Phylum Annelida: Segmented Worms

Vocabulary practice flashcards covering the taxonomy, body plans, physiological adaptations, and major classes (Polychaeta, Oligochaeta, Hirudinea) of Phylum Annelida.

Phylum Annelida

A major group of triploblastic, coelomate, protostome invertebrates characterized by segmented bodies and a hydrostatic skeleton.

Metamerism

The repetition of homologous body segments that increases burrowing efficiency and allows independent contraction and movement of individual segments.

Hydrostatic Skeleton

A fluid-filled true coelom that provides structural support, transmits muscular pressure during locomotion, and serves as a medium for circulatory transport.

Setae

Chitinous, bristle-like rods embedded in sacs with tiny muscles that anchor body segments during locomotion to prevent backward slipping.

Septa

Internal membranous partitions that separate the coelomic cavity into distinct compartments between successive segments (absent in leeches).

Nephridia

Paired, kidney-like excretory tubular organs situated in each segment that eliminate metabolic wastes through external pores.

Nephridiopore

An external excretory opening located near the setae through which waste collected by the nephridial bladder is expelled.

Closed Circulatory System

A circulatory system in which blood is fully enclosed within vessels; blood flows anteriorly in the dorsal vessel and posteriorly in the ventral vessel.

Cerebral Ganglia

A pair of dorsal ganglia situated anteriorly that function as the primitive annelid brain, connected to the double ventral nerve cord.

Trochophore

A free-swimming, ciliated larval stage produced during the reproductive cycle of certain annelids, primarily marine polychaetes.

Class Polychaeta

A large class of mostly marine annelids (>10,000 species) characterized by a distinct head with eyes and tentacles, and numerous setae borne on lateral parapodia.

Parapodia

Paired, lateral paddle-like appendages found on most polychaete segments that bear clusters of setae, assist in locomotion, and function in gas exchange as gills.

Scale Worms

Carnivorous polychaetes featuring flattened bodies covered by broad, shield-like scales.

Fireworms

Marine polychaetes that feed on cnidarians and possess hollow, brittle setae packed with poison for defense.

Class Oligochaeta

A class of primarily terrestrial and freshwater annelids (>3,000 species), such as earthworms, possessing few setae per segment, no parapodia, and no distinct head.

Clitellum

A thickened, non-segmented glandular section of the body wall in oligochaetes and leeches that secretes a mucous cocoon for eggs and sperm during reproduction.

Direct Development

A life cycle seen in earthworms where juvenile worms hatch directly from a protective cocoon without passing through a free-living larval stage.

Castings

Soil and undigested organic matter mixed with digestive mucus that earthworms egest through the anus, enriching soil fertility and improving texture.

Class Hirudinea

A class of primarily freshwater annelids (leeches) lacking both setae and internal septa, adapted as predators or blood-sucking ectoparasites.

Hirudin

A chemical anticoagulant secreted by blood-feeding leeches that prevents host blood from clotting while they feed.