Segmented Worms

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.