Phylum Platyhelminthes (flatworms) & Phylum Gastrotricha

Key Traits

• ~30,000 species

• Triploblast (acoelomate)

• Most parasitic; few land-dwelling

• Cephalization; excretory flame cells

• Key trait: being flat!

Flatworm Body Plan

• Cleavage: spiral

• Cell Fate: mosaic

• Protostome

Body Complexity

• 3 germ layers

• Acoelomate

• Bilateral; cephalization

• Hydroskeleton

Digestion

• Variable

  • Planarians - incomplete

    • Pharynx extends

    • Intestine/gastrovascular cavity branches

  • Flukes - complete (?)

    • Mouth anterior

  • Tapeworms - none

Circulation & Respiration

Across body surface (moist environment required)

Excretion (Osmoregulation)

Flame cells

Nervous

• Nerve cords + nerve net

• Sensory: ocelli (light), statocysts (equilibrium), auricles (chemicals)

Movement

• Rhabdites secrete mucous, move by ciliated epidermis

• Dual-glands (secretions to adhere or release)

Reproduction: Asexual & Sexual

• Asexual by fission

• Sexual = hermaphroditic

Defense

• Regeneration

• Toxins and aposematic coloration

• Parasites: “host camouflage” & “distractions”

4 Classes of Platyhelminthes (The Mighty Tiny Critters)

• Turbellaria* — planaria; free-living

• Trematoda — digenetic (2 hosts) flukes

• Monogenea — monogenetic (1 host) flukes

• Cestoda — tapeworms

Class Turbellaria

• Most: marine or freshwater bottom-dwellers

• Move by cilia/mucus

• E.g. planaria → carnivorous

Class Trematoda (“flukes”)

• ~18,000 species

• Endoparasites

  • In vertebrate = sexual

  • In mollusc = asexual

• Adaptations for parasitism include:

  • Enzymes to enter skin; hooks/suckers to attach

  • Increased # of offspring (100,000x)

  • Sense organs poorly developed

General Trematoda Life Cycle

• Adults in vertebrate sexually reproduce; vertebrate poops zygotes → water

• Egg hatches → larva (miracidia)

• Miracidium penetrates tissues of snail

• Asexually reproduce → cercaria

• Cercariae emerge from snail

• Ingested by vertebrate → adult

Example: Clonorchis (human liver flukes)

• Common in E. Asia

• May cause cirrhosis, cancer, death

• Life Cycle

  • Adults in liver; zygote develops in egg → water

  • Larvae (miracidium) enters snail; develop in snail liver → cercariae

  • Cercariae exits snail → water; enter fish

  • Mammal eats raw fish → adult

Example: Schistosoma (blood fluke)

• Over 200 million people infested w/ schistosomiasis (common in Africa, South America, and Middle/Far East)

• Sexes separate

• Life Cycle

  • Adults in blood; zygote/egg → water

  • Larvae (miracidium) ingested by snails → cercaria

  • Cercariae exit snail → contact skin

  • Pierce skin, enter blood vessels → adult

Class Monogenea (“gill fluke”)

• Single host

• External parasites of fish, especially gills

• Posterior hooks = attachment organ (opisthaptor)

Class Cestoda (tapeworms)

• ~5,000 species of tapeworms

• Most require 2 hosts; infect vertebrates digestive tract

• Anatomy:

  • Long, flat body with scolex (anchor) and proglottids (reproductive units) in chain (strobila)

  • No digestive system (surface of tapeworm covered in microtriches)

• Proglottids orginate behind scolex

• Cross-fertilize by exchanging sperm

• Shelled embryos form in the uterus

  • Expelled OR whole proglottid shed

Example: Beef Tapeworm

• Adult in human intestine; juvenile in cattle muscle

• Avoid infection: thoroughly cook meat

• Life Cycle

  • Gravid proglottids in feces

  • Cattle ingest larvae from grass; larvae burrow through intestine to muscle and encyst

  • Infected muscle (meat) eaten → scolex attaches to intestine wall

Evolutionary History

• “Turbellarians” = paraphyletic

• Parasitic = monophyletic

Natural History

• Habitat/distribution: all habitats

• Biological community: predators/parasites

• Aposematic coloration or mimicry or both (??)

Human Interactions

• Disease!!

• Tolerant of poor habitat (indicator sp.)

• Invasive species

Phylum Gastrotricha

• ~800 species

• VERY tiny

• Complete digestive tract