in soil or fresh water.

Key Concept

• Leeches are typically external parasites that suck the blood and body fluids of their host.

Key Concept

• Polychaetes are marine annelids that have paired, paddlelike appendages tipped with setae.