CHAPTER 17

1. CHAPTER 17 Annelids

2. Characteristics Diversity • Annelids exhibit segmentation or metamerism • Bodies composed of serially repeated units • Each unit contains components of most organ systems • Evolution of metamerism allowed much greater complexity in structure and function • Increased burrowing efficiency • more sophisticated nervous system • Provided a safety factor • If one segment failed, others could still function

3. Characteristics • Segmented worms living in marine, freshwater, and moist terrestrial habitats • Include marine bristle worms, leeches, and earthworms Classification • Class Polychaeta (scale, fire, tube worms) • Class Oligochaeta (Eathworms) • Class Hirudinida (leeches)

5. Phylum Annelida Characteristics • About 15,000 species • Coelomates - 1st phyla to have tru coelom • Segmentation • Body segments marked by circular grooves called annuli • Metamerism • Repetition of organs in segments called metameres or somites • Septa - internal tissue separate segments

6. Phylum Annelida • Setae • Tiny bristles called setae • Absent in leeches • Short setae anchor segments in earthworms • Prevent it from slipping backward • Long setae help aquatic worms swim • Class Polychaetes • Primarily marine • Class Oligochaetes (earthworms) and Hirudinida (leeches) • Freshwaters, or terrestrial soils • Many leeches are predators

7. 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 • Strong circular and longitudinal muscles underlie the body wall

9. Coelomates

10. Phylum Annelida • Hydrostatic Skeleton • Except in leeches, coelom is filled with fluid and serves as a hydrostatic skeleton • Fluid volume remains constant • Contraction of longitudinal and circular muscles • Causes body to shorten and expand, narrow and lengthen • By separating this force into sections, widening and elongation move the whole animal • Alternate waves of contraction, allow efficient burrowing

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

12. 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 • Hermaphroditic (monoecious) animals • Young develop inside a cocoon secreted by the clitellum, and emerge as small worms

13. Clade Clitellata Class Oligochaeta- earthworm • Diversity • Over 3000 species • Occur in habitats from soil to freshwater • Few are marine or parasitic • 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

15. Clade Clitellata • Setae • Bristlelike rods moved by tiny muscles • Project outward through small pores in cuticle • Aid anchoring by digging into walls of burrow • 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 • Typhlosole increases surface area

17. 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

18. Excretory System

19. Clade Clitellata • 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 • Fused ganglia in each segment contain both sensory and motor fibers • Lack eyes but have many photoreceptors in the epidermis • Free nerve endings in tegument are tactile structures

20. Nervous System: Brain, Nerve Cord, Peripheral

21. Nerve Cord

22. Clade Clitellata • General Behavior • Avoid bright light • Chemical stimuli are important in locating food • Limited learning ability • Primarily trial-and-error learning

23. 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

25. 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

26. Freshwater Oligochaetes

27. 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

28. Largest Leech

29. Clade Clitellata • Form and Function • Usually have a fixed number of segments • Lack distinct compartments • No septa • No setae • Developed suckers for attachment • Gut specialized for storage of large quantities of blood • Most use suckers to attach and inch along surfaces

30. 2 suckers: Posterior and Anterior

31. 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 • Some parasitic leeches leave a host during breeding season • Fish leeches may remain on a host permanently

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