Lophotrochozoa II: Flatworms Flashcards
How can we tell that Platyhelminths are Bilateria?
- three germ layers
- bilateral symmetry
- centralised nervous system with ‘brain’
What are the 3 germ layers?
endoderm, mesoderm, exoderm
Describe bilateral symmetry
along anterior-posterior, dorsal-ventral, left-right axes
Describe radial symmetry
- oral and aboral
Describe the connectome of Platyhelminths
- two nerve cords
- transverse commissures
- sense organs and integration at front
Transverse commissures
= sideways links
Describe the features seen in other Bilateria that Platyhelminths lack
- no coelom
- no hydrostatic skeleton
- no gills/respiratory organs
- no blood system
- no skeleton
- no appendages
- no anus (secondarily lost)
Coelom
body cavity
What are the constraints of having no hydrostatic skeleton
limits power of locomotion
What are the constraints of having no blood system
- oxygen must reach cells by diffusion
- flat and (usually) small
What are the constraints of having no anus
food and waste use same opening
Platyhelminth =
flatworms
Describe the ecology of the Platyhelminths
- some are free-living
- some are parasitic
Describe the free-living Platyhelminths
- triclads or planarians (freshwater flatworms)
- polyclads (marine flatworms)
Describe the parasitic Platyhelminths
- monogeneans (flukes with one host)
- trematodes (flukes with >1 host)
- cestodes (tapeworms)
Describe the morphology of free-living flatworms
- small (usually 1-10 mm)
- cilia on the epidermis
- eversible pharynx
Describe the ecology of free-living flatworms
- glide along by cilia and muscular ripples
- hunters and scavengers
Describe the ecology of planarians/triclads
- rivers/ponds
- hermaphrodite
- reciprocal mating (using a hatching egg capsule)
- no larvae
- scavenge and hunt (worms and snails)
- three-branched-gut
- pharyngeal cavity and cylindrical plicate pharynx
Describe the terrestrialised Platyhelminths (Hugh Jones (2005))
- ~3 in the UK including invasive species
- hunt earthworms
- e.g. Rhyncodemus sylvaticus (1.5cm)
Give an invasive terrestrialised Platyhelminth
- 15 cm “New Zealand flatworm”
- (Arthurdendyus triangulatus
- coiled (2cm diameter) resting specimen
Describe the ecology of the polyclads
- marine
- hermaphroditic mating
- hunt (worms and snails etc)
- diverse colour signalling
- some can swim
Describe triclad regeneration
- memory of ‘Polarity’
- adult stem cells (neoblasts)
- adaptation hypothesised to berelated to starvation response
Describe the monogeneans
- etoparasites on fish
- <3 cm
- one host
- suckers
- specialised protective, non- ciliated, syncytial skin
Describe trematodes
- most are endoparasites
- > 1+ hosts for life-cycle
- specialised, protective, non- ciliated, syncytial skin
- many infect humans (e.g. human liver fluke)
