Flatworms and Bilaterians

Movement > Cephalization

Bilateral symmetry, head vs. tail.

Bilateria =

triploblasts

What phylums are important to us?

Xenacoelomorpha and Platyhelminthes

What are the subphylums of phylum Xenacoelomorpha?

Acoelomorpha

Xenoturbellida

Phylum Xenacoelomorpha; Subphylum Acoelomorpha

• Small, flat, marine worms.

• 350 spp.

• Ciliated epidermis

• Ventral mouth opens into gut cavity, no anus.

  • Muscles present: longitudinal, circular, diagonal

Phylum Xenacoelomorpha; Subphylum Xenoturbellida

Deep sea worms

Phylum Platyholminthes: Classes

• Lophotrochozoan protosome

  • Class Turbellaria

  • Class Trematoda

  • Class Monogenea

  • Class Cestoda

Class Turbellaria

Paraphyletic group; free-living

3500 spp.

Marine, freshwater, terrestrial

triclad vs. polyclad gut

penis fencing. Feeding

Class Trematoda

Parasites, generally of vertebrates; digenetic flukes.

11,000 spp.

• struct. sim to turbellarians, with mods for parasitism (eg. suckers)

Class Monogenea

parasites, typically ectoparasites of fish gills; monogenetic flukes

1100 spp.

• Simple Life Cycle, single host

  • egg > oncomiracidium > adult parasite

Class Cestoda

parasites (in gut of vertebrates); tapeworms

3400 spp.

Class Cestoda: Body Plan

• Composed of posterior scolex, repeating segments (proglottids), making the strobila

• No gut, absorb nutrients through skin

• Each proglottid contains both male and female organs

Class Cestoda: Beef tapeworm

Taenia saginata

• Life Cycle

  • Adult (in human) > zygote > shelled larva (in grass) > cysticercus juvenile (in cow) > human

Phylum Platyhelminthes: Basic Bodyplan

• Bilateral symmetry

• Triploblastic

• Acoelomate

• Dorsoventrally Flattened

• Cephalized

Flatworm Structure

“Planarian” - freshwater turbellarian

• ciliated epidermis with rhabdites

• body wall muscles, also muscles around pharynx

Planarian

• extendable pharynx

• branched gut or gastrovascular cavity

• CNS

• Sense organs

• Excretory system of protonephiridia

• Well developed reproductive system

• No respiratory or circulatory systems

Planarian Regeneration

Important model organism for developmental biology

Class Trematoda: Chinese Liver Fluke

Clonorchis sinensis

• Complex life cycle

  • adult > egg > miracidium > sporocyst > redia > cercaria > metacercaria

  • snails (and fish) are intermediate hosts, vertebrates are definitive hosts

Class Trematoda: Blood Flukes

Schistosoma spp.

• responsible for schistosomiasis

  • affects 200 million people worldwide

• Life Cycle:

  • adult > egg > miracidium > sporocyst > cercaria