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CHAPTER 14 Flat Worms, Mesozoans and Ribbonworms: Phylum Acoelomorpha Phylum Mesozoa 14-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features Animals that actively seek food, shelter, home sites, and mates require a different set of strategies and body organization than radially symmetrical sessile organisms Two major evolutionary advances 14-2 Cephalization Concentrating sense organs in the head region Primary bilateral symmetry Body can be divided along only 1 plane of symmetry to yield 2 mirror images of each other Active, directed movement most efficient with an elongated body form with anterior (head), posterior (head), dorsal, and ventral sides Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features Position and Biological Contributions Simplest animals with primary bilateral symmetry Mesoderm well-defined Triploblastic Mesozoans 14-3 No clearly defined body layers Development does not include gastrulation Highly specialized parasites Some argue this group were derived from complex free-living organisms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features Acoelomates Typical acoelomates have only one internal space, the digestive cavity Region between the epidermis and digestive cavity is filled with parenchyma Some members of Acoelomorpha are atypical acoelomates: No digestive cavity 14-4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features Developmental character suites define two metazoan clades: Protostomia and Deuterostomia Protostomes 14-6 Spiral or centrolecithal cleavage Development is mosaic Embryonic blastopore becomes the mouth Coelom forms by schizocoely Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features Deuterostomes 14-7 Radial cleavage Development is regulative Blastopore becomes the anus Coelome forms by enterocoely Platyhelminth: acoelomate protostomes Nemertea: coelomate protostomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Characteristics 14-8 Small flat worms less than 5 mm in length Typically live in marine sediments; few are pelagic Some species live in brackish water Most symbiotic but some parasitic Group contains ~350 species Members were formerly in Class Turbellaria within phylum Platyhelminthes Have a cellular ciliated epidermis Parenchyma layer contains small amount of ECM and circular, longitudinal, and diagonal muscles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Digestion and Nutrition Some have digestive system from a mouth to a tube-like pharynx followed by a sack-like gut No anus In many acoels, the gut and pharynx are absent 14-10 Mouth leads into either an endodermally derived mass of cells or syncytial mass Phagocytotic cells digest food intracellularly when food is passed into temporary spaces Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Reproduction Monoecious Female produces yolk-filled eggs Endolecithal eggs Following fertilization 14-12 Some or all cleavage events produce a duet-spiral pattern of new cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Nervous System Other defining features proposed for acoelomorphs Biochemical (patterns of neurotransmitters) Cellular ultrastructure such as formation of a network of interconnecting rootlets from epidermal cilia Acoelomorphs lack a “true” brain Have a radial arrangement of nerves instead of a ladder-like pattern seen within Phylum Platyhelminthes 14-13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Phylogeny of Acoelomorpha Phylogenetic studies describe acoelomorphs as early-diverging bilaterally, symmetrical triploblasts Have only four or five Hox genes 14-14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Clades within Protostomia Divided into two large clades: Lophotrochozoa and Ecdysozoa Before use of molecular phylogeny studies Molecular phylogenies Protostomes grouped on the basis of body plan Group acoelomate and coelomate taxa together within the protostomes Ecdysozoa possess a cuticle that is molted as their bodies grow 14-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Acoelomorpha Lophotrochozoa Share either an odd horse-shoe shaped feeding structure, the lophophore or Larval form called the trochophore Trochophore larvae 14-16 Minute, translucent, and roughly top-shaped Have a prominent circlet of cilia and sometimes one or two accessory circlets Occur in the early development of marine members of Annelida and Mollusca Assumed to be the ancestors of such groups Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Characteristics Commonly called flatworms Vary from a millimeter to many meters in length Some free-living; others parasitic Some argue that the phylum Platyhelminthes is not a valid monophyletic phylum The parasitic clades 14-17 Share an external body covering called a syncytial tegument or neodermis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Platyhelminthes is divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda Class Turbellaria Mostly free-living forms Most are bottom dwellers in marine or freshwater Freshwater planarians Found in streams, pools, and hot springs Terrestrial flatworms limited to moist places 14-19 All members of Monogenea and Trematoda (flukes) and Cestoda (tapeworms) are parasitic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Form and Function Epidermis and Muscles Most have cellular, ciliated epidermis on a basement membrane Rod-shaped rhabdites Swell and form a protective mucous sheath Most turbellarians have dual-gland adhesive organs Viscid gland cells fasten microvilli of anchor cells to substrate Secretions of releasing gland cells provide a quick chemical detachment 14-21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Some turbellarians, and all other members of this phylum Have a syncytial epidermis Non-turbellarians Lack cilia and have a tegument Form the subphylum Neodermata Under the basement membrane Nuclei are not separated by cell membranes Muscle fibers run circularly, longitudinally and diagonally Parenchyma cells fill spaces in the body 14-24 In some, if not all, these are noncontractile portions of muscle cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Nutrition and Digestion Cestodes have no digestive system Others have a mouth, pharynx, and intestine In planarians Intestine has three branches Pharynx may extend through the ventral mouth One anterior and two posterior Gastrovascular cavity lined with columnar epithelium Mouth of trematodes and monogeneans 14-26 Opens near the anterior end Pharynx is not extensible Intestine ends blindly, varies in degree of branching Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Planaria 14-30 Carnivorous and detect food by chemoreceptors Food trapped in mucous secretions from glands and rhabdites Wrap themselves around prey Extend the proboscis to suck up bits of food Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Monogeneans and Trematodes Feed on host cells, cellular debris, and body fluids Proteolytic enzymes from the intestine are secreted for extracellular digestion Phagocytic cells in gastrodermis complete digestion at intracellular level Undigested food egested out the pharynx Cestodes 14-31 Rely on the host’s digestive tract Absorb digested nutrients Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Excretion and Osmoregulation Flatworms have protonephridia Used for osmoregulation Beating flagella drive fluids down collecting ducts Wall of the duct beyond the flame cell bears folds or microvilli to resorb ions and molecules Majority of metabolic wastes Removed by diffusion across the cell wall Collecting ducts join and empty at nephridiopores Marine turbellarians 14-32 Lack these units No need to expel excess water Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Monogeneans have two excretory pores Open laterally near anterior end Flame cell protonephridia present also parasitic taxa Ducts of trematodes open into excretory bladder that opens to a terminal pore Cestodes have two main excretory canals on each side Metabolic wastes are removed largely by diffusion through the body wall 14-33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Nervous System Subepidermal nerve plexus resembles nerve net of cnidarians One to five pairs of longitudinal nerve cords lie under the muscle layer More derived flatworms have fewer nerve cords Freshwater planarians 14-34 One ventral pair of nerve cords forming a laddertype pattern Brain is a bilobed ganglion anterior to the ventral nerve cords Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Sense Organs Active locomotion favored cephalization and evolution of sense organs Ocelli (light-sensitive eyespots) Tactile and chemoreceptive cells Present in turbellarians, monogeneans, and larval trematodes Abundant, especially in the ear-shaped auricles Statocysts (equilibrium) and rheoreceptors (sense direction of water currents) in some Sensory nerve endings found in 14-35 Oral suckers and genital pores of parasitic groups Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Reproduction and Regeneration Fission Many turbellarians constrict behind the pharynx and separate into two animals Each half regenerates the missing parts Provides for rapid population growth Some do not separate immediately, creating chains of zooids Regeneration If the head and tail are cut off 14-36 Each end grows the missing part; it retains polarity Extract of heads added to a culture of headless worms prevents regeneration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes 14-37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Some asexual reproduction occurs in intermediate hosts Nearly all are monoecious but cross-fertilize Endolecithal eggs with spiral determinate cleavage are typical and ancestral Some turbellarians and all other groups have female gametes with little yolk 14-38 Yolk is contributed by separate organs, vitellaria Vitelline ducts bring yolk cells to the zygote (ectolecithal development) A cleavage pattern cannot be distinguished Zygote and yolk cells surrounded by eggshell move into the uterus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Male Structures One or more testes are connected to vasa efferentia that connect to one vas deferens The vas deferens runs to a seminal vesicle A papilla-like penis or extensible cirrus is the copulatory organ Turbellarians develop male and female organs opening at a common pore After copulation, eggs and yolk cells enclosed in small cocoon 14-39 Attach by a stalk to plants Embryos emerge and resemble little adults Embryos of some marine forms are ciliated, freeswimming larvae Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Larval trematodes emerge as ciliated larvae Cestodes hatch only after being consumed by a host Penetrate a snail or eaten by a host Many different animals can serve as intermediate hosts Trematoda, Monogenea, and Cestoda 14-40 United into a single clade called Neodermata Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Class Turbellaria Mostly free-living Range from 5 mm to 50 cm long Except for polyclads, endolecithal turbellarians Polyclads have a folded pharynx and a gut with many branches 14-41 Simple gut or no gut and a simple pharynx Larger polyclads have more highly branched intestines Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-42 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Members of order Tricladidia are ectolecithal Have a three-branched intestine Very small planaria swim by cilia Others move by cilia Glide over a slime track secreted by adhesive glands Rhythmical muscular waves pass backward from the head 14-43 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Class Trematoda All trematodes are parasitic flukes Most adults are endoparasites of vertebrates They resemble ectolecithal turbellaria but the tegument lacks cilia in adults Adaptations for parasitism include: Penetration glands Glands to produce cyst material Hooks and suckers for adhesion Increased reproductive capacity 14-45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Some trematodes retain ancestral characteristic of Alimentary canal and reproductive, excretory and nervous systems Sense organs are poorly developed 14-46 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Subclass Aspidogastrea Least well-known Most have only a single host Usually a mollusk If there is a second host 14-47 Usually a fish or turtle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Subclass Digenea Indirect life cycle in most Definitive or final host First intermediate host a mollusc Vertebrate Organisms reproduce sexually in this host A 2nd or 3rd intermediate host may be required in the life cycle Parasitize a wide range of hosts 14-48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes General Digenean Life Cycle Egg passes from definitive host in excreta and must reach water Hatches into a free-swimming ciliated larva, the miracidium Miracidium penetrates tissues of a snail Transforms into a sporocyst Sporocyst reproduces asexually to form sporocysts or rediae Rediae reproduce asexually and form rediae or cercariae 14-49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Cercariae emerge from the snail Penetrate a 2nd intermediate host or encyst on objects Develop into metacercariae (juvenile flukes) Metacercaria develop into adults when eaten by definitive host when Some serious parasites of humans and domestic animals are digeneans 14-50 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Sheep Liver Fluke Fasciola hepatica Adult fluke lives in bile passageways in the liver of sheep and other ruminants Eggs are pass out in feces Miracidia hatch and penetrate snails to become sporocysts After two generations of rediae First digenean whose life cycle was described Cercaria encyst on vegetation and await being eaten by sheep When eaten, metacercariae develop into young flukes 14-51 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Clonorchis sinensis: Human Liver Fluke Most important human liver fluke Common in China, Japan, and Southeast Asia Also infects cats, dogs, and pigs Adult fluke is 10–20 mm long with an oral and ventral sucker Digestive system includes pharynx, esophagus, and two long intestinal ceca Excretory system has two protonephridial tubules with branches with flame cells 14-52 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Nervous system Males Two cerebral ganglia and longitudinal cords with transverse connectives Testes, two vasa efferentia uniting to a vas deferens, seminal vesicle, and ejaculatory duct No cirrus Females 14-53 Branched ovary, and a short oviduct joined by ducts from seminal receptacle and vitellaria at the ootype Ootype is surrounded by Mehlis’ gland Uterus then extends to the genital pore Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Clonorchis Life Cycle Adults live in bile passageways of humans and other fish-eating mammals Eggs containing a complete miracidium are shed into water with feces The eggs hatch only when ingested by snails of specific genera Miracidium enters snail tissue and transforms into a sporocyst Sporocyst produces one generation of rediae, which begin differentiation 14-54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-55 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Rediae pass into the snail liver Cercariae escape into water Make contact a fish in the family Cyprinidae Bore into fish muscles or under scales Shed tail and encyst as metacercariae A mammal eats raw fish Continue embryonation into tadpole-like cercariae Cyst dissolves and flukes migrate up bile duct Heavy infection can destroy the liver and result in death Control of parasites 14-56 Destroy snails and thoroughly cook fish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Schistosoma: Blood Flukes Over 200 million people infested with schistosomiasis Common in Africa, South America, West Indies, and the Middle and Far East Sexes are separate 3 species account for most human schistosomiasis: 14-57 S. mansoni in venules of large intestine S. japonicum in venules of small intestine S. haemotobium in venules of urinary bladder Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Schistosoma Life Cycle Eggs discharged in human feces or urine In water, eggs hatch as ciliated miracidia Must contact a particular species of snail to survive In the snail, they transform to sporocysts Sporocysts produce cercaria directly Cercariae escape the snail and swim until they contact bare human skin Cercariae pierce the skin and shed their tails 14-58 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Enter blood vessels and migrate to the hepatic portal blood vessels Develop in the liver and they migrate target sites Eggs released by females are extruded through gut or bladder lining and exit with feces or urine Eggs that remain behind become centers of inflammation 14-59 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-60 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-61 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Eggs of S. mansoni and S. japonicum Eggs of S. haematobium Damage the intestinal wall Damages the bladder wall Control: proper disposal of human wastes Schistosome dermatitis (swimmer’s itch) 14-62 Occurs when cercariae penetrate an unsuitable host such as a human Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Paragonimus: Lung Flukes Paragonimus westermani Lung fluke that parasitizes humans, pigs, rodents, etc. Eggs are coughed up in sputum, then swallowed and eliminated in feces Zygotes develop in water and miricidia penetrate a snail host Within the snail, miricidia give rise to sporocysts, which develop into rediae 14-63 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-64 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Cercariae are shed into the water and ingested by freshwater crabs Metacercariae develop in freshwater crabs Human infection occurs by eating uncooked crabmeat 14-65 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Other Trematodes Fasciolopsis buski Lives in human intestines contracted from eating raw aquatic vegetation Leucochloridium 14-66 Produces remarkably colorful sporocysts in snails’ heads Attracts birds to eat snails and continue the life cycle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Classification of Phylum Platyhelminthes Class Turbellaria Class Trematoda Class Monogenea Class Cestoda 14-67 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Class Monogenea Monogenetic flukes were originally placed in Trematoda Some now argue they are sister taxa, both having a posterior attachment with hooks External parasites of fish, especially gills, but a few are found in bladders of frogs and turtles Have direct life cycle in a single host Oncomiracidium attaches to host by posterior hooks 14-68 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Posterior hooks may become the posterior attachment organ of the adult, the opisthaptor Opisthaptors vary widely (hooks, suckers, clamps) Withstand the force of water flow Some serious economic problems in fish farming 14-69 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-70 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Class Cestoda Tapeworms have long flat bodies with scolex Holdfast structure with suckers and hooks Scolex is followed by a linear series of reproductive units or proglottids Lack a digestive system Muscles, excretory and nervous systems similar to other flatworms Lack sensory organs except for modified cilia 14-71 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-72 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-73 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Tegument is syncytial and has no cilia Entire surface of cestodes is covered with projections (microtriches) similar to microvilli seen in the vertebrate small intestine 14-74 Microtriches increase the surface area for food absorption Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Subclass Eucestoda Aside from two orders of lesser importance, all have proglottids and are polyzoic Larvae have six hooks on the scolex Chain of proglottids is called a strobila Proglottids originate in the germinative zone just behind the scolex Some practice self-fertilization, although the norm is cross-fertilization Shelled embryos form in the uterus 14-75 Either expelled or the whole proglottid is shed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Proglottid formation is not “true” segmentation Nearly all cestodes require two hosts Replication of sex organs not equivalent to metamerism in annelids, etc. Adult is parasitic in the digestive tract of the vertebrate Over 1000 species of tapeworms known, infecting almost all vertebrates Most tapeworms do little harm to host 14-76 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Taenia saginata: Beef Tapeworm Lives as an adult in the alimentary canal of humans Juvenile form found in intermuscular tissue of cattle Mature adults can reach over 10 meters in length with over 2000 proglottids Scolex has four suckers but no hooks Gravid proglottids (with shelled, infective larvae) pass in feces 14-77 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Excretory canals run from scolex along proglottids Flame cells attach to excretory ducts Nerve cords from a nerve ring in the scolex run along proglottids Each mature proglottid has muscles and parenchyma plus male and female organs This order contains vitellaria in a single vitelline gland Gravid proglottids usually crawl out of feces Proglottids rupture as they dry 14-78 Embryos are viable for five months and are picked up by grazing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Life Cycle Cattle swallow shelled larvae that hatch as oncospheres Oncospheres use hooks to burrow through the intestinal wall into blood or lymph vessels When they reach voluntary muscle, they encyst to become bladder worms (cysticerci) When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal mucosa 14-79 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes New proglottids develop in 2–3 weeks Infected individuals expel numerous proglottids daily Infection can be avoided by eating only thoroughly cooked beef 14-80 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-81 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-82 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Taenia solium: Pork Tapeworm Adults live in small intestine of humans Juveniles live in muscles of pigs Scolex has both suckers and hooks on the rostellum If eggs or proglottids are ingested Embryos migrate to organs and form cysticerci Cysticercosis commonly occurs in eyes or the brain blindness, serious neurological symptoms or death Infection can be avoided by eating thoroughly cooked pork 14-83 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Diphyllobothrium latum: Fish Tapeworm Adults found in intestines of humans, dogs, cats and other mammals Immature stages found in crustaceans and fish Largest cestode of humans, reaching up to 20 meters in length 14-84 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Echinococcus granulosus: Unilocular Hydatid Adults parasitize dogs and other canines Juveniles infest many mammals Humans may serve as intermediate host Juveniles are a special cysticercus, a hydatid cyst, that grows for up to 20 years Main cyst maintains a single chamber Daughter cysts bud off with thousands of scolices, each able to produce a worm if eaten Surgical removal is the only treatment 14-85 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-86 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Phylogeny and Adaptive Diversification Of Platyhelminthes Phylogeny Even excluding acoels, the taxon Turbellaria is paraphyletic Ectolecithal turbellarians 14-87 Allied with trematodes, monogeneans, and cestodes as the sister group to endolecithal turbellarians Neodermatans (trematodes, monogeneans, and cestodes) form a monophyletic group Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Platyhelminthes Features of the most recent common ancestor is debated Planuloid ancestor probably gave rise to sessile branch that was radial and another branch that became creeping and bilateral Bilateral symmetry provided cephalization Ancestral form would have had a simple body with a blind gut 14-88 Advantage where sensory structures move to the head Body shape and metabolic requirements ideal for parasitic lifestyles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Mesozoa Considered a “missing link” between protozoa and metazoa Have a simple level of organization All live as parasites in marine invertebrates Most composed of only 20 to 30 cells arranged in two layers Minute, ciliated, and wormlike animals Layers are not homologous to germ layers of other metazoans Two classes, Rhombozoa and Orthonectida, are so different that some authorities place them in separate phyla. 14-89 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Mesozoa Rhombozoans Live in kidneys of benthic cephalopods Adults called vermiforms and are long and slender Inner, reproductive cells give rise to vermiform larvae When overpopulated, reproductive cells develop into gonad-like structures producing male and female gametes Larvae are shed with host urine into the seawater 14-90 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-91 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-92 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Mesozoa Orthonectids Parasitize variety of invertebrates Reproduce sexually and asexually 14-93 Asexual reproduction consists of a multinucleated mass called a plasmodium Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Mesozoa Phylogeny of Mesozoans Some consider these organisms primitive flatworms and place them in phylum Platyhelminthes Molecular evidence groups them with flatworms in superphylum Lophotrochozoa However, molecular phylogeny that included an orthonectid and two species from a rhombozoan subgroup, the dicyemids, did not show members of the two classes to be sister taxa 14-94 The phylum may not be monophyletic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Characteristics Often called ribbon worms Have a long muscular tube, the proboscis Over 1000 species Most are less than 20 cm long General body plan similar to that of turbellarians Epidermis is ciliated with many gland cells Excretory system has flames cells; several have rhabdites Mostly dioecious 14-95 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-96 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-97 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-98 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Helmet-shaped pilidium larva Adult has an anus, producing a complete digestive system that is more efficient Simplest animals with a blood-vascular system Most are marine Ventral mouth but no anus resembling flatworms and trochophore larvae of annelids and molluscs Some are found in moist soil and freshwater Few are commensals or parasites 14-99 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Form and Function Slender and fragile Amphiporus is a common example Dorsoventrally flattened with rounded ends Body wall is ciliated columnar cells and layers of circular and longitudinal muscles Partly gelatinous parenchyma fills space around organs Anterior end has ocelli, a mouth and a separate opening of the proboscis 14-100 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14-101 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Proboscis is an eversible organ Protruded from a rhynchocoel for defense and catching prey Proboscis is everted by fluid pressure and retracted by muscles Has a sharp-pointed stylet at the tip 14-102 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Locomotion Movement is by both musculature and cilia Some glide on the substrate Some use the proboscis to attach and draw the body forward 14-103 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Feeding and Digestion Carnivorous: Feed on dead or living prey Slime-covered proboscis wraps around prey Stylet pierces and holds prey until it is thrust into mouth Pours a neurotoxin, tetrodotoxin (the toxin in puffer fishes) on its prey Complete digestive system has a dilated stomach and an intestine with lateral ceca The tract is lined with ciliated epithelium and glandular cells in the esophagus Food digested in the intestinal tube is absorbed into the blood-vascular system 14-104 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Circulation Blood-vascular system Single dorsal vessel and two lateral vessels Blood is colorless and contains nucleated corpuscles Some have colored pigments with unknown functions No heart 14-105 Blood moved by muscular walls of blood vessels and by body movements Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Excretion and Respiration Near the edge of body is a lateral tube with branches and flame cells Wastes picked up from parenchymal spaces by flame cells are carried out excretory ducts Protonephridia are so closely associated with circulatory system that they are truly excretory rather than simply osmoregulatory in function as in flatworms Respiration occurs through the body surface 14-106 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Nervous System Brain composed of Four fused ganglia, one pair dorsal and one pair ventral Five longitudinal nerves extend backward from the brain The proboscis, ocelli and other sense organs have nerves leading to the brain Sense organs include tactile papillae, sensory pits and grooves, and probably auditory organs 14-107 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Reproduction and Development Amphiporus is dioecious Gonads discharge eggs or sperm through short ducts Fertilization occurs in the water As eggs are produced, other visceral organs degenerate Cleavage is spiral and determinate 14-108 Mesoderm derived from both endoderm and ectoderm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Rhynchocoel develops from mesoderm but is not homologous to the coelom in other coelomate phyla Pilidium larvae bears a dorsal spike of fused cilia and lateral lobes 14-109 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Regeneration Nemerteans regenerate readily Some fragment themselves during certain seasons Tail sections can regenerate a new proboscis within a short time 14-110 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Nemertea Classification of Phylum Nemertea Class Enopla Class Anopla Phylogeny of Nemertea Much debate about the phylogenetic position of nemerteans Larval forms vary Nemertean body plan is controversial Are they coelomate or acoelomate? 14-111
