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CHAPTER 17. Annelids. 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
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CHAPTER 17 Annelids
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
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)
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
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
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
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
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
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
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
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
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
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
Clade Clitellata • General Behavior • Avoid bright light • Chemical stimuli are important in locating food • Limited learning ability • Primarily trial-and-error learning
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
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
Freshwater Oligochaetes
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
Largest Leech
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
2 suckers: Posterior and Anterior
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
Medicinal Leech: Used to relieve congestion in the veins due to injury.