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Annelida
• Paleontologists believe in the pre-Cambrian era the only animals on Earth were sponges, cnidarians and ancestral bilateral worms.
• A group of animals called annelid worms developed during the Cambrian
Explosion. Today, about 15,000 species of annelids exist including earthworms, marine bristle worms, and leeches.
• Scientists believe that burrowing worms play a vital role in maintaining life on Earth by recycling plant and animal remains into carbon dioxide gas.
This gas helps modify the climate of the biosphere.
• Before active burrowers appeared, organic remains became buried in sediments and depleted the atmosphere of carbon dioxide.
• With sufficient carbon dioxide in the air, land plants can thrive and the oceans remain free of ice across much of the planet.
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Phylum Annelida
• Segmented body
– metameric
• Setae
– Bristles or hairs
– Absent in leeches
• Coelom divided by septa
• Closed circulatory system
• Nephridia for each segment (metamere)
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Segmented Worms
• Body divided into repeating segments
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Sedentary- don’t move
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Terrestrial
• Earthworm
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• Leech
Feed on Blood
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Echinodermata
Vertebrata
Lophophores
Other Chordata
Hemichordata
Uniramia
Arthropoda
Chelicerata
Crustacea
Annelida
Mollusca
Other pseudocoelomates
Nemertea
Nematoda
Platyhelminthes
Ctenophora
Cnidaria
Placozoa
Mesozoa
Sarcomastigophora
Ciliophora
Apicomplexa
Microspora
Porifera
Myxozoa
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Mouth
Trochophore larva
Apical tuft
Stomach
Ciliary band
Anus
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Annelid development
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Phylum Annelida Body Plan
• Cylindrical, bilateral, & segmented
• True coelom
• Complete digestive system
• Closed circulatory system

Phylum Annelida Body Plan
• Annelids have a true body cavity or coelom that is located between layers of mesoderm
• Annelids are the first major phyla showing segmentation (metamerism) which is advantageous to movement, safety, and tagmatization (specialization of body regions)

Open
Circulatory Systems
Closed- Annelids
Annelids have 5 hearts
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Feeding and Digestive System
• Annelids have a complete digestive system
• Their system contains mouth, pharynx, esophagus, crop, gizzard, intestine and anus
• They use muscles to help in the mechanical digestion of food

Digestive System
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Metamerism
Pygidium
Septa
Prostomium
Peristomium
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Movement
• Each annelid segment contains its own longitudinal and circular muscles
• These muscles work antagonistically, such that when the longitudinal muscles contract the circular muscles relax and the segment become short and thick
• In contrast, when the circular muscles contract, the longitudinal muscles relax and the segment becomes long and thin

Reproduction
• Class Oligochaeta are hermaphroditic and during copulation they line up facing in the opposite direction
• The clitellum secretes a
Oligochaeta mucous that holds the worms in place
• Later, the clitellum acts as a cocoon where the fertilized egg develops

Classification
• Class Polychaeta (many hairs)
• Class Oligochaeta (few hairs)
• Class Hirudinae (leeches)

Class Polychaeta
• Hairy worms
• Many setae
– Hairs or bristles made of chitin
• Parapodia
– Paired projections on each segment
– Like feet or flippers
– Used for locomotion
• Mostly marine
• 1mm – 3m in length
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