Flatworms, Mesozoans, and Ribbon Worms

1. Flatworms, Mesozoans, and Ribbon Worms Chapter 14

2. Flatworms, Mesozoans, and Ribbon Worms • Recall: • Sponges- First Animal Specialized Cells Organization, Asymmetric • Cnidarians/Ctenophorans- Tissue Organization, Diploblastic, Radial Symmetry Now: EvolutionaryMilestones: Bilateral, Triploblastic, Acoelomate

3. Simple To More Complex

4. Bilateria • Most animals have bilateral symmetryand triploblastic development.

5. Bilateral Symmetry • Radially symmetrical animals have the world coming at them from all directions. • They can catch prey coming from any direction. (ex. Sea Anemone) • Animals that begin to move about actively seeking food need a different body organization. • Distinct head (anterior) end with sensory structures is called Cephalization!

6. Bilateral Symmetry • Animals with bilateral symmetryhave a distinct head end and can be divided into right and left halves.

7. Triploblastic • 3 embryonic germ layers • Ectoderm • Endoderm • Mesoderm

8. Acoelomate Bilateral Animals • Animals that have no space between their gut and body wall are said to be acoelomate. • Organ level of organization

9. Acoelomates • Although flatworms undergotriploblasticdevelopment, they are acoelomates.

10. Acoelomates • These acoelomate phyla are protostomes and have spiral cleavage. • Protostomes = “first mouth” • Blastopore becomes mouth

11. Four Phyla (Simple to Complex) • Phylum Acoelomorpha • Small flatworms • Phylum Platyhelminthes • The flatworms (planarians, parasitic) • Phylum Mesozoa • Ciliated worm-like animals • Phylum Nemertea • Ribbon worms

12. Phylum Acoelomorpha • General Characteristics: • Bilateral, Triploblastic, Acoelomate • Group contains ~350 species. • Small flat worms less than 5 mm in length. • Typically live in marine sediments; few are pelagic (open ocean) • Most symbiotic but some parasitic.

13. Acoels: Locomotion • Have a cellular ciliated epidermis. • Parenchyma layer contains small muscles cells (from mesoderm). • Slow Swimming • Very Primitive

14. Acoels: Digestion and Nutrition • Incomplete digestive system - no anus. • Mouth—Pharynx--Gut • In many acoels, the gut and pharynx are absent. Then: • Phagocytotic cells digest food intracellularly when food is passed into temporary spaces.

15. Acoels: Reproduction • Asexual: • Fragmentation: Splits off and regrows • Sexual: • Monoecious (can produce both egg and sperm-hermaphroditic) • Fertilization occurs in water

16. Acoels: Sensory, Excretory, Respiratory • Nervous “System”: • Simple cephalization- collection of nerve cells with radial arrangement (not ladder pattern) • Statocysts present (balance) • NO Excretory and Respiratory Systems!! • Diffusion!

17. Phylum Platyhelminthes • Members of Phylum Platyhelmintheslive in marine, freshwater, and damp terrestrial habitats. • Aquatic or Terrestrial

18. Platyhelminthes • General Characteristics: • Bilateral, Triploblastic, Acoelomate • Free-Swimming (planarian) or Parasitic (tapeworms) • Length Range: mm to meters • Free-Swimming are 1-3 cm Four Classes: Turbellaria, Trematoda, Monogenea, Cestoda

19. Platys: Locomotion • Free-Living: • Ciliated epidermis • Rhabdites (cells that swell and form a mucus sheath when in water) • Cells involved with attachment (adhesive organs); these are connected to the nervous “system” as well

20. Platys: Locomotion • Parasitic: • Muscle Cells in body wall • Has tegument- outer covering • Early form has ciliated covering but once in its host, shed so as to avoid the host immune system • Resistant to host immune system • Tegument can be Absorptive or Secretory • Absorbs nutrients from host • Secretes enzyme to lessen effectiveness of host’s digestive system

21. Platys: Digestion • Mouth (Middle of Body)—Pharynx—Intestine (gastrovascular cavity) • Carnivores • Detect food far away with -chemoreceptors • Entangle prey in mucus secretion from rhabdites (cells that swell) • Wraps body around prey, extends pharynx, and sucks up food in small amounts.

22. Platys: Digestion • Both extracellular and intracellular digestion • Digestive enzymes in intestines • Parasites: graze on host • Exception: Cestodans have no digestive tract but absorbs nutrients directly

23. Platys: Excretion, Osmoregulatory • The osmoregulatory system consists of protonephridia with flame cells. • Cells that aid in excretion. Most metabolic wastes removed by diffusion across the body wall aided by cilia.

24. Platys: Sensory • Simplest Flatworms’ sensory systems resemble nerve net of Cnidarians • Most Others: The nervous system consists of a ladder-like network of nerves and a bilobed brain (ganglion). • Many have large ocelli (EYESPOTS)– light sensing organs.

25. Platys: Reproduction • Many can reproduce asexually as well as sexually. • Asexual reproduction via fission. • Also- regeneration- unique in that regenerates head or tail depending on what was lost! • Budding in intermediate host

26. Platys: Reproduction • Sexual: • Most hermaphroditic • But cross fertilize (do not fertilize self!) • Genital pore opens to both male and female organs • Copulation, fertilized egg enclosed in cocoon, attaches to stones or underside of plants

27. Taxonomy • Flatworms (phylum Platyhelminthes) are divided into four classes: • Class Turbellaria – ex. Planaria • Class Trematoda – parasitic flukes • Class Monogenea – parasitic monogenetic flukes • Class Cestoda - tapeworms

28. Phylum Platyhelminthes

29. Class Turbellaria • Turbellarians are nearly all free-living and mostly marine.

30. Class Turbellaria • Free-swimming; 5 mm to 50 cm in length • Marine, Freshwater, Terrestrial • Creeps or glides

31. Class Turbellaria • The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.

32. Class Turbellaria • Planarian Dugesia

33. Class Trematoda • Trematodes live as parasites in or on other animals. • They parasitize a wide range of hosts. • Leaflike in form; one or more suckers (hook and suckers) • Special ability to increase reproduction

34. Class Trematoda • Subclass Digenea, digenetic flukes, have a complex life cycle with a mollusc (snail) as the first host and a vertebrate as the final, or definitive, host. • Can live in digestive, respiratory, circulatory, urinary reproductive systems/tracts

35. Class Trematoda • Can acquire directly from water or ingest aquatic vegetation • “liver rot” in sheep

36. Liver Flukes (Class of Trematodes) • Human parasite • Cats, dogs and pigs also • 10-20 mm in length, oral sucker • 80% body devoted to reproduction • hermaphroditic • Bile passageway of humans • Effects: abdominal pain; serious- cirrhosis of liver and possible death • To Avoid: cook fish, snail control • Species of Digenean!

37. Blood Flukes and Others • Schistosomiasis- infection of blood flukes (Digenean!) • Major infectious disease- 200 million infected • Africa, South America, Middle East • Live in veins (endoparasite) • Effects: ulcers, abscesses, bloody diarrhea (from eggs) • To Avoid: improved sanitation (avoid contaminated water and poorly disposed waste) • Others: swimmer’s itch (from infected lakes); lung flukes

38. Life Cycle of Blood Fluke • 1. Egg passes from feces into water • 2. Miracidium penetrates snail (Intermediate Host!) • 3. Sporocyst develops in snail • 4. Cercaria (larva) released in water, penetrate human

39. Class Monogenea • All monogeneans are parasites. • Often found in the gills or external surfaces of fishes. (mostly ectoparasites) • Also, bladder of frogs or turtles; eye of hippos • Little damage to host done unless over crowding occurs (fish farms) • Life Cycle: Egg—Free Swimming Larva attaches to host---Reproduce • ONE HOST!

40. Class Cestoda • Tapeworms (Class Cestoda) are also parasitic and lack a digestive system. • The scolex is equipped with suckers and hooks for attachment to the host. • Each proglottid contains a set of reproductive organs.

41. Class Cestoda • Cestodes usually require at least two hosts. • Body lined with microvilli to increase surface area for absorption • Possible to self-fertilize • 1000 known species

42. Other Tapeworms • Life Cycle • Human intestines as adults • In cattle muscle • Can get 10 m long • Can get from raw or rare meat (Beef/Pork) • 1% of American cattle infected • 20% cattle meat not inspected

43. Phylum Mesozoa • General Characteristics: • Phylum Mesozoa is 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(Crabs,Crayfish) • Most composed of only 20 to 30 cells arranged in two layers.

44. Phylum Mesozoa • Some live in kidneys of benthic cephalopods. • Larvae are shed with host urine into the seawater.

45. Phylum Nemertea • Ribbon worms, phylum Nemertea, use a proboscis to capture prey. • Almost completely marine. • Active predators (NONPARASITIC). • General body plan similar to turbellarians (Has flame cells!)

46. Phylum Nemertea • An anusis present providing these worms with a complete digestive system. • Nermeteans are the simplest animals to have a closed loop blood-vascular system. • Reproduction: Dioecious (Egg or Sperm)

47. Ribbon Worms • Types of Ribbon Worms

48. Phylogeny • A planuloid ancestor(like the planula larva of cnidarians?) may have given rise to a branch of descendents that were sessile or free floating and radial Cnidaria. • Another branch acquired a creeping habit and bilateral symmetry Bilateria.