Flatworms (Chapter 14)

Flatworms

• new branch

  • common ancestor for the rest of the animals

    • bilateral, triploblastic animals

    • cephalization

Triploblast

• 3 germ layers

Acoelomate

• no coelom

• acoelomorpha

• platyhelminthes

• mesoderm completely fill coelom

• gut still present

Pseudocoelomate

• body cavity not lined by mesoderm

• rotifers

• roundworms

Coelomate

• protostomes (lophotrochozoa and ecdysozoans) vs. deuterostomes

Coelomate Protostomes

• spiral cleavage

• blastopore becomes mouth

• mesoderm band fills blastocoel

Coelomate Deuterostome

• radial cleavage

• blastopore becomes anus

Cephalization

• evolutionary innovation — the head

• key feature for triploblasts

Phylum Acoelomorpha

• acoelomate

• bilateral symmetry

• triploblast

• organs (because they have mesoderm)

• flatworms, less than 5mm in length

• mostly free-living in marine sediment (not parasites)

• ciliated epidermis

• asexual and sexual reproduction

Clade Lophotrochozoa

• includes multiple phyla

  • Platyhelminthes

  • Rotifera

  • Polyzoa

  • Brachiozoa

  • Mollusca

  • Annelida

• have either a lophophore (horseshoe-shaped feeding structure) and/or trochophore larva

  • free swimming ciliated larva

• monophyletic

Phylum Platyhelminthes

• acoelomate

• bilateral symmetry

• triploblast

• protostome (= spiral cleavage, etc.)

• primitive nervous system

• probably not monophyletic

• <1mm-meters in length, flatworm

• can be free-living or parasitic

• sexual/asexual reproduction

Phylum Platyhelminthes — 4 Classes

• Turbellaria — parasitic

• Trematoda — non parasitic

• Monogena — non parasitic

• Cestoda — non parasitic

  • loss of digestive tract

  • scolex

Excretion and Osmoregulation

• protonephridia (greek for “first kidneys”)

  • primitive organs — series of tubules

    • no mesoderm so there are no real organs

• contain “flame cells” that draw in excess water to be excreted

Reproduction

• asexual reproduction usually by fission

  • split near the center and each half will regenerate the missing piece

• sexual reproduction

  • most are hermaphroditic

  • cross-fertilization (often happens)

Sense Organs

• active locomotion

  • further improvement of sense organs

• Ocelli

• Statocysts

• Rheoreceptors

• Chemoreceptors

• sensory nerves around the mouth

  • because many are parasites and need to get into their hosts via their mouth

Ocelli

• light detection (eye spot)

Statocysts

• equilibrium

Rheoreceptors

• determine direction of water current

Chemoreceptors

• olfaction

Class Turbellaria

• 5mm to 50cm

• free-living

• marine, freshwater, terrestrial

• movement via ciliary beating and rhythmic muscle contractions

Class Trematoda

• all endoparasitic (= living in organs or tissues of host) of vertebrates

• structural adaptations

  • penetration glands

  • adhesion organs — hooks or suckers

  • increased reproductive capacity

  • cyst production

• complex life cycle: multiple hosts at different life stages

Human Liver Fluke

• found in freshwater in Asia

  • comes from ingesting uncooked or undercooked fish (particularly freshwater fish)

  • ex; sushi

• 80% of body dedicated to reproduction

• many people infected without any symptoms

Blood Fluke

• “snail fever”

• chronic infections

• hundreds of millions infected worldwide

  • Africa

  • South America

  • Middle East

• Schistsome Dermatitis

  • swimmer’s itch

Class Monogenea

• parasites primarily of gills or external fish surfaces

  • one parasitizes the eye of a hippopotamus

• have opisthaptor as attached organ in the posterior end

• often causes little damage to host

  • can cause disease

Opisthaptor

• helps attach to host

Scolex

• slightly barbed

• for attachment

Class Cestoda

• tapeworms (can get very long)

• scolex

• no digestive system

  • absorb everything from the host

• some concern to humans

  • from eating rare or uncooked meat

Beef Tapeworms

• 1% of American cattle

• 25% of cases in inspected meat are missed

Fish Tapeworm

• can grow up to 20cm

• largest tapeworm to infect humans