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Segmented Worms

Segmented Worms. Chapter 17. Annelids and Allied Taxa. Phylum Annelida Class Polychaeta Class Oligochaeta Class Hirudinida Phylum Echiura Phylum Sipuncula. Annelids and Allied Taxa. Members of phyla Echiura and Sipuncula are benthic marine animals with unsegmented bodies.

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Segmented Worms

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  1. Segmented Worms Chapter 17

  2. Annelids and Allied Taxa • Phylum Annelida • Class Polychaeta • Class Oligochaeta • Class Hirudinida • Phylum Echiura • Phylum Sipuncula

  3. Annelids and Allied Taxa • Members of phyla Echiura and Sipuncula are benthic marine animals with unsegmented bodies. • Molecular sequence data place echiurans within phylum Annelida. • Echiurans - Sister taxon to Annelida. • Sipunculans - Sister taxon to a clade composed of Annelida and Echiura.

  4. Phylum Annelida • Annelids are protostome coelomates in superphylum Lophotrochozoa. • Spiral, determinate cleavage. • Nervous system more centralized & circulatory system more complex than in previous phyla.

  5. Phylum Annelida • Annelids are segmented worms. • They have bodies composed of a series of fused rings. • Earthworms, leeches, clam worms.

  6. Phylum Annelida • The evolutionary innovation shown by annelids is segmentation (metamerism). • Segmentation evolved separately in annelids, arthropods, and chordates. • The body is divided into a series of segments, each having similar components of all major organ systems. • Built in fail-safe. • Allows for specialization.

  7. Phylum Annelida • Many annelids have chitinous bristles called setae. • Help in locomotion • Anchor worm in place • Deter predators

  8. Phylum Annelida • Annelids can be found worldwide in marine, freshwater, and terrestrial habitats.

  9. Phylum Annelida – Body Plan • Prostomium – anterior part followed by segmented body. • Pygidium – terminal portion.

  10. Phylum Annelida – Body Plan • Peritonia (layers of mesodermal epithelium) of adjacent segments meet to form septa. • Fluid-filled coelom acts as a hydrostatic skeleton.

  11. Phylogeny • Traditionally, annelids are divided among 3 classes: • Class Polychaeta • Class Oligochaeta • Class Hirudinida • Polychaeta is a paraphyletic class because ancestors of the clitellates arose from within it. • Oligochaeta and Hirudinida form a monophyletic group called Clitellata. • Characterized by reproductive structure called a clitellum. • Class Oligochaeta is a paraphyletic group because ancestors of leeches arose from within it.

  12. Class Polychaeta • Polychaeta is the largest, most diverse class. • May be brightly colored, variable shape.

  13. Class Polychaeta • Polychaetes have some features other annelids do not: • A well developed head. • Paired appendages, parapodia, that function as gills and aid in locomotion. • No clitellum. • Many setae

  14. Class Polychaeta • Polychaetes are mostly marine and mostly benthic. • May live under rocks, burrow into sediment, or build their own tubes. • Some are planktonic.

  15. Class Polychaeta • Sedentary and errant (free-moving) forms. • Sedentary forms often have elaborate devices for feeding and respiration. • Filter or deposit feeders.

  16. Class Polychaeta • Errant forms include pelagic and benthic types and are often predators or scavengers.

  17. Class Polychaeta - Reproduction • Gonads are temporary structures in polychaetes. • Sexes usually separate. • Fertilization is external. • Early larva is a trochophore.

  18. Circulation and Respiration • Most have parapodia and gills for gaseous exchange. • Others use the body surface. • Circulation varies. • In Nereis a dorsal vessel carries blood forward and a ventral vessel carries blood posteriorly. • Blood flows across between these major vessels in networks around the parapodia and intestine. • In some, septa are incomplete and coelomic fluid serves circulatory function. • Many polychaetes have respiratory pigments - Hemoglobin, chlorocruorin or hemerythrin.

  19. Excretion • Excretory organs vary, from protonephridia to metanephridia, and mixed forms. • One pair per metamere. • Inner end (nephrostome) opens into the coelomic cavity. • Coelomic fluid enters the nephrostome. • Selective resorption occurs along the nephridial duct.

  20. Nervous System and Sense Organs • Double ventral nerve cord runs length of the worm with ganglia in each metamere. • Sense organs include: • Eyes, nuchal organs and statocysts. • Eyes vary from simple eyespots to well-developed image-resolving eyes similar to mollusc eyes. • Nuchal organs are ciliated sensory pits that are probably chemoreceptive. • Some burrowing and tube-building polychaetes use statocysts to orient their body.

  21. Representative Polychaetes • Clam Worms: Nereis • Errant polychaetes • Live in mucus-lined burrows near low tide level. • Come out of hiding places at night to search for food. • Prostomium bears a pair of palps sensitive to touch and taste, a pair of short sensory tentacles, and two small dorsal eyes sensitive to light. • Peristomium has a ventral mouth, a pair of jaws, and four pairs of sensory tentacles.

  22. Representative Polychaetes • Scale worms • Flattened bodies are covered with broad scales. • Some are large, all are carnivores and some are commensals in burrows of other organisms.

  23. Representative Polychaetes • Fireworms • Have hollow, brittle setae that contain poisonous secretions. • Feed on cnidarians.

  24. Representative Polychaetes • Tubeworms • Tube-dwellers • May line their burrows with mucus • Use cilia or mucus to obtain food

  25. Representative Polychaetes • Fanworms or Featherduster worms • Unfurl tentacular crowns to feed. • Food moved from radioles to mouth by ciliary action.

  26. Representative Polychaetes • Parchment Worms • Lives in a U-shaped tube. • Modified segments pump water through tube.

  27. Clade Siboglinidae (Pogonophorans) • Formerly members of phylum Pogonophora (beardworms). • Discovered in 1900. • 150 species described. • Most are small, less than 1 mm in diameter. • Giant beardworms that live in deepwater hydrothermal vents are 3 m long and 5 cm in diameter.

  28. Clade Siboglinidae (Pogonophorans) • Most live in mud on ocean floor at depths of 100 to 10,000 m. • Sessile animals that secrete and live in long chitinous tubes. • Tubes have general upright orientation in bottom sediments. • Tubes are generally three or four times the length of the animal. http://www.youtube.com/watch?v=2FFnrW_SUdM

  29. Clade Siboglinidae (Pogonophorans) • Long cylindrical body covered with cuticle. • Divided into a short anterior forepart, a long slender trunk, and a small, segmented opisthosoma. • Tentacles are hollow extensions of the coelom and bear minute pinnules.

  30. Clade Siboglinidae (Pogonophorans) • No mouth or digestive tract. • Nutrients such as glucose and amino acids absorbed from seawater through pinnules and microvilli of tentacles.

  31. Clade Siboglinidae (Pogonophorans) • Most energy derived from a mutualistic relationship with chemoautrophic bacteria that oxidizes hydrogen sulfide. • Trophosome, derived embryonically from midgut, houses the bacteria.

  32. Clade Siboglinidae (Pogonophorans) • Sexes are separate. • Research suggests that cleavage is unequal and atypical. • Appears to be spiral. • Coelom formed by schizocoely. • Embryo • Worm-shaped and ciliated. • Poor swimmer. • Probably carried by water currents until it settles.

  33. Osedax – bone eating worms http://www.youtube.com/watch?v=URi8KccVkks

  34. Clade Clitellata • Class Oligochaeta and Class Hirudinida • Form reproductive structure called a clitellum. • Ring of secretory cells found in a band around the body. • Permanent in oligochaetes but visible only during reproductive season in leeches. • Members are derived annelids that lack parapodia. • Hermaphroditic (monoecious) animals that exhibit direct development. • Young develop inside a cocoon secreted by the clitellum, and emerge as small worms.

  35. Class Oligochaeta • Class Oligochaeta includes earthworms and many freshwater worms. • They possess setae, but not as much as polychaetes.

  36. Class Oligochaeta • Earthworms are the most familiar oligochaetes, found in moist, rich soil. • They can burrow deep underground and remain dormant in a slime chamber during dry weather. • Setae help prevent slipping while burrowing.

  37. Class Oligochaeta • Darwin wrote about earthworms in The Formation of Vegetable Mould Through the Action of Worms. • He noted the beneficial activities of worms, aeration, moving nutrients up from subsoil, adding nitrogenous products, breakdown of organic matter in dead leaves etc. • An earthworm can ingest its own weight in soil every 24 hours.

  38. Class Oligochaeta - Reproduction • Earthworms are hermaphroditic – male and female organs in the same animal. • When mating, two worms are held together by mucus secreted by the clitellum.

  39. Class Oligochaeta - Reproduction • After mating, a cocoon forms around the clitellum, as it passes forward it gathers both gametes, and fertilization occurs inside.

  40. Class Oligochaeta - Reproduction • Development occurs inside the cocoon and young worms hatch out. • Development is direct, no larval stage.

  41. Class Oligochaeta - Feeding • Food is stored in a thin-walled crop. • Muscular gizzard grinds food into small pieces. • Digestion and absorption occur in intestine.

  42. Class Oligochaeta - Excretion • Each segment, except the 1st three and terminal one, have a pair of metanephridia. • A ciliated funnel, the nephrostome, draws in wastes and leads through the septum. • These coil until the nephridial duct ends at a bladder that empties outside at nephridiopore. • Wastes from both the coelom and the blood capillary beds are discharged. • Aquatic oligochaetes excrete toxic ammonia.

  43. Class Oligochaeta - Circulation and Respiration • Coelomic fluid and blood transport food, wastes, and respiratory gases. • Blood circulates in a closed system with five main trunks running lengthwise in the body. • Dorsal vessel contains valves and functions as a true heart. • Pumps blood anteriorly into 5 pairs of aortic arches. • Aortic arches ensure steady pressure in ventral vessel.

  44. Class Oligochaeta - Nervous System and Sense Organs • Central nervous system and peripheral nerves. • Pair of cerebral ganglia connect around the pharynx to the ganglia of the ventral nerve cord. • Neurosecretory cells in brain and ganglia secrete neurohormones. • Regulate reproduction, secondary sex characteristics, and regeneration. • Lack eyes but have many photoreceptors in the epidermis. • Free nerve endings in tegumentare probably tactile structures.

  45. Class Oligochaeta - General Behavior • Avoid bright light (negative phototaxis). • Chemical stimuli are important in locating food. • Limited learning ability - primarily trial-and-error learning.

  46. Class Hirudinida • Class Hirudinida includes the leeches. • Primarily freshwater, a few marine & terrestrial. • More common in tropical climates.

  47. Class Hirudinida • Many leeches live as carnivores on small invertebrates. • Some are temporary parasites. • Some are permanent parasites – they never leave their host.

  48. Class Hirudinida • Leeches are hermaphroditic and have a clitellum (only appears during breeding season), like oligochaetes. • Leeches do not have setae. • They’ve developed suckers for attachment and a specialized gut for storing large amounts of blood.

  49. Class Hirudinida - Respiration and Excretion • Some fish leeches have gills. • All other leeches exchange gases across epidermis. • 10 to 17 pairs of nephridia. • Coelomocytes and other special cells may assist in excretion.

  50. Class Hirudinida - Nervous and Sensory Systems • Two “brains” • Anterior fused ganglia form a ring around the pharynx. • Seven pairs of posterior fused ganglia. • 21 pairs of segmental ganglia in between along a double nerve cord. • Epidermis contains free sensory nerve endings and photoreceptor cells. • Pigment-cup ocelli are present.

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