Chapter 8 ACOELOMATE BILATERAL ANIMALS 14-1 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 – 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 14-2 General Features Position and Biological Contributions • • • • Simplest animals with primary bilateral symmetry Mesoderm well-defined Triploblastic-3 primary germ layers Highly specialized parasites 14-3 General Features • Acoelomates-no body cavity – Typical acoelomates have only one internal space, the digestive cavity • Region between the epidermis and digestive cavity is filled with parenchyma 14-4 15-5 Insert new figure 14.5 14-6 Phylum Platyhelminthes Characteristics • • • • Commonly called flatworms Vary from a millimeter to many meters in length Some free-living; others parasitic The parasitic clades – Share an external body covering called a syncytial tegument 14-7 Phylum Platyhelminthes • Platyhelminthes is divided into four classes: • Turbellaria-planaria • Trematoda- digenetic ( 2 hosts) flukes • Monogenea- monogentic ( 1host) flukes – Cestoda-tapeworms (1-3 hosts) – All members of Monogenea and Trematoda (flukes) and Cestoda (tapeworms) are parasitic 14-8 Phylum Platyhelminthes – 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 14-9 Phylum Platyhelminthes Form and Function • Epidermis and Muscles – Most tubellarians 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 14-10 14-11 14-12 Phylum Platyhelminthes • Some turbellarians, and all other members of this phylum – Turbellians have a syncytial epidermis • Nuclei are not separated by cell membranes • Non-turbellarians- parasitic classes – Lack cilia and have a tegument • Under the basement membrane – Muscle fibers run circularly, longitudinally and diagonally • Parenchyma cells fill spaces in the body 14-13 14-14 Phylum Platyhelminthes Nutrition and Digestion • Cestodes-tapeworms have no digestive system • Others have a mouth, pharynx, and intestine • In planarians – Pharynx may extend through the ventral mouth • Intestine has three branches – One anterior and two posterior • Mouth of trematodes and monogeneans – Opens near the anterior end – Pharynx is not extensible – Intestine ends blindly, varies in degree of branching 14-15 Phylum Platyhelminthes • Planaria – 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 14-16 Figure 8_07 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-tapeworms – Rely on the host’s digestive tract – Lack a digestive system – Absorb digested nutrients 14-18 Phylum Platyhelminthes Excretion and Osmoregulation • Flatworms have protonephridia-flame cells – Used for osmoregulation – Beating flagella drive fluids through the weirs to tubules, into collecting ducts, excreted through pores – 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 • Marine turbellarians – Lack these units – No need to expel excess water 14-19 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 • Freshwater planarians – One ventral pair of nerve cords forming a ladder-type pattern – Brain is a bilobed ganglion anterior to the ventral nerve cords 14-20 Phylum Platyhelminthes Sense Organs • Active locomotion favored cephalization and evolution of sense organs • Ocelli (light-sensitive eyespots) – Present in turbellarians, monogeneans, and larval trematodes • Tactile and chemoreceptive cells – Abundant, especially in the ear-shaped auricles • Statocysts (equilibrium) and rheoreceptors (sense direction of water currents) in some 14-21 Phylum Platyhelminthes Reproduction and Regeneration • Fission – Many turbellarians constrict behind the pharynx and separate into two animals – Each half regenerates the missing parts – Some do not separate immediately, creating chains of zooids • Regeneration – If the head and tail are cut off • Each end grows the missing part – Extract of heads added to a culture of headless worms prevents regeneration 14-22 Phylum Platyhelminthes Class Trematoda • All trematodes are parasitic flukes • Most adults are endoparasites of vertebrates • They resemble turbellaria but the tegument lacks cilia in adults • Adaptations for parasitism include: – Penetration glands-lytic enzymes to enter skin – Glands to produce cyst material – Hooks and suckers for adhesion – Increased reproductive capacity 14-23 Phylum Platyhelminthes • Some trematodes share several characteristics of turbellarians • Similar reproductive, excretory and nervous systems • Sense organs are poorly developed 14-24 Phylum Platyhelminthes Subclass Digenea • Indirect life cycle in most – First intermediate host a mollusc • Definitive or final host – 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 14-25 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 14-26 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 14-27 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 Heavy infection may cause cirrhosis of the liver and death of human • Adult fluke is 10–20 mm long with an oral and ventral sucker 14-28 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 14-29 Phylum Platyhelminthes • Rediae pass into the snail liver – Continue embryonation into tadpole-like cercariae • 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 – Cyst dissolves and flukes migrate up bile duct • Heavy infection can destroy the liver and result in death • Control of parasites – Destroy snails and thoroughly cook fish 14-30 14-31 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: – Schistosoma mansoni in venules of large intestine – S. japonicum in venules of small intestine – S. haemotobium in venules of urinary bladder 14-32 Figure 8_10a 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 14-34 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 14-35 Figure 8_10b Phylum Platyhelminthes • Eggs of S. mansoni and S. japonicum – Damage the intestinal wall • Eggs of S. haematobium – Damages the bladder wall • Control: proper disposal of human wastes • Schistosome dermatitis (swimmer’s itch) – Occurs when cercariae penetrate an unsuitable host such as a human 14-37 Figure 8_11 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 14-39 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 14-40 14-41 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 14-42 14-43 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 – Microtriches increase the surface area for food absorption 14-44 14-45 Phylum Platyhelminthes • 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 from one proglottid to another • Shelled embryos form in the uterus – Either expelled or the whole proglottid is shed 14-46 Figure 8_15 Phylum Platyhelminthes • Nearly all cestodes require two hosts – 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 14-48 Phylum Platyhelminthes Taenia saginata: Beef Tapeworm • Lives as an adult in the alimentary canal-digestive tract) of humans • Juvenile form found in intermuscular tissue of cattle • Mature adults can reach over 10 meters (32 feet) in length with over 2000 proglottids • Scolex has four suckers but no hooks • Gravid proglottids (with shelled, infective larvae) pass in feces 14-49 Phylum Platyhelminthes • Each mature proglottid has muscles and parenchyma plus male and female organs • Gravid proglottids usually crawl out of feces • Proglottids rupture as they dry – Embryos are viable for five months and are picked up by grazing 14-50 Phylum Platyhelminthes Life Cycle • Cattle swallow shelled larvae that hatch • They 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 and waits for humans to eat undercooked beef. 14-51 Phylum Platyhelminthes • Once eaten by humans new proglottids develop in the intestines in 2–3 weeks • Infected individuals expel numerous proglottids daily • Infection can be avoided by eating only thoroughly cooked beef 14-52 14-53 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 • If eggs or proglottids are ingested by a human instead of pig – 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 14-54 Brain of person who died from cerebral cysticercosis 14-55 Phylum Rotifera Characteristics • Possess a ciliated crown, the corona, that beats like a rotating wheel • Approximately 2000 species • Aquatic species are mostly benthic 15-56 15-57 Phylum Rotifera • Rotifer body has a head, trunk and a foot • Corona is often a pair of cilated discs – Beating of the cilia help in feeding and locomotion – Mastax is a muscular pharynx equipped with hard jaws for grasping and chewing – Narrow foot has one to four toes and may be retractile Attaches with pedal glands that secrete an adhesive Pair of protonephridial tubules with flame cells empty into a cloacal bladder that collects excretory and digestive waste Bilobed brain and sensory organs include eyespots, sensory pits and papillae 15-58 Phylum Rotifera Reproduction • Dioecious • Males are unknown in many species • Females are parthenogenetic, producing diploid eggs that hatch into diploid females • Females produce – amictic eggs that form diploid females by parthenogenesis or – Haploid mictic eggs that, if not fertilized, become haploid males 15-59