Phylum Platyhelminthes

1. Phylum Platyhelminthes The Flatworms Part 1 Free-living Flatworms

2. Emerging Patterns in Evolution • Evolution of a head end: cephalization • Mesoderm • Kidney • Internal fertilization

3. Bauplan • Dorsoventrally flattened • Triploblastic • Embryo has 3 cell layers • Ectoderm (cnidarians) • Endoderm (cnidarians) • AND NOW, introducing MESODERM! • Acoelomate - Hey, where’s my cavity?

4. endoderm

5. Bauplan • Bilaterally symmetrical • Nervous system • Anterior brain • Paired ventral nerve cords • Longitudinal • Incomplete gut • No anus • Food exits via mouth

6. Bauplan • Excretory system & osmoregulation • Diffusion across body wall • Protonephridia • Specialized excretory organs • May also help osmoregulate

7. Life History • Most are simultaneous hermaphrodites • Function as male and female at the same time • Transfer sperm and receive sperm at same time • Most are not self-fertile • (A few exceptions)

8. Classification • Phylum Platyhelminthes • Classes • Turbellaria: free-living flatworms • Trematoda: flukes • All are parasites • Cestoda: tapeworms • All are parasites • Monogenea? (Maybe belong with cestoda) • Ectoparasites of fish

9. Class Turbellaria • Free living • Freshwater • Planarians such as Dugesia • A few inhabit terrestrial (moist) habitats

11. Class Turbellaria • Marine species (beautiful colors) • Significant members of coral reef ecosystems • Some are major predators of colonial ascidians (sea squirts) • Others are pests of commercial clams and oysters • Some live symbiotically on a variety of reef invertebrates

18. Class Turbellaria • Locomotion • Usually by cilia • Dugesia can move at about 1.5 mm/sec • Peristaltic waves can achieve higher velocities • Marine flatworms can use this to swim gracefully

19. Class Turbellaria • Locomotion • Terrestrial planarians • Glide smoothly on the substrate by the action of powerful, closely spaced cilia in a special medial ventral strip (creeping sole), on a thin coat of mucus secreted on the substrate by glands opening into the creeping sole • Planarians that migrate on plants or objects above the ground sometimes lower themselves to the ground on a string of mucus.

20. Class Turbellaria • Body construction • See lab notes • Live Dugesia • Slides of whole planaria and cross sections • Respiration • Diffusion • Must be less than 0.5-1 mm thick for diffusion to be effective. Flatworms are flat by necessity

21. Class Turbellaria • Excretion • NH4 excretion mostly by diffusion through epidermis • Protonephridia • Mostly osmoregulatory but may also help with excretion • Pair of longitudinal canals • Open to outside through two dorsal pores • Tributaries to excretory canals highly branched, ramify throughout the body;

22. Class Turbellaria • Protonephridia • One end of the tubule opens through a small pore to the exterior.  The other end of the tube ends blindly within the body in a spherical structure containing long cilia - these are called flame cells • Excess water (and possibly wastes) enters the flame cell system and is propelled through the tubules toward the outside by the beating of the cilia (the "flame").

23. Excretory pores

24. Class Turbellaria • Protonephridia • Branches terminate in blind flame cells. • Flame cells have slits that penetrate the cell • Slits are crossed by filaments or a membrane that reduce the effective pore size • Slits act as an ultrafilter to keep back large proteins • Filtrate of mesenchymal intercellular fluid enters tubule. • Inorganic and organic materials actively reabsorbed in tubule; remainder is excreted

26. Class Turbellaria • Feeding Ecology • Most turbellarians are carnivorous predators or scavengers. • Carnivores feed on organisms that they can fit into their mouths, such as protozoans, copepods, small worms, and minute mollusks. • Some species use mucus that may have poisonous or narcotic chemicals to slow or entangle prey. • Some have specific diets and feed on sponges, ectoprocts, barnacles, and tunicates.

27. Class Turbellaria • Feeding Ecology • Several species have commensal relationships with various invertebrates and few actually border on being parasitic because they graze on their live hosts. • Land planarians devour earthworms, slugs, insect larvae, and are cannibalistic. Prey are located by chemoreceptors located in a single ciliated pit under the head or in a ciliated ventral groove.

28. Class Turbellaria • Feeding Ecology • Land planarians • Struggling prey are held to the substrate and entangled in slimy secretions from the planarian. • A few species have symbiotic algae that supply the worm with carbohydrates and fats and the worm supplies the algae with nitrogen waste products and a home.

29. Class Turbellaria • Feeding and Digestion • The pharynx is protruded from the mouth and into the prey. • The pharynx and gut cells produce digestive enzymes that breakdown food extracellularly. • Nutritive cells in the gastrodermis then phagotize partially digested material that is distributed throughout the body. • Because these worm lack a circulatory system, larger species have extensive anastomosing guts to aid in distribution. • Since these worms have incomplete guts, all waste must pass back out of the mouth.

31. Class Turbellaria • Nutrition • Planaria store food in digestive epithelium and can survive many weeks shrinking slowly in size without feeding. • They are capable of utilizing their own tissues such as reproductive tissue for food when reserves are exhausted. • Lab animals often tend to shrink in size when not fed properly • Liver or egg yolk • Dugesia feeds various invertebrates, including mosquito larvae

33. Class Turbellaria • Digestive system • Ventral mouth • Muscular pharynx • A blind intestine (details depend on order) • Tricladida — 10-20 mm long • Tripartite gut; one main anterior branch and two main posterior branches with numerous blind pockets off all three. Dugesia • Polycladida — up to 5 cm long • Gut has numerous branches ramifying throughout the body.

34. Polycladida digestive system

37. Class Turbellaria • Nervous system and sensory organs • Dorsal, anterior eyes • Ciliated pits behind auricles on head are probably chemosensors • Dorsal, bilobed brain underlays eyes • Primitively 3-4 pairs of longitudinal nerve cords • Reduced to two longitudinal ventral nerve cords • Run down length of body with numerous cross connections and branches in most groups.

40. BRAIN Auricle Cerebral ganglion Paired nerve cords

41. (connectives)

42. Retinular cells Light sensitive region Pigment cups

43. Class Turbellaria • Reproduction • Hermaphroditic • Worms are male and female at same time • Most do not self fertilize • Fertilization is internal • Stab penis through body wall in marine flatworms • See film notes

46. nervous system male female gut

48. Class Turbellaria • Development • Turbellarians have either direct development or produce a pelagic larva. • Polyclads often produce a pelagic Muller's larva that settles to the bottom and goes through metamorphosis in a few days. • This larva has eight ventrally directed ciliated lobes, which it uses to swim.

49. Metamorphosis of Muller’s larva into a free-living Turbellarian (above) A living larvae (right)

50. Class Turbellaria • Asexual reproduction • Architomy • Type of fission in which the worm divides into two fragments without prior differentiation of new parts. • Transverse cleavage just posterior to the pharynx divides the worm into an anterior, nearly normal, worm with head, mouth, pharynx and most of the gut, and an incomplete, headless posterior mass of tissues which must replace its missing parts. • Following division, the anterior end behaves normally but the posterior end remains immobile until regeneration is complete and the missing parts replaced.