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Platyhelminthes

Dr Natalie Welden

By the end of this lecture

By the end of this lecture

  • Describe and recognise the main characteristics of the platyhelminths

  • Describe the main classes and list their general features

  • Understand their main biological traits including feeding, reproduction and locomotion

Diagnostic Characteristics

  • ‘platy’ : flat ‘helminth’: worm

  • 26,500 identified species

  • Bilaterally symmetrical

Diagnostic Characteristics

Earliest metazoan group to have 3 germ layers

  • Endoderm

  • Ectoderm

  • mesoderm

Diagnostic Characteristics

Exhibit spiral cleavage

"holoblastic cleavage that is typical of protostomes and that is characterized"

Diagnostic Characteristics

  • Acoelomate

Diagnostic Characteristics

  • Gut (if present) only a single opening

  • Reproduction is complex

  • Encephalisation = Having a head

Diagnostic Characteristics

  • Complex organ systems

  • Gas exchange is diffusion
  • Disadvantages

Excretion and Osmoregulation

Through body wall by diffusion

  • Advantages

  • Disadvantages

Majority have protonephridia - flame cells

Excretion and Osmoregulation

Nucleus

Excretory canal

Bundle of cillia

Cytoplasm

Nephridial duct

Excretory Pore

Excretory Panal

Flame Cell

Classification of Platyhelminths

Turbellaria

Cestoda

Trematoda

Monogenea

Tubellaria - 4500 species

Rhabdocoela

Tricladida

Polycladida

Tubellaria

Rhabdocoela

Tricladida

Polycladida

Sac like gut

Most free-living

Some symbiotic

Tubellaria

Rhabdocoela

Tricladida

Polycladida

Tubellaria

Rhabdocoela

Tricladida

Polycladida

Tubellaria

  • Majority are free-living predators, some are symbioytic or parasitic

  • Rhabdites =

Tubellaria: feeding

  • Eversible pharynx

  • Digestion

Tubellaria: reproduction

  • Simultaneous hermaphrodites

  • Example: Pseudoceros bifurcus

Hypodermic insemination

Penis fencing

Müller's larvae

Tubellaria: regeneration

  • Reproduction via fission!

  • Connective tissue (parenchyma) contains mesenchymal cell types embedded in their associated extracellular matrices (ECM)

  • Polarity maintained

Tubellaria: defence

  • Avoidance

  • Aposematic colouration:

  • Batesian mimicry:

  • Mullerian mimicry:

Tubellaria: defence

  • Avoidance

  • Aposematic colouration:

  • Batesian mimicry:

  • Mullerian mimicry:

Tubellaria: defence

Philinopsis gardineri-

I'm a nasty tasting slug!

  • Avoidance

  • Aposematic colouration:

  • Batesian mimicry:

  • Mullerian mimicry:

Pseudoceros sapphirinus-

I'm a tasty flatworm

Tubellaria: defence

We both taste baaaaaaaaaad!

  • Avoidance

  • Aposematic colouration:

  • Batesian mimicry:

  • Mullerian mimicry:

Pests: New Zealand Flatworm

nom nom nom... earthworms

Cestodes

Majority are members of subclass Eucestoda (Tapeworms)

All parasitic

All hermaphroditic

Indirect development

Cestodes

Majority are members of subclass Eucestoda (Tapeworms)

Head - Scolex

Head cups - Bothria

Cestodes

Majority are members of subclass Eucestoda (Tapeworms)

Proglottids - body segemts

Strobila - all proglottids

Cestodes: Diphyllobothrium

Trematoda

  • up to 24,000 extant species

  • Hermaphroditic

  • parasitic flukes

  • life cycle: indirect, staged development, often in more than one host

Schistosomiasis

Affected about 252 million people worldwide in 2015

Trematoda

Fasciola hepatica- Sheep Liver fluke

Monogenea

Ectoparasites

Direct development

Haptor - attachment

Recap and Review

  • Acoelomate, triploblastic, mouth only, gas exchange, bilaterally symmetrical, complex reproduction, regeneration

  • Turbellarians, cestodes, trematodes, monogenea

  • Parasites

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