Lophotrochozoa II: Flatworms

Question 1

How can we tell that Platyhelminths are Bilateria?

Answer 1

• three germ layers
• bilateral symmetry
• centralised nervous system with ‘brain’

Question 2

What are the 3 germ layers?

Answer 2

endoderm, mesoderm, exoderm

Question 3

Describe bilateral symmetry

Answer 3

along anterior-posterior, dorsal-ventral, left-right axes

Question 4

Describe radial symmetry

Answer 4

• oral and aboral

Question 5

Describe the connectome of Platyhelminths

Answer 5

• two nerve cords
• transverse commissures
• sense organs and integration at front

Question 6

Transverse commissures

Answer 6

= sideways links

Question 7

Describe the features seen in other Bilateria that Platyhelminths lack

Answer 7

• no coelom
• no hydrostatic skeleton
• no gills/respiratory organs
• no blood system
• no skeleton
• no appendages
• no anus (secondarily lost)

Question 8

Coelom

Answer 8

body cavity

Question 9

What are the constraints of having no hydrostatic skeleton

Answer 9

limits power of locomotion

Question 10

What are the constraints of having no blood system

Answer 10

• oxygen must reach cells by diffusion
• flat and (usually) small

Question 11

What are the constraints of having no anus

Answer 11

food and waste use same opening

Question 12

Platyhelminth =

Answer 12

flatworms

Question 13

Describe the ecology of the Platyhelminths

Answer 13

• some are free-living
• some are parasitic

Question 14

Describe the free-living Platyhelminths

Answer 14

• triclads or planarians (freshwater flatworms)
• polyclads (marine flatworms)

Question 15

Describe the parasitic Platyhelminths

Answer 15

• monogeneans (flukes with one host)
• trematodes (flukes with >1 host)
• cestodes (tapeworms)

Question 16

Describe the morphology of free-living flatworms

Answer 16

• small (usually 1-10 mm)
• cilia on the epidermis
• eversible pharynx

Question 17

Describe the ecology of free-living flatworms

Answer 17

• glide along by cilia and muscular ripples
• hunters and scavengers

Question 18

Describe the ecology of planarians/triclads

Answer 18

• 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

Question 19

Describe the terrestrialised Platyhelminths (Hugh Jones (2005))

Answer 19

• ~3 in the UK including invasive species
• hunt earthworms
• e.g. Rhyncodemus sylvaticus (1.5cm)

Question 20

Give an invasive terrestrialised Platyhelminth

Answer 20

• 15 cm “New Zealand flatworm”
• (Arthurdendyus triangulatus
• coiled (2cm diameter) resting specimen

Question 21

Describe the ecology of the polyclads

Answer 21

• marine
• hermaphroditic mating
• hunt (worms and snails etc)
• diverse colour signalling
• some can swim

Question 22

Describe triclad regeneration

Answer 22

• memory of ‘Polarity’
• adult stem cells (neoblasts)
• adaptation hypothesised to berelated to starvation response

Question 23

Describe the monogeneans

Answer 23

• etoparasites on fish
• <3 cm
• one host
• suckers
• specialised protective, non- ciliated, syncytial skin

Question 24

Describe trematodes

Answer 24

• most are endoparasites
• > 1+ hosts for life-cycle
• specialised, protective, non- ciliated, syncytial skin
• many infect humans (e.g. human liver fluke)

Question 25

Describe the life cycle of the blood fluke (Schistoma)

Answer 25

• larvae in water
• egg case in faeces
• amplified in the marine environment

Question 26

Describe the life cycle of the Chinese liver fluke (Clonorchis)

Answer 26

• humans eat via raw fish
• deposited from humans in egg case via faeces
• amplified in marine environment
• larvae eaten by fish

Question 27

Describe cestodes

Answer 27

• tapeworms
• endoparasites
• multiple hosts for life-cycle
• many infect humans
• scolex to attach to gut
• proglottids
• no gut: absorb nutrients through special skin

Question 28

Describe proglottids

Answer 28

• chain of repeated units with testes and ovaries
• youngest units near neck, older and
  bigger further down
• break off when full of fertilized eggs

Question 29

Describe the Cestoda scolex

Answer 29

with hooks and suckers

Question 30

Describe the longest tapeworm in humans

Answer 30

• Diphyllobothrium latum
• caught from sushi
• up to 10 metres

Question 31

Give the life cycle of Diphyllobothrium latum

Answer 31

• 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

Question 32

When do the infective phases begin in the life cycle of Diphyllobothrium latum

Answer 32

Development into plerocercoid larva

Question 33

Describe the evolutionary relationship between tapeworms and flukes

Answer 33

monophyletic