3. PLATYHELMINTHES

AIMS – FREE LIVING PLATYHELMINTHES

Understand the basic morphology of Platyhelminthes.

Describe the locomotion and reproductive system of free-living flatworms.

Describe the typical life history strategies seen.

Describe the adaptions for parasitism in Trematodes

Platyhelminthes - The basics

Platy = flat

Helminth = worm

30000 species

0.2 mm - 40 m

simple animals - some specialised features

Bilateral (first ones to be called that)

Free living and parasitic

HOX Genes

Used to look at how species evolve

Group of related genes that conrol how embryo develops - along aterior-posterior end

Embryo segments have associated HOX protiends that determine what the segment becomes

Free Living PLATYHELMINTHES

Thought to be the first of Bilateral - bilaterally symmetrical

Dorso-ventrally flattened

Sagittal planes - head to tail - similar halves on either side

Secondarily Simple

eg. cave fish that lost eyes

lost traits from ancestral form due to evolution

Does simple form= ancestral form?

EVOLUTION OF BILATERIA

CEPHALISATION and DIFFERENTIATION- allowed the Platyhelminthes to pioneer a new way to survive

allow for movement - first hunters

Paired sense organs and a locomotory surface, they were able to move in a forward direction; they were able to hunt for food and for mates

• find food

• find mate

CEPHALISATION

Sense organs at head end. • Anterior end meets the environment first • Anterior/Posterio

DIFFERNTIATION

Of upper surface – dorsal, and lower surface- ventral.

• The ventral surface becomes the locomotor surface

Terrestrial Movement of Platyhelminths

• small cillia and slime (muscus)

• anterior end senses where it is

• mouth is at middle of body

• inverts pharynx (tounge) into prey and puts digestive enzymes into prey

Marine Movement of Platyhelminths

undulating muscular contractions to swim

still use gliding action over surfaces

Classes of Platyhelminthes

1. Turbellaria - Mostly free living hunters and scavenger

2. Monogenea - Flukes, all parasitic, most species are ectoparasites on fish -will be covered more in the platyhelminth lab

3. Cestoda - Tapeworms, all endoparasitic in vertebrates

4. Termatoda - Flukes, all parasitic, found in digestive tracts of vertebrates

PLATYHELMINTHES GENERAL ANATOMY

• muscular structures - allow for size and variety

• Rabdite cells - slime production

• sensory: cerebral ganglion and ventral nerve cord

LOCOMOTION

Smaller - Turbellaria generally glide over secreted mucous using ciliary action.

Larger - species are too heavy to glide using cilia alone and use muscular contraction to swim or creep, raising and lowering their ventral surface

Platyhelminth digestive system

• similar to sponge canal system - but not relying on water coming in and out

Platyhelminth EXCRETORY system

• Flame Cells: Protonephridial network of tubules - act like kidneys - processing and purifying nutrients (get rid of ammonia)

• Active - draw stuff in and get rid of rest

• Osmoregulatory process- regulates water

Platyhelminth Nervous system

simple to complex

Cerebral gangalia (brain)

complex ones have lateral nerve systems

• key to them being the first hunters - able to make decions on where to go

Platyhelminthe Sensory Organs

Photoreceptors - “cup eyes” - see light and dark

Statocysts - gravity senses

Mechanoreceptors - can feel what they’ve touch before

Cempreceptors - tasting environment

Frequently arranged to locate the source of sensory stimuli, useful for a hunter!