Platyhelminthes

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