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)
Describe the life cycle of the blood fluke (Schistoma)
- larvae in water
- egg case in faeces
- amplified in the marine environment
Describe the life cycle of the Chinese liver fluke (Clonorchis)
- humans eat via raw fish
- deposited from humans in egg case via faeces
- amplified in marine environment
- larvae eaten by fish
Describe cestodes
- tapeworms
- endoparasites
- multiple hosts for life-cycle
- many infect humans
- scolex to attach to gut
- proglottids
- no gut: absorb nutrients through special skin
Describe proglottids
- chain of repeated units with testes and ovaries
- youngest units near neck, older and
bigger further down - break off when full of fertilized eggs
Describe the Cestoda scolex
with hooks and suckers
Describe the longest tapeworm in humans
- Diphyllobothrium latum
- caught from sushi
- up to 10 metres
Give the life cycle of Diphyllobothrium latum
- unembryonated eggs passed in humans faeces
- eggs embryonate in water
- coracidia hatch from eggs
- coracidia are ingested by crustaceans
- procercoid larvae found in coelom of crustaceans
- infected crustacean ingested by small freshwater fish
- procercoid larvae released from infected crustacean
- develop into plerocercoid larva
- predator fish eats small infected fish
- human ingests raw or undercooked infected fish
When do the infective phases begin in the life cycle of Diphyllobothrium latum
Development into plerocercoid larva
Describe the evolutionary relationship between tapeworms and flukes
monophyletic
