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Annelida: Understanding the Segmented Body Structure and Characteristics of Worms

The Annelida phylum encompasses segmented worms believed to have originated in the sea, with two-thirds of species still residing in marine environments. These organisms can range in size from 1 mm to 3 meters and show remarkable diversity. Each segment includes essential organ systems for digestion, excretion, and locomotion. Unique to annelids are the closed circulatory systems, coeloms, and specialized structures like setae and parapodia. Classification is based on the number of setae and the presence of parapodia, leading to groups like Polychaeta, Oligochaeta (earthworms), and Hirudinea (leeches).

bradley-manning
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Annelida: Understanding the Segmented Body Structure and Characteristics of Worms

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  1. Chapter 13 Annelida: The Merametric Body Form

  2. Phylum Annelida • Segmented worms believed to have evolved in the sea where two-thirds still live • Other one third are terrestrial • Fossils found up to 530 million years old • Range from 1 mm to 3 M long

  3. Characteristics of Annelids • Each segment contains digestive, excretory, circulatory, and locomotor organs while some are modified for reproduction, feeding, or sensation • Anterior segment contains a cerebral ganglion (primitive brain) which is connected to a ventral nerve cord that runs along the underside of the body • Delivers sensory info. To brain

  4. Characteristics of Annelids • Septa- Internal body walls separate the segments through which nutrients and other materials pass • Coelom- Large, fluid-filled cavity located completely within mesoderm

  5. Characteristics of Annelids • Organ systems include: • Closed circulatory system • Excretory structures called nephridia • Different regions of gut for different functions • Setae- External bristles paired on each segment provide traction for crawling • Found on most • Parapodia- Fleshy appendages on some

  6. Annelid Groups • Two characteristics used to classify Annelids: • Number of setae on each segment • Presence of parapodia • Marine Worms • Earthworms • Leeches

  7. Marine Worms • Class Polychaeta- Parapodia with numerous setae • Largest group; live in ocean; eyes • Distinctive pair of paddle-like parapodia on each segment Ex: Nereis • Many burrow or are free-swimming, others live in protective tubes formed by gland secretions • Ex: Feather-dusters; tubes

  8. Class Clitellata- No parapodia; few to no setae • Subclass Oligochaeta- Earthworms and some freshwater worms • No parapodia and few setae on each segment • No head region, nor eyes • Aortic arch- 5 tube-like hearts • Light sensitive and touch sensitive organs at each end; sensory cells detect moisture • Consume soil with pharynx which travels through one-way gut • Crop-gizzard-intestine-anus • Waste is called casting which fertilizes soil

  9. Earthworms • Fluid of coelem creates a hydrostatic skeleton • Each segment has circular and longitudinal muscles which pull against this skeleton • Alternately contract the two sets of muscles to move • Clitellum- Contains male and female gametes; develops into cocoon after fertilization • Seminal vescicles- store earthworm’s own sperm • Seminal receptacles- Store the sperm of another worm

  10. Class Clitellata • Subclass Hirudinea- Leeches • eye spots • Lack setae and parapodia • Flattened body; segments not separated internally • Suckers at both ends of its body • Most species are predators or scavengers; some are parasites

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