CHAPTER 11 STUDY GUIDE 11.1 Dividing Up the Body A. A spacious, fluid-filled coelom provides an efficient hydrostatic skeleton. B. When septa divided

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1 CHAPTER 11 STUDY GUIDE 11.1 Dividing Up the Body A. A spacious, fluid-filled coelom provides an efficient hydrostatic skeleton. B. When septa divided this coelom into a series of compartments, metamerism first arose. C. Metamerism may have arisen independently in the deuterostome line, which includes the vertebrates. D. Metamerism makes independent segmental movements possible. E. If one segment should fail, another could still function Characteristics A. General 1. About 15,000 species of segmented worms are classified in the phylum Annelida. 2. About two-thirds are more obscure marine worms. 3. Annelids are true coelomates in the protostome branch with spiral cleavage and mosaic development (see Inset, position in animal kingdom, page 204). 4. The nervous system is more centralized and the circulatory system is more complex than in other worms. 5. Except for leeches, annelids have tiny chitinous bristles called setae. 6. Short setae anchor a segment in an earthworm so it prevents slipping backward. 7. Long setae help aquatic worms swim. B. Ecology Relationships 1. Annelids occur in the sea, freshwater, and on land. 2. Marine annelids borrow in the mud or live in tubes. 3. Some feed on organic matter in the mud, some are suspension feeders, many are predators. 4. Freshwater annelids burrow in the mud, live among vegetation, or swim freely. 5. Leeches are bloodsuckers or are carnivorous. C. Economic Importance: 1. Many annelids are grazers; many are preyed upon by fish. 2. Burrowing on land or in oceanic mud and sand, annelids are incredibly important for nutrient redistribution in the biosphere (see Figure 11.10). 3. Blood-sucking leeches are used medicinally. D. Biological Contributions: (see Inset, page 198) 11.3 Body Plan A. Body Wall 1. The anterior tip contains the prostomium and peristomium, a two part head (Figure 11.1). 2. The terminal portion bearing the anus is the pygidium. 3. The prostomium and pygidium are not considered metameres like the other segments 4. New metameres form just in front of the pygidium; thus the newest segments are at the posterior. 5. The coelom develops embryonically as a split in mesoderm on each side of the gut (schizocoel). 6. Peritoneum (mesodermal epithelium) lines the body wall and forms dorsal and ventral mesenteries (Figure 11.2). 7. Peritonea of adjacent segments meet to form the septa. 8. Except in leeches, the coelom is filled with fluid and serves as a hydrostatic skeleton. a. The fluid volume remains constant. b. Therefore contraction of longitudinal muscles causes the body to shorten and fatten. c. Contraction of circular muscles causes the body to narrow and lengthen. d. By separating this force into sections, widening and elongation move the whole animal. e. Alternate waves of contraction, or peristalsis, allow efficient burrowing. f. Swimming annelids use undulatory movements. 9. Strong circular and longitudinal muscles underlie the body wall (Figure 11.1). 10. The surface is covered with an epidermis and a thin outer layer of non-chitinous cuticle. 11. The gut and longitudinal blood vessels extend through the septa. C. Characteristics of Phylum Annelida (see Inset page 199) Class Polychaeta A. Characteristics 1. Polychaeta is the largest class of annelids with more than 10,000 species, mostly marine. 2. They vary from 1 mm to 3 meters long; most are 5 to 10 cm long. 92

2 3. Some live in crevices; others inhabit tubes or are pelagic. 4. Polychaetes are an important part of marine food chains. 5. Polychaetes have a well-differentiated head with sense organs (Figure 11.3). 7. Paired appendages called parapodia are on most segments (Figure 11.1). 8. They have no clitellum. 9. Many setae are arranged in bundles on the parapodia. 10. A head or prostomium may or may not be retractile; it often bears eyes, tentacles and sensory palps. a. The first segment or peristomium surrounds the mouth and may have setae, palps or chitinous jaws. b. Ciliary feeders may bear a tentacular crown that opens like a fan but can be withdrawn in to the tube. 11. Most segments of the trunk bear parapodia with lobes, cirri, setae and other parts (Figure 11.1). a. Parapodia have two lobes, the dorsal notopodium and the ventral neuropodium. b. Usually the parapodia are the chief respiratory organ although the worm may also possess gills. c. Parapodia help crawl, swim, and anchor the worm in a tube. d. Along with gills, they serve is chief respiratory organs. e. Amphritite has plumelike gills (Figure 11.4). 12. Sense organs include eyes and statocysts that are more developed than oligochaetes. a. Eyes vary from simple eyespots to well-developed image-resolving eyes. 13. In contrast to clitellates, polychaetes have no permanent sex organs and have separate sexes. a. Gonads appear as simple temporary swellings of the peritoneum. b. Gametes are shed into the coelom and exit by gonoducts, metanephridia or rupturing of the body. c. Fertilization is external and the early larva is a trochophore 14. Diversity a. Polychaetes are pelagic, burrowers, and sedentary; some live in tubes. a. The clamworm Nereis is an example of a predatory polychaete with jaws. b. Scale Worms (Figure 11.5) are carnivores and some are commensals living in burrows of other organisms. c. Tube-dwellers may line their burrows with mucus and use cilia to obtain food. d. Fireworms feed on cnidarian (Figure 11.6). e. Amphitrite traps particles with extensible tentacles. (Figure 11.4) c. It lives in a U-shaped tube; modified segments pump water through the tube (Figure 11.9). d. Featherduster worms or fan worms live in tubes but unfurl tentacular crowns to feed. e. Fanworms such as Sabella, a polychaete ciliary feeder, directs small food balls along grooved radioles to the mouth by ciliary action (Figure 11.7A,B). d. Chaetopterus, a sedentary polychaete, has wing-like notopodia that secretes a mucous net to strain out food particles from the water (Figure 11.9). e. As the net fills with food, the food cup rolls it into a ball, which is passed into a groove toward the mouth. e. Arenicola, the lugworm, lives in an L-shaped burrow in intertidal mudflats and ingests foodladen sand (Figure 11.10) d. Eunice viridis, the Samoan palolo worm, has posterior somites making up the epitokal region consisting of segments packed with gametes (Figure 11.8 and Inset, page 208). e. During the swarming period, which occurs at the beginning of the last quarter of the October-November moon, these epitokes break off and swim to the surface; swarming provides for synchronous maturation of all epitokes. f. Tubes may be parchmentlike, attached to rocks, and composed of sand, shell, or seaweed Clade Siboglinidae (Pogonophorans) A. General Description (Figure 11.11) 1. Pogonophorans or beardworms have been found from Indonesia coasts to the Atlantic Ocean. 2. Some 150 species have been described from marine environments. 93

3 3. Molecular and morphological analysis indicates they are derived from polycheates. 4. Segmentation is present in only one body region. 5. They are sessile, bottom living animals found at depths of more than 200 m. 6. Body characteristics: a. They have a forepart, trunk, and opisthosoma. b. A cuticle, setae, coelomic compartments, and tentacles are present. c. A mouth and digestive tract are absent. 7. Soboglinids absorb nutrients from seawater through pinnules and tentacles. 8. They form a mutual association with chemoautotrophic bacteria that oxidize hydrogen sulfide. 9. The trophosome of the midgut bears bacteria. 10. Sexes are separate. 11. A closed vascular system and photoreceptor cells are present Class Oligochaeta A. Diversity 1. Over 3000 species occur in habitats from soil to fresh water; a few are marine or parasitic. 2. Nearly all bear setae; although highly varied, setae are fewer than in polychaetes. 3. Earthworms (Figure 11.12). a. Earthworms burrow in moist, rich soil; they emerge at night. b. In wet weather they stay near the surface; in dry weather they burrow deep and become dormant (see Inset, page 212). c. Lumbricus terrestris is commonly studied in school laboratories. c. Darwin studied earthworms and estimated tons [9-16 metric tonnes] of dry earth passed through earthworm intestines per acre [0.4 hectares] annually. d. Earthworms have an important role in churning the soil, mixing materials and adding nutrients. e. Giant tropical earthworms may reach 4 meters long and live in interconnected tunnels. B. Form and Function 1. The prostomium overhangs the mouth at the anterior end (Figures 11.12A,B). a. In most earthworms, each segment bears four pairs of chitinous setae; some may bear over 100 (Figure 11.13). 2. Each seta is a bristle-like rod set in a sac and moved by tiny muscles. 3. Setae anchor segments during burrowing. 4. Earthworms move by peristalsis. a. Circular muscles contract, the anterior end lengthens, setae anchor the forward end. b. Longitudinal muscles contract, body shortens, posterior end is pulled forward. 5. Most oligochaetes are scavengers, feeding on decayed organic matter, leaves, refuse, etc. a. Food is moistened by the mouth and drawn in by a sucking action of the muscular pharynx. b. Digestion and absorption occur in the intestine; an infolded typhlosole increases surface area. c. Chloragogen tissue surrounds the intestine and synthesizes glycogen and fat; cells full of fat float free in the coelom as eleocytes. d. Chloragogen cells also function in excretion. 6. Circulation, Respiration, Excretion a. Both coelomic fluid and blood carry food, wastes and respiratory gases. b. Blood circulates in a closed system with five main trunks running lengthwise in the body. c. The dorsal vessel above the alimentary canal has valves and functions as a true heart (Figure 11.12A, C). d. The dorsal vessel pumps blood anteriorly into five pairs of aortic arches. e. Blood contains colorless ameboid cells and dissolved hemoglobin. f. Earthworms have no special gaseous exchange organs; the moist skin handles all exchanges. g. Each somite except the first three and last one have a pair of nephridia (Figure 11.14). h. Each nephridium occupies parts of two adjacent somites. i. A ciliated funnel, the nephrostome, draws in wastes and leads through the septum. j. These coil until the nephridial duct ends at a bladder that empties outside at the nephridiopore (Figure 11.14). k. Wastes from the coelom are discharged. 7. Nervous System and Sensory Organs 94

4 a. A pair of cerebral ganglia connects around the pharynx to the ganglia of the ventral nerve cord. b. Fused ganglia in each somite contain both sensory and motor fibers. c. One or more giant axons are located in the ventral nerve cord to increase the rate of conduction and stimulate contractions of muscles in many segments. 8. Reproduction and Development a. Earthworms are monoecious (Figure 11.15). b. In Lumbricus, reproductive systems are in somites 9 to 15 (Figure 11.15A-E). c. Earthworms mate at night during warm, moist weather. d. They mate by aligning in different directions with ventral surfaces together. e. Mucus secreted by the clitellum holds them together. f. Sperm travel to the seminal receptacles of the other worm along seminal grooves. g. After mutual copulation, each worm secretes a mucous tube and chitinous band to form a cocoon. h. As the cocoon passes forward, eggs, albumin and sperm pour into it. i. Fertilization and embryogenesis takes place in the cocoon; young worms emerge. C. Freshwater Oligochaetes 1. These oligochaetes are generally smaller with longer setae than on earthworms. 2. Aquatic oligochaetes are important food for fishes; a few are ectoparasites. 3. A wide variety occur in water, mu d, and moist soil (Figure 11.16). 4. Some have long slender gills, others have ciliated posterior gills that extend fro m tubes. 5. Chief foods are algae and detritus; burrowers swallow mud and digest the organic material. 6. Aeolosoma are ciliary feeders that use currents produced by cilia at the anterior end of gills (Figure 11.16A-D) Class Hirudinea: Leeches A. Diversity 1. Most leeches live in fresh water but a few are marine or in moist terrestrial environments. 2. About 500 species, they are more common in the tropics than temperate zones. 3. Leeches vary in color: black, brown, red and olive green. 4. Most are flattened; they range in size from 2 cm to 20 cm or more (Figure 11.17). 5. Leeches usually have 34 somites, but appear to have more due to superficial annuli (Figure 11.18). 6. Most leeches lack distinct coelomic compartments and septa have disappeared. 7. The coelomic cavity is filled with connective tissue. 8. Feeding a. Some are carnivores on small invertebrates; others are temporary or permanent parasites. b. Leeches feed on prey with a protrusible proboscis or muscular pharynx armed with teeth. c. Most are fluid feeders that prefer tissue fluids and blood pumped from open wounds. d. Bloodsuckers secrete an anticoagulant in their salvia. e. They depend mostly on bacteria in their gut for digestion of their blood meal. f. Leeches are hermaphroditic and have a clitellum during the breeding season. g. The clitellum secretes a cocoon for reception of eggs. h The cocoons are buried in mud or damp soil, and development is similar to that oligochaetes. 9. Leeches are being used again medically to relieve congestion until veins grow back in severed digits (Figure 11.19) 11.7 Phylogeny and Adaptive Radiation A. Phylogeny (Figure 11.20) 1. Molecular and morphological analysis suggests Annelida is paraphyletic without pognophorans and perhaps echiurans. 2. Pogonophorans are in the clade Siboglinidae within Polychaeta. 3. Echiurans are not included with Polychaeta due to loss of segmentation. 4. Class Polychaeta is paraphyletic even with siboglinids. 5. Oligochaeta arose from within the polychaete clade. 6. Oligochaeta may also be paraphyletic because leeches arose from this clade. 7. Hirudinea and Oligochaeta form the clade Clitellata. 8. Molluscs and Arthropoda may be sister taxons to Annelida 95

5 a. Annelids and arthropods share a similar nervous system and metamerism. b. Annelids and arthropods may not share the same mechanism of segmentation. c. Molluscs and annelids have similar embryos and trochophore larva. d. Ribosomal analysis supports placement of annelids and mollusks in the clade Lophotrochozoa. B. Adaptive Radiation 1. Septal arrangement with fluid-filled compartments has been varied for precise movements. 2. Powerful circular and longitudinal muscles shorten and lengthen the body. 3. Oligochaetes are constrained by the terrestrial soil environment. 4. Polychaetes inhabit a wide range of habitats and are not constrained by the physical environment to a great degree. 5. Feeding adaptations vary widely, from chitinous jaws to specialized tentacles. 6. Leeches have developed both parasitic and predatory adaptations. C. Classification Class Polychaeta Class Oligochaeta Class Hirudinea 96

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 17. Annelids 17-1

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 17. Annelids 17-1 CHAPTER 17 Annelids 17-1 Characteristics of the Phylum Annelida Diversity Exhibit segmentation or metamerism Bodies composed of repeated units Each unit contains components of most MAJOR organ systems