CHAPTER 8

1. CHAPTER 8 Acoelomates

2. 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

3. General Features • Triploblastic • Acoelomates • Typical acoelomates have only one internal space, the digestive cavity • Region between the epidermis and digestive cavity is filled with parenchyma

5. Phylum Acoelomorpha Characteristics • Small flat worms less than 5 mm in length • Typically live in marine sediments; few are pelagic • Have a cellular ciliated epidermis • Parenchyma layer contains small amount of ECM and circular, longitudinal, and diagonal muscles

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 or neodermis

8. 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 • All members of Monogenea and Trematoda (flukes)and Cestoda (tapeworms) are parasitic

10. 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

13. Phylum Platyhelminthes Nutrition and Digestion • Cestodes have no digestive system • Others have a mouth, pharynx, and intestine Excretion and Osmoregulation • Flatworms have protonephridia • Used for osmoregulation • 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 • Lack these units • No need to expel excess water

15. 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 • Sensory nerve endings found in • Oral suckers and genital pores of parasitic groups

16. 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 • Each end grows the missing part; it retains polarity • Extract of heads added to a culture of headless worms prevents regeneration

17. Phylum Platyhelminthes • Some asexual reproduction occurs in intermediate hosts • Nearly all are monoecious but cross-fertilize • Endolecithal eggs • Some turbellarians and all other groups have female gametes with little yolk • Yolk is contributed by separate organs, vitellaria • Vitelline ducts bring yolk cells to the zygote (ectolecithal development) • Zygote and yolk cells surrounded by eggshell move into the uterus

18. Phylum Platyhelminthes • Larval trematodes emerge as ciliated larvae • Penetrate a snail or eaten by a host • Cestodes hatch only after being consumed by a host • Many different animals can serve as intermediate hosts

19. Phylum Platyhelminthes Classification of Phylum Platyhelminthes • Class Turbellaria • Class Trematoda • Class Monogenea • Class Cestoda

20. Phylum Platyhelminthes Class Turbellaria • Mostly free-living • Range from 5 mm to 50 cm long • Others move by cilia • Glide over a slime track secreted by adhesive glands • Rhythmical muscular waves pass backward from the head

22. 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

23. Phylum Platyhelminthes Sheep Liver Fluke • Fasciola hepatica • First digenean whose life cycle was described • 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 • Cercaria encyst on vegetation and await being eaten by sheep • When eaten, metacercariae develop into young flukes

24. Phylum Platyhelminthes Clonorchis sinensis 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

25. 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

27. 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

28. 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

30. Phylum Platyhelminthes Class Monogenea • External parasites of fish, especially gills, but a few are found in bladders of frogs and turtles • Have a single host • 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

32. 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 • 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

34. Phylum Platyhelminthes Taenia saginata: Beef Tapeworm • Lives as an adult in the digestive system of humans • Juvenile form found in intermuscular tissue of cattle • Mature adults can reach over 10 meters in length with over 2000 proglottids • Gravid proglottids (with shelled, infective larvae) pass in feces

35. 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 attaches to intestinal mucosa

38. Phylum Mesozoa • Considered a “missing link” between protozoa and metazoa • Have a simple level of organization • Minute, ciliated, and wormlike animals • All live as parasites in marine invertebrates • Most composed of only 20 to 30 cells arranged in two layers • Layers are not homologous to germ layers of other metazoans

39. Phylum Nemertea Characteristics • Often called ribbon worms • Have a long muscular tube, the proboscis • Most are less than 20 cm long • 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

44. 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

45. 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

46. CHAPTER 9 Gnathiferans and Smaller Lophotrochozoans

47. Protostomia Phylogeny • Large clade sometimes called a superphylum • Protostome phyla are divided between two large clades • Lophotrochozoa and Ecdysozoa • Lophotrochozoa (10 Phyla) • Ancestors possessed complex cuticular jaws • Gnathostomulida • Micrognathozoa • Rotifera • Acanthoephala

48. Protostomia • 6 other lophotrochozoan phyla • Gastrotricha • Tiny aquatic animals that may be closely related to gnathiferans • Molecular characteristics place the following with Lophotrochozoa • Cycliophora • Entoprocta • Ectoprocta • Brachiopoda • Phoronida