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Phylum Platyhelminthes (flatworms). A phylogeny of Animalia. Platyhelminthes. Bilateria. Phylum Platyhelminthes. The Flatworms Part 1: Free-living Flatworms. Emerging Patterns in Evolution. Bilateral symmetry Dorsal & ventral, anterior & posterior

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Phylum Platyhelminthes (flatworms)

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Phylum platyhelminthes flatworms

Phylum Platyhelminthes (flatworms)

Phylum platyhelminthes flatworms

A phylogeny of




Phylum platyhelminthes

Phylum Platyhelminthes

The Flatworms

Part 1: Free-living Flatworms

Emerging patterns in evolution

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



  • Dorsoventrally flattened

    • 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 THEMESODERM!

  • Acoelomate - Hey, where’s my cavity?

Phylum platyhelminthes flatworms

Phylum Platyhelminthes – tissue organization

  • Triploblastic = 3 tissue layers

    • Ectoderm (outer)Endoderm (inner)Mesoderm (between)

Muscles and movement

Muscles and Movement

Hydrostatic Skeleton



  • Nervous system

    • Anterior brain

      • Paired ganglia and nerves

    • Pairedventral nerve cords

      • Longitudinal

      • Lateral

  • Incomplete gut

    • No anus

    • Food exits via mouth

Phylum platyhelminthes flatworms

Phylum Platyhelminthes - digestive system

  • Gastrovascular cavity

  • Protrusible pharynx in some taxa

  • Digestion is both intracellular and extracellular



  • 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

Phylum platyhelminthes excretory system

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 ofextracellular fluid

  • Flatworms have extracellular fluid compartmentcells protected from external environment

    • Cnidaria lack this- cells are in contact with water (in freshwater cnidaria, each cell has prominent contractile vacuoles)

Phylum platyhelminthes flatworms

Protonephridial osmoregulatory/excretory system

“flame cell”





  • 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.

Phylum platyhelminthes flatworms


Life history

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

Asexual reproduction by transverse fission budding

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

Phylum platyhelminthes flatworms

Class Cestoda(tapeworms) all parasitic

Class Monogenea(monogenetic trematodes, gill flukes) all parasitic

Class Trematoda(digenetic trematodes, flukes) all parasitic

Class Turbellaria(turbellarians)mostly free-living

Platyhelminthes- 4 classes traditional



  • 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

Phylum platyhelminthes flatworms


“Neodermata” The parasitic flatworms)




  • Phylum Platyhelminthes

    • Classes

      • Acoelomorpha:

        • Acoels (free-living) + Neodermata (parasites)

      • Turbellaria: free-living flatworms

      • Trematoda: flukes

        • All are parasites

      • Cestoda: tapeworms

        • All are parasites

      • Monogenea? (Maybe belong with cestoda)

        • Ectoparasites of fish

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

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

Acoels are not platyhelminthes

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:

Phylum platyhelminthes flatworms

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 283:1919-1923


Class turbellaria

Class Turbellaria

  • Order Acoela

    • Small flatworms with no permanent gut cavity.

    • Free-living, marine and brackish water.

Rhabodocoel turbellarians

Rhabodocoel turbellarians

  • Diverse and important predators on meiofauna in freshwater ecosystems

  • Recent research on predation on freshwater bivalves

Phylum platyhelminthes flatworms

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

Phylum platyhelminthes flatworms

C. Barnhart

Mesostoma – close-up showing the babies

Mom’s oral sucker


Mom’s eyes

Phylum platyhelminthes flatworms

Macrostomum – predator on juvenile bivalves

Phylum platyhelminthes flatworms

Eggs of Macrostomum

Phylum platyhelminthes flatworms

  • Platyhelminthes

    • Turbellaria

      • Tricladida


A land planarian common in greenhouses world-wide – causing problems in Britain

Phylum platyhelminthes flatworms

Bipalium feeding on an earthworm

Class turbellaria1

Class Turbellaria

  • Order Tricladida

    • Gut with three branches

    • Free living

      • Freshwater

        • Planarians such as Dugesia

      • A few inhabit terrestrial (moist) habitats

    • Neodermata

      • Exclusively parasitic

Class turbellaria2

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

Phylum platyhelminthes flatworms

Bdelloura- a marine triclad that is commensal with horseshoe crabs

(most turbellarians are not parasites)

Class turbellaria3

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

Phylum platyhelminthes flatworms

Rhabdites secrete mucus

Class turbellaria4

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.

Class turbellaria5

Class Turbellaria

  • Body construction

    • Lab notes

      • Live Dugesia

      • Slides of whole planaria and cross sections

Class turbellaria6

Class Turbellaria

  • Respiration

    • Diffusion

      • Must be less than 0.5 -1 mm thick for diffusion to be effective. Flatworms are flat by necessity

Class turbellaria7

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;

Class turbellaria8

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").

Phylum platyhelminthes flatworms

Excretory pores

Class turbellaria9

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

Class turbellaria10

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.

Class turbellaria11

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.

Class turbellaria12

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.

Class turbellaria13

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.

Class turbellaria14

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

Class turbellaria15

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.

Phylum platyhelminthes flatworms

Polycladida digestive system

Class turbellaria16

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.

Phylum platyhelminthes flatworms



Cerebral ganglion

Paired nerve cords

Phylum platyhelminthes flatworms

Retinular cells

Light sensitive region

Pigment cups

Class turbellaria17

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

Phylum platyhelminthes flatworms

nervous system




Class turbellaria18

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.

Phylum platyhelminthes flatworms

Metamorphosis of Muller’s larva into a free-living Turbellarian (above)

A living larvae (right)

Class turbellaria19

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.

Class turbellaria20

Class Turbellaria

  • Additional sketches from R. Fox.

Phylum platyhelminthes flatworms

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.

Ecology and evolution

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)

Ecology and evolution1

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 .

Ecology and evolution2

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.

Phylum platyhelminthes flatworms

Phyllidia - a sea slug (Phylum Mollusca)

Phylum platyhelminthes flatworms

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

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