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CHAPTER 17

CHAPTER 17. Annelids. Characteristics. Diversity Annelids exhibit segmentation or metamerism Each unit contains components of most organ systems Evolution of metamerism allowed: Increased burrowing efficiency more sophisticated nervous system Provided a safety factor

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CHAPTER 17

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  1. CHAPTER 17 Annelids

  2. Characteristics Diversity • Annelids exhibit segmentation or metamerism • Each unit contains components of most organ systems • Evolution of metamerism allowed: • Increased burrowing efficiency • more sophisticated nervous system • Provided a safety factor • If one segment failed, others could still function

  3. Characteristics Classification • Class Oligochaeta (Eathworms) • Class Hirudinida (leeches)

  4. Phylum Annelida Characteristics • About 15,000 species • Coelomates - 1st phyla to have true coelom • Platyhelminthes = acoelomate • Nematoda = pseudocoelomate Ectoderm Mesoderm Endoderm

  5. Phylum Annelida • Triploblastic Protostome • Complete Gut • Platyhelminthes - incomplete • Nematodes- complete • Have a Circulatory System - closed • Heart-like structure : aortic arches • Other worms - no circulatory system • Setae (se’-tah) • Tiny bristles called setae • Absent in leeches • Short setae anchor segments in earthworms • Prevent it from slipping backward • Long setae help aquatic worms swim

  6. Phylum Annelida Body Plan • Body Wall • Head is composed of • Prostomium • Terminal portion bearing the anus is the pygidium • Head and pygidium are not considered metameres (segments) • New metameres form in front of the pygidium • Surface is covered with an epidermis and a thin outer layer cuticle

  7. Marine Annelids

  8. Coelomates

  9. Phylum Annelida Phylogeny • Traditionally, annelids are divided among 3 classes • Class Polychaeta (marine) • Class Oligochaeta (earthworms) • Class Hirudinida (leeches) • Oligochaeta and Hirudinida • Characterized by reproductive structure called a clitellum • Clade clitellata

  10. Clade Clitellata • Class Oligochaeta and Class Hirudinida • Form reproductive structure called a clitellum • Permanent in oligochaetes but visible only during reproductive season in leeches • Hermaphroditic (monoecious) animals • Young develop inside a cocoon secreted by the clitellum, and emerge as small worms

  11. Clade Clitellata Class Oligochaeta- earthworm • Diversity • Over 3000 species • Nearly all have setae (bristle like hairs) • Form and Function • Sometimes called “night crawlers” • Burrow in moist rich soil and usually live in branched interconnected tunnels • Damp, rainy weather: Remain near surface • Dry weather: Burrow deep underground and go dormant coiled in a slime chamber

  12. Clade Clitellata Nutrition • Scavengers, feeding on decayed organic matter, leaves, etc. • Food moistened by mouth and drawn in by a sucking action of muscular pharynx • Food is stored in a thin-walled crop • Muscular gizzard grinds food into small pieces • Digestion and absorption occur in intestine

  13. Clade Clitellata • Circulation and Respiration • Coelomic fluid and blood transport food, wastes, and respiratory gases • Blood circulates in a closed system • Dorsal vessel and Ventral vessel present. • Heart consists of 5 pairs of aortic arches • No special gaseous exchange organs • Gas exchanged across body surface • Excretion • Each segment, except the 1st three, have a pair of metanephridia (excretory organ) - releases waste into nephridiopore

  14. Excretory System

  15. Nervous System: Brain, Nerve Cord, Peripheral

  16. Nerve Cord

  17. Clade Clitellata • Reproduction and Development • Monoecious (hermaphroditic) • Reproductive structures are located in segments 9 through 15 • Sperm produced by testes mature in seminal vesicles and pass into sperm ducts • Eggs are discharged by ovaries • Two pairs of seminal receptacles receive and store sperm

  18. Clade Clitellata • Mate at night during warm, moist weather • Aligning in opposite directions, ventral surfaces together • Mucus secreted by clitellum holds worms together • Sperm from each worm are transported to the seminal receptacles of the other along seminal grooves • After mutual copulation, each worm secretes a mucus tube to form a cocoon • Cocoon passes forward and eggs, and sperm are added • Fertilization and embryogenesis occur in cocoon • Young worms emerge from cocoon

  19. Clade Clitellata Class Hirudinida: Leeches • Hirudinea • 34 segments, lack setae but possess anterior and posterior suckers • Diversity • Most freshwater, few marine or live in moist terrestrial environments • More common in the tropics temperate zones • Vary in color: black, brown, red, and olive green • Some carnivores feeding on small invertebrates • Others are temporary or permanent parasites • Hermaphroditic • Form a clitellum during breeding season • Secretes a cocoon for reception of eggs

  20. Largest Leech Gut specialized for storage of large quantities of blood

  21. 2 suckers: Posterior and Anterior

  22. Clade Clitellata • Nutrition • Not all are parasites, many are predators • Freshwater leeches have a proboscis (pharynx) for ingesting small invertebrates as well as to suck blood • Some terrestrial leeches feed on insect larvae, earthworms, and slugs • Other terrestrial leeches climb trees or bushes to reach warm-blooded vertebrates such as baby birds • Most are fluid feeders that prefer tissue fluids and blood pumped from open wounds

  23. Medicinal Leech: Used to relieve congestion in the veins due to injury.

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