Unsegmented worms = 1. Acoelomates
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2. Pseudocoelomates
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Platyhelminthes
aka Flatworms
Nematoda
aka Roundworms
Coelom = fluid-filled body cavity between body wall & digestive tract
ACOELOMATES
= “without a coelom”
Kingdom: Animalia
Phylum: Platyhelminthes
Class : Turbellaria - planarians
Class : Trematoda - flukes
Class : Cestoda - tapeworms
Phylum Nemertea – ribbon worms
Phylum Gnathostomulida- jaw worms
PLATYHELMINTHES = “platy” = flat “helminth” = worm
1. Body structure
size range 1mm ------► many meters (Cestoda)
♦ Acoelomate = no coelom
♦ No cavity between body wall and digestive tract \
♦ Space between body wall and digestive tract is filled with cells
(parenchyma)
♦ Shape – flattened dorso-ventrally
2. Level of Organization –organ system
3. Symmetry – bilateral (has forward movement & cephalization)
4. Habitat – aquatic (freshwater or marine), moist soil, guts of other animals
5. Feeding - depends on class
¾ Turbellaria – planarians
• Mostly free-living carnivore/scavengers
• Incomplete digestive tract – one opening (no anus)
• Mouth on ventral side
• Protrusible pharynx
• Gastrovascular cavity (GVC)
• “Intestine” very branched
• Enzymes digest food
¾ Trematoda – parasitic flukes
• Endo-parasites
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E.g. Human Liver fluke
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•
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Endo-parasites
Scolex w/ hooks & suckers
NO need for dig. tract – absorbs
nutrients from gut of host
¾ Cestoda – tapeworms
6. Respiration - via diffusion (only a few cell layers thick)
7. Internal transport – via diffusion
8. Excretion – via diffusion
EXCEPTION: planarians have flame cells ( w/ flagella that help
move waste to excretory pores, then out of the body
9. Response – (Nervous system)
• Primitive brain – anterior ganglia ganglion = cluster of nerve cells
• Ladder-like crossbridges of nerves
• Sense organs:
1. Ocelli – eyespots / photodetectors
2. Auricles - contain chemoreceptors & thigmoreceptors
3. Statocysts – balance
4. Rheoreceptors – sense direction of water current
10. Locomotion –
o Planaria – use cilia, slime, circular and longitudinal muscles
to GLIDE
o Trematoda, Cestoda – little motion
11. Reproduction
♦ Asexual – Regeneration (Planaria only)
♦ Sexual – monoecious (hermaphrodites)
- cross- fertilization (swap sperm)
12. Ecological Roles
♦ Scavengers/ predators – recycle nutrients to ecosystem
♦ Prey for fish & birds
♦ Endo-parasites (cause disease)