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CHAPTER 17 Annelids and Allied Taxa 17-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 17-2 Increased burrowing efficiency by permitting independent movement of segments Evolution of a more sophisticated nervous system Provided a safety factor If one segment failed, others could still function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics 17-3 Cleavage is spiral mosaic Mesoderm forms from derivatives of the 4d cell Coelom forms by schizocoely Share a trochophore as the ancestral larval form Segmented worms living in marine, freshwater, and moist terrestrial habitats Include marine bristle worms, leeches, and earthworms, pogonophorans, and vestimentiferan worms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Members of phylum Echiura and Sipuncula are benthic marine animals with unsegmented bodies Molecular sequence data place echiurans within phylum Annelida Echiurans Sipunculans 17-4 Sister taxon to Annelida Sister taxon to a clade composed of Annelida and Echiura Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Annelida Characteristics About 15,000 species 2/3 are the more obscure marine worms. Segmentation Body segments marked by circular grooves called annuli Metamerism 17-6 Repetition of organs in segments called metameres or somites Septa separate segments Found in arthropods, probably homologous with annelids, and in vertebrates, where it evolved independently Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Annelida Setae Tiny chitinous bristles called setae Absent in leeches Short setae anchor segments in earthworms Primarily marine and usually benthic Oligochaetes and leeches 17-7 Long setae help aquatic worms swim Polychaetes Prevent it from slipping backward Freshwaters, or terrestrial soils Many leeches are predators Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Annelida Body Plan Body Wall 17-8 Head is composed of Prostomium Perstonium Terminal portion bearing the anus is the pygidium Head and pygidium are not considered metameres New metameres form in front of the pygidium Surface is covered with an epidermis and a thin outer layer of non-chitinous cuticle Strong circular and longitudinal muscles underlie the body wall Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Annelida Coelom Forms by schizocoely Peritoneum (mesodermal epithelium) lines body wall and forms dorsal and ventral mesenteries Peritonea of adjacent segments meet to form the septa Gut and longitudinal blood vessels extend through the septa 17-10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 muscles Contraction of circular muscles 17-12 Causes body to shorten and expand Causes body to narrow and lengthen By separating this force into sections, widening and elongation move the whole animal Alternate waves of contraction, or peristalsis, allow efficient burrowing Swimming annelids use undulatory movements Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Annelida Phylogeny Traditionally, annelids are divided among 3 classes Class Polychaeta Class Oligochaeta Class Hirudinida Polychaeta is a paraphyletic class Oligochaeta and Hirudinida form a monophyletic group called Clitellata Characterized by reproductive structure called a clitellum 17-13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Annelida Some now consider Clitellata to be an annelid class containing oligochaetes and leeches as orders We retain the three classes and consider Clitellata a clade whose members are class Oligochaeta and class Hirudinida Class Oligochaeta is a paraphyletic group because ancestors of leeches arose from within it 17-14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Diversity Largest class of annelids More than 10,000 species, mostly marine Vary from 1 mm to 3 meters long Differentiation of some somites More specialization of sensory organs than in clitellates Tolerate a wide range of salinity Warmer regions have more freshwater polychaetes Some live in crevices, others inhabit tubes, or are pelagic 17-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Important part of marine food chains Well-differentiated head with sense organs Paired appendages called parapodia on most segments No clitellum Many setae arranged in bundles on parapodia Sedentary polychaetes mainly tube-living Errant polychaetes may be free-moving, burrowing, or crawling Clamworm Nereis is an example of a predatory polychaete with jaws on an eversible, muscular pharynx 17-16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Form and Function Prostomium may or may not be retractile Often bears eyes, tentacles, and sensory palps Surrounds mouth and may have setae, palps, or chitinous jaws Ciliary feeders may bear a tentacular crown that opens like a fan but can be withdrawn into the tube Most segments of trunk bear parapodia with lobes, cirri, setae, and other parts 17-18 Aid crawling, swimming, and anchor worm in a tube Usually the chief respiratory organ although the worm may also possess gills Amphritite and Arenicola Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Nutrition 17-20 Polychaetes have a foregut, midgut and hindgut Foregut has a stomodeum, pharynx, and anterior esophagus lined with cuticle Midgut derived from mesoderm secretes enzymes and absorbs nutrients Short hindgut derived from ectoderm and leads to anus on the pygidium Errant polychaetes are predators or scavengers Sedentary polychaetes feed on suspended particles or particles in sediment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Circulation and Respiration 17-22 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Excretion 17-23 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Nervous System and Sense Organs Dorsal cerebral ganglia connect to subpharyngeal ganglia by a circumpharyngeal commissure Double ventral nerve cord runs length of the worm with ganglia in each metamere Sense organs include 17-24 Eyes, nuchal organs and statocysts Eyes vary from simple eyespots to well-developed imageresolving eyes similar to mollusc eyes Alciopid eyes have accessory retinas specialized for the wavelengths that penetrate deep seas Nuchal organs are ciliated sensory pits that are probably chemoreceptive Some burrowing and tube-building polychaetes use statocysts to orient their body Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Reproduction and Development 17-25 Polychaetes have no permanent sex organs Monoecious Gonads appear as simple temporary swellings of peritoneum Gametes are shed into coelom and exit by gonoducts, metanephridia, or rupturing of the body Fertilization is external and the early larva is a trochophore Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Representative polychaetes Clam Worms: Nereis 17-28 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Parapodia Each has two lobes One or more chitinous spines (acicula) support each lobe Abundant blood vessels assist respiration Function in creeping and swimming Oblique muscles in each somite Undulatory movements of the body provide freeswimming and burrow-pumping actions Feed on small animals, other worms, and larval forms Food moved through alimentary canal by peristalsis 17-29 Dorsal notopodium Ventral neuropodium Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Scale worms Belong to the family Polynoidae Flattened bodies are covered with broad scales Some are large, all are carnivores and some are commensals in burrows of other organisms Fireworms 17-30 Have hollow, brittle setae that contain poisonous secretions Feed on cnidarians Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Class Polychaeta Tubeworms Fanworms Tube-dwellers May line their burrows with mucus Use cilia or mucus to obtain food Unfurl tentacular crowns to feed Food moved from radioles to mouth by ciliary action Parchment Worms 17-32 Lives in a U-shaped tube Modified segments pump water through tube Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Siboglinidae (Pogonophorans) Formerly members of phylum Pogonophora (beardworms) Discovered in 1900 Characteristics 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 17-36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 17-37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-38 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Siboglinidae (Pogonophorans) Body Long cylindrical body covered with cuticle Divided into a short anterior forepart, a long slender trunk, and a small, segmented opisthosoma Tentacles located on a cephalic lobe 17-39 Hollow extensions of the coelom and bear minute pinnules Lie parallel to one another, enclosing a cylindrical intertentacular space Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Siboglinidae (Pogonophorans) Internal body No mouth or digestive tract Mode of digestion puzzling Nutrients such as glucose and amino acids absorbed from seawater through pinnules and microvilli of tentacles Most energy derived from a mutualistic relationship with chemoautrophic bacteria that oxidizes hydrogen sulfide Trophosome, derived embryonically from midgut, houses the bacteria 17-40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Siboglinidae (Pogonophorans) Reproduction and Development Sexes are separate Research suggests that cleavage is unequal and atypical Pair of gonads and gonoducts in trunk section Appears to be spiral Coelom formed by schizocoely Embryo 17-42 Worm-shaped and ciliated Poor swimmer Probably carried by water currents until it settles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-43 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 17-44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Class Oligochaeta Diversity Over 3000 species Occur in habitats from soil to freshwater Few are marine or parasitic Nearly all bear setae Fewer in number than in polychaetes Form and Function 17-45 Sometimes called “night crawlers” Burrow in moist rich soil and usually live in branched interconnected tunnels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-46 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Damp, rainy weather: Remain near surface Dry weather: Burrow deep underground and go dormant coiled in a slime chamber Peristaltic movements 17-47 Contractions of circular muscles in the anterior end lengthen the body, pushing the anterior end forward where it anchors Anchoring is accomplished by contraction of the longitudinal muscles in forward segments Causes the segments to become short and wide, pushing against the burrow Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Setae Bristlelike rods set in a sac and moved by tiny muscles Project outward through small pores in cuticle Aid anchoring by digging into walls of burrow Nutrition 17-48 Scavengers, feeding on decayed organic matter, leaves, refuse, etc. Food moistened by mouth and drawn in by a sucking action of muscular pharynx Calcium in soil leads to high blood Ca+2 Calciferous glands along the esophagus reduce calcium ion concentration in the blood Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata 17-50 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 Chloragogen tissue surrounds the intestine Synthesizes glycogen and fat Cells full of fat float free in the coelom as eleocytes Also function in excretion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Circulation and Respiration 17-51 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata 17-52 Ventral vessel serves as an aorta, delivering blood to body walls, nephridia, and digestive tract Blood contains colorless ameboid cells and dissolved hemoglobin No special gaseous exchange organs Gas exchanged across body surface Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Excretion 17-53 Each somite, except the 1st three and terminal one, have a pair of metanephridia Each unit occupies parts of two adjacent somites 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata 17-55 Terrestrial worms excrete less toxic urea Chloragogen cells that break off and enter nephridia produce urea and ammonia Salts pass across the integument, apparently by active transport Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Nervous System and Sense Organs 17-56 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 somite contain both sensory and motor fibers Neurosecretory cells in brain and ganglia secrete neurohormones Regulate reproduction, secondary sex characteristics, and regeneration One or more giant axons located in ventral nerve cord increase rate of conduction and stimulate contractions of muscles in many segments Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-57 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-58 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata 17-59 Lack eyes but have many photoreceptors in the epidermis Free nerve endings in tegument are probably tactile structures Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata General Behavior 17-60 Avoid bright light (negative phototaxis) Chemical stimuli are important in locating food Limited learning ability Primarily trial-and-error learning Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Reproduction and Development 17-61 Monoecious In Lumbricus, reproductive structures are located in somites 9 through 15 Sperm produced by testes mature in seminal vesicles and pass into sperm ducts Eggs are discharged by ovaries into coelomic cavity Ciliated funnels transport them outside Two pairs of seminal receptacles receive and store sperm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-62 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata 17-63 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 and chitinous band to form a cocoon Cocoon passes forward and eggs, albumin, and sperm are added Fertilization and embryogenesis occur in cocoon Young worms emerge from cocoon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-64 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Class Hirudinida: Leeches Divided into 3 orders Hirudinea 34 segments, lack setae and possess anterior and posterior suckers Acanthobdellidae Hirudinea (‘true” leeches) Acanthobdellidae Branchiobdellidae 27 segments, setae only present on the first five segments, and have a posterior sucker Branchiobdellidae 17-65 14 or 15 segments, no setae, and an anterior sucker Commensal or parasitic on crayfish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-66 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Diversity 17-67 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 Most are flattened 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Form and Function Usually have a fixed number of segments 17-68 Appear to have more due to superficial annuli Lack distinct coelomic compartments No septa In most, coelomic cavity filled with connective tissue and spaces (lacunae) Lacunae channels may serve as auxiliary circulatory system No setae Developed suckers for attachment Gut specialized for storage of large quantities of blood Most use suckers to attach and “inchworm” along surfaces Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-69 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Nutrition 17-70 Not all are parasites, many are predaceous Freshwater leeches have a proboscis 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-71 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Respiration and Excretion 17-72 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Nervous and Sensory Systems Two “brains” 17-73 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 Row of sensillae in central annulus of each segment. Pigment-cup ocelli are present Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Reproduction 17-74 Hermaphroditic and practice cross-fertilization Sperm transferred by hypodermic impregnation Clitellum secretes cocoon to receive sperm and egg Cocoons are buried in mud or damp soil Development is similar to that of oligochaetes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Circulation 17-75 Coelom reduced by invasion of connective tissue and chloragogen tissue Forms system of coelomic sinuses and channels Some have a typical oligochaete circulatory system Coelomic system is auxiliary Some lack blood vessels and coelomic sinuses serve as only vascular system Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clade Clitellata Classification Class Polychaeta Class Oligochaeta Class Hirudinida 17-76 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Echiura Diversity Approximately 140 species of marine worms that burrow into mud or sand Live in empty snail shells or sand-dollar tests, or rocky crevices Found in all oceans Length varies from a few millimeters to 40 or 50 cm 17-77 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-78 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Echiura Form and Function Sausage-shaped Inextensible proboscis anterior to the mouth Often called spoon worms” Simple nervous system with a ventral nerve running length of the body Ciliated groove on the proboscis allows them to gather detritus over the mud while lying buried Muscular body wall is covered by a cuticle and epidermis which may be smooth or covered by papillae 17-79 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-80 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Echiura Large coelom Digestive tract long and coiled Pair of anal sacs may serve an excretory and osmoregulatory function Most have a closed circulatory system with colorless blood Hemoglobin found in certain cells and in coelomic corpuscles Respiration probably occurs in hindgut which is continually filled and emptied by cloacal irrigation 17-81 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-82 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Echiura Sexes are separate Gonads produced by special regions in peritoneum in each sex Fertilization usually external Early cleavage and trochophore stages similar to annelids 17-83 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Sipuncula Diversity Approximately 250 species of benthic marine worms Sedentary, living in burrows of mud or sand, snail shells, coral crevices, or among vegetation More than ½ restricted to tropical zones Some are tiny, slender worms, but most range from 3 to 10 cm in length Some are known as “peanut worms” because when disturbed, they contract to a peanut shape 17-84 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-85 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Sipuncula Form and Function No segmentation or setae Slender, retractable introvert or proboscis at anterior end Walls of the trunk are muscular 17-86 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Sipuncula Nutrition 17-87 Little known about feeding habits Some appear to be detritivores and others suspension feeders Some nutrition may come from dissolved organic matter in the surrounding water From burrow or hiding place, they extend tentacles to explore and feed Collected organic matter moved from mucus on tentacles to mouth by ciliary action Retractor muscles retracts the introvert to allow tubular compensation sacs along the esophagus to accept fluid from tentacles Large fluid-filled coelom Digestive tract is U-shaped Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17-88 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Sipuncula Respiration Lack a circulatory and respiratory system Gas exchange appears to occur across the introvert and tentacles Nervous and Sensory Systems 17-89 Bilobed cerebral ganglion behind tentacles Ventral cord extends the length of body Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Sipuncula Reproduction 17-90 Sexes are separate Sex organs develop seasonally within the connective tissue covering the origins of the retractor muscles Sex cells are released through the nephridia. Asexual reproduction occurs by transverse fission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Evolutionary Significance of Metamerism Origins of Metamerism and the Coelom 17-91 No satisfactory explanation for origins of metamerism and coelom has gained acceptance Coelom may have been advantageous as a hydrostatic skeleton Coelomic fluid would have acted as a circulatory fluid and reduce need for flame cells everywhere Coelom could store gametes for timed release Would require nervous and endocrine control Unlikely that segmentation is homologous among annelids, arthropods, and chordates Current evidence supports the hypothesis that segmentation arose independently multiple times Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Evolutionary Significance of Metamerism 17-92 Selective advantage of a segmented body for annelids appears to lie in the efficiency of burrowing However, does not explain segmentation in arthropods given the rigidity of the exoskeleton Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Molluscs and annelids share many developmental features so are presumed to be closely related However, shared features are likely to be a retained ancestral feature for lophotrochozoan protostomes Pogonophorans and vestimentiferans were once placed in Annelida but are now in clade Siboglinidae Molecular analyses place sipunculids and echiurans closely related to the annelids 17-93 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Some biologists place echiurans within Annelida because they have serially repeated structures One recently developed phylogenetic tree place echiurans near capitellid polychaetes because both dwell in sediments 17-94 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification With more studies, Echiura, like Pogonophora, may no longer be a valid phylum Placement of Sipuncula is contentious Members are not metameric and lack setae Larval development similar to annelids, molluscs, and echiurans Molecular data may clarify position within Lophotrochozoa Presently, depicted as the sister taxon to a clade of annelids and echiurans Polychaeta is a paraphyletic group 17-95 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Adaptive Diversification 17-96 Oligochaetes are constrained by terrestrial soil environment Polychaetes inhabit a wide range of habitats Septal arrangement with fluid-filled compartments has been varied for precise movements Feeding adaptations vary widely, from chitinous jaws to specialized tentacles Leeches have developed both parasitic and predatory adaptations