Annelids
Segmented Worms
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Phylum Annelida
Characteristics

• Includes duster worms, earthworms, & leeches
• Abundant in all habitats
• Have a true coelom fully lined with mesoderm
• Body divided into external segments called metameres (metamerism)

• Metameres correspond to internal segments
• Have a one-way digestive system with a mouth & anus
• Well developed brain & sensory organs
• Fluid-filled coelom provides hydrostatic skeleton
• Most have external bristles or setae that aid movement
• Setae may be modified into flashy appendages called parapodia

PARAPODIA

• Includes 3 classes based on number of setae & presence or absence of parapodia

• Classes of segmented worms --- Oligochaeta, Polychaeta, & Hirudenia

Class Oligochaeta
Characteristics

• Have no parapodia & few setae

• Includes earthworms

EARTHWORM

•  Bodies may have over 100 metameres

• Internal partitions called septa

• Distinct anterior & posterior ends

• Cephalization (head with sense organs) shows specialization for burrowing

• Have both circular & longitudinal muscles for movement

• Have external, saddle-shaped structure called clitellum that forms a cocoon containing eggs & sperm

• Prostomium or lip digs through soil as earthworm feeds on organic matter

• Pharynx is a muscular organ behind the mouth to help suck in food

• Food temporarily stored in crop, ground in gizzard, and digested & absorbed in intestine

• Wastes called castings pass out through anus

• Closed circulatory system with 5 pairs of aortic arches or hearts

• Dorsal blood vessel carries blood posteriorly to cells & ventral blood vessel returns blood anteriorly

• Secrete mucus to keep skin moist so oxygen will dissolve & diffuse into body

• Long tubules called nephridia filter wastes from blood & excrete it through pores

• Simple brain, no eyes, & dorsal and ventral nerve cords

• Sensitive to light, touch, moisture, chemicals, temperature, & vibrations

• Hermaphrodites exchange sperm & cross-fertilize

• Sperm sacs store the worm's own sperm & seminal receptacles store exchanged sperm

Class Hirudenia
Characteristics

• No setae or parapodia

• Includes leeches

• Have anterior & posterior suckers for attachment

LEECH

• Some suck blood from hosts, while others are scavengers or predators

• Mouth's of blood-sucking leeches with chitinous teeth & secrete anticoagulant

• Found in freshwater

• Flattened dorso-ventrally

• Hermaphrodites that cross-fertilize

Class Polychaeta
Characteristics

• Marine

• Includes sandworms & clamworms

• Have paddle-like parapodia to move

• Take in oxygen through parapodia

• Some are free-swimming predators with strong jaws to feed on small animals

• Many live commensally with sponges, mollusks, & echinoderms

• Well-developed head with antenna & specialized mouthparts

SANDWORM