1. Phylum Platyhelminthes (flatworms)

2. A phylogeny of
Animalia
Platyhelminthes
Bilateria

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

5. Emerging Patterns in Evolution
Bilateral symmetry
Dorsal & ventral, anterior & posterior
Associated with locomotion on a solid surface
Evolution of a head end: cephalization
Possess a head (leading end with sense organs)
Internal fertilization

7. Bauplan Dorsoventrally flattened Triploblastic
Lack circulatory system for transport‑ must rely on diffusion for gas exchange
Triploblastic
Embryo has 3 cell layers
Ectoderm (cnidarians)
Endoderm (cnidarians)
AND NOW, introducing THE MESODERM!
Acoelomate - Hey, where’s my cavity?

8. Phylum Platyhelminthes – tissue organization
Triploblastic = 3 tissue layers
Ectoderm (outer) Endoderm (inner) Mesoderm (between)

9. Muscles and Movement
Hydrostatic Skeleton

10. Bauplan Nervous system Incomplete gut Anterior brain
Paired ganglia and nerves
Paired ventral nerve cords
Longitudinal
Lateral
Incomplete gut
No anus
Food exits via mouth

13. Phylum Platyhelminthes - digestive system
Gastrovascular cavity
Protrusible pharynx in some taxa
Digestion is both intracellular and extracellular

14. Bauplan Excretory system & osmoregulation Diffusion across body wall
Protonephridia
Specialized excretory organs
Probably mainly for water balance - produces urine- a filtrate of extracellular fluid
May also help osmoregulate

15. Phylum Platyhelminthes- excretory system
“Protonephridial” system: blind‑ending tubes with flame cells & fenestrae at ends, connected with excretory pores
Probably mainly for water balance produces urine- a filtrate of extracellular fluid
Flatworms have extracellular fluid compartment cells protected from external environment
Cnidaria lack this- cells are in contact with water (in freshwater cnidaria, each cell has prominent contractile vacuoles)

16. Protonephridial osmoregulatory/excretory system
“flame cell”
fenestrae
duct

17. Bauplan Lack cuticle or other rigid skeletal support
Lack fluid-filled body cavity
Spermatozoa possess a pair of flagella that arise from the paired centrioles
Epidermal cells are typically multiciliated.
Neoblasts---stem cells that give rise to the other cell types in the body.

18. endoderm

19. Life History Most are simultaneous hermaphrodites
Function as male and female simultaneously
Transfer sperm and receive sperm simultaneously
Monoecious
Most are not self-fertile
(A few exceptions)
Most are oviparous (= produce shelled eggs that are released and hatch outside the body).
Most can also reproduce asexually by budding or transverse fission

20. Asexual reproduction by transverse fission (budding)
Interesting in 2 ways:
Another flatworm class (cestoda) is segmented ‑ they grow by serial reproduction of body parts
If you cut a turbellarian in half, it can regenerate
Regeneration involves undifferentiated cells in mesoderm called neoblasts

21. Platyhelminthes- 4 classes traditional
Class Turbellaria (turbellarians) mostly free-living
Class Trematoda (digenetic trematodes, flukes) all parasitic
Class Monogenea (monogenetic trematodes, gill flukes) all parasitic
Class Cestoda (tapeworms) all parasitic

22. Platyhelminthes
Inability of flatworms to synthesize fatty acids and sterols may explain their need for establishing symbiotic relationships
The parasitic classes of flatworms are linked by a synapomorphy- the tegument (see next slide)
Clade Neodermata

23. “Neodermata” The parasitic flatworms)
Turbellaria
“Neodermata” The parasitic flatworms)
(Tegument)

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

25. Turbellarians were traditionally classified based on the form of the gastrovascular cavity

26. Acoels are not Platyhelminthes???
rRNA analysis (see cladograms) shows that Acoela is basal to other bilateral clades
Digestive syncytium instead of a gut
Netlike nervous system, lack brain
Kinked cilia
Acoel eggs cleave only once and the two resulting cells immediately generate many small cells.
Link to more info on Acoela:

27. 18S rDNA-based maximum-likelihood tree of 61 metazoan species
From:Ruiz-Trillo et al. (1999) Acoel flatworms: earliest extant bilaterian metazoans, not members of Platyhelminthes. Science 283:
Platyhelminthes

28. Class Turbellaria Order Acoela
Small flatworms with no permanent gut cavity.
Free-living, marine and brackish water.

29. Rhabodocoel turbellarians
Diverse and important predators on meiofauna in freshwater ecosystems
Recent research on predation on freshwater bivalves

30. Mesostoma – a viviparous rhabdocoel – note the embryos within the uterus

31. Mesostoma – close-up showing the babies
C. Barnhart
Mom’s oral sucker
Babies
Mom’s eyes

32. Macrostomum – predator on juvenile bivalves

33. Eggs of Macrostomum

34. Platyhelminthes Turbellaria Tricladida Bipalium
A land planarian common in greenhouses world-wide – causing problems in Britain

35. Bipalium feeding on an earthworm

36. Class Turbellaria Order Tricladida Gut with three branches Free living
Freshwater
Planarians such as Dugesia
A few inhabit terrestrial (moist) habitats
Neodermata
Exclusively parasitic

38. 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 with/on a variety of reef invertebrates

39. Bdelloura- a marine triclad that is commensal with horseshoe crabs
(most turbellarians are not parasites)

46. Class Turbellaria Locomotion Usually by cilia on a layer of mucus
Dugesia can move at about 1.5 mm/sec
Peristaltic waves can achieve higher velocities
Marine flatworms can use this to swim gracefully
Also use cilia same way as protists do

47. Rhabdites secrete mucus
S. mediterranea: an experimental model for ciliated epithelia. (a) A typical S. mediterranea adult flatworm used in this study. (b) Cross section of paraffin-embedded planarian tissue counterstained with hematoxylin and eosin. Dorsoventral muscle bands (mb) divide the body into compartments that are traversed by branches of the gastrovascular (gv) cavity. The mesenchyme (me) of the planarian body is populated with numerous undifferentiated pluripotent cells, known as neoblasts. The epidermis consists of a simple cuboidal epithelium, composed of ciliated (ventral) or mostly nonciliated (dorsal) cells. Dispersed throughout the epithelium are secretory ducts (SD) and rhabdites (rhb); rhabdites in the dorsal surface appear larger in size. (c) Dorsal view of a live planarian. Motile cilia are readily visible on the lateral sides of the head region. The inset at right shows a magnified area of the left lateral side of the head where a row of cilia may be seen.

48. 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.

49. Class Turbellaria Body construction Lab notes Live Dugesia
Slides of whole planaria and cross sections

51. Class Turbellaria Respiration Diffusion
Must be less than mm thick for diffusion to be effective. Flatworms are flat by necessity

52. 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;

53. 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").

54. Excretory pores

55. 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

57. 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.

58. 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.

59. 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.

60. 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.

62. 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

64. Class Turbellaria Digestive system Ventral mouth Muscular pharynx
A blind intestine (details depend on order)
Tricladida — 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.

65. Polycladida digestive system

68. 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.

71. BRAIN
Auricle
Cerebral ganglion
Paired nerve cords

74. Light sensitive region
Retinular cells
Light sensitive region
Pigment cups

75. 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
Penis Fencing
See film notes

78. male
female
gut
nervous system

80. 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.

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

82. 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.

83. Class Turbellaria
Additional sketches from R. Fox.

85. The dorsal epidermis contains numerous secretory vesicles and rod-shaped membrane enclosed secretions, the rhabdites (rhabd = rod). Rhabdites are synthesized by epidermal gland cells submerged below the basal lamina into the parenchyma. When expelled at the surface, rhabdites absorb water and expand to become sticky mucus which may help trap small invertebrate prey.

90. Ecology and Evolution Many flatworms are brightly colored
Warning coloration (aposomatic coloration)
These flatworms are believed to be toxic or distasteful
Other species are mimics of toxic nudibranchs (sea slugs)

91. Ecology and Evolution Evolution of mimicry
Imagine that the nudibrach (a type of shell-less mollusc) Phyllidia is the 'model’
Has extremely toxic and distasteful secretions which deter fish from eating it
Fish learn to avoid the Phyllidia shape and color and so Phyllidia are somewhat protected from predation
Any animal that looks like Phyllidia will gain some protection from predation by 'tricking' predatory fish into thinking they are a Phyllidia .

92. Ecology and Evolution Evolution of mimicry
Imagine that the nudibrach Phyllidia is the 'model’
If a flatworm species has a vaguely similar shape and color to Phyllidia then those individuals that look most like a Phyllidia are most likely to escape fish attack
Gradually the surviving flatworms of each generation will become more and more like Phyllidia simply because only those with the genes to look like Phyllidia will survive.

93. Phyllidia - a sea slug (Phylum Mollusca)

95. Explain how this scenario fulfills the 3 conditions necessary for evolution to occur.