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