Platyhelminthes
Kingdom: Animal
Phylum: Platyhelminthes
Class: Turbella
Class: Trematoda
Class: Cestoda
Phylum Characteristics:
General Characteristics
aka. Flatworms
Invertebrate
Least complex worms
14,500 species
Marine, freshwater, moist terrestrial environments
Body Structure
Bilateral symmetry
Cephalization : have a clearly-defined anterior end,
characteristic of motile animals, implies that the nervous tissue is
concentrated at the anterior end
Triploblastic : body wall consists of three cellular layers
No body cavity , spaces between internal organs are filled
(acoelomate)
Digestion
o Incomplete (mouth, but no anal opening)
o 1st Pharynx: tube like, muscular organ, extends out of
mouth, sucks in food
o 2nd Gastrovascular cavity: food broken up by enzymes
o 3rd Cells lining GC absorb nutrients through phagocytosis
o 4th individual cells digest food
o 5th some species (Planaria) have flame cells
remove waste through pores on body surface by
using cilia
Nervous system
Some species have a nerve net
Most have very simple nervous system
Planaria
o Two nerve chords run down length of body, has sensory
pits that detect chemicals and movement in water
o Two sensors that detect light (eyespots)
o Ganglion : located at head, receives messages from
eyespots and sensory pits & can communicate with the rest
of the body along the nerve chords
Reproduction
Asexually : Planaria - can re-grow body parts or an entire
new body (regeneration)
Sexual
Life Cycle
Sessile species usually parasites: obtain nutrients from inside
host
Platyhelminthes Classes
Class: Tubellaria (Planarians)
General Characteristics
Free-living
 Mostly aquatic
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marine or freshwater, bottom dwellers (live in algae masses or
under stones)
few terrestrial live in humid forests
Body Structure
o Elongated, with triangular shaped head
o Two anterior eye spots
o Mouths in the middle of the ventral side
Example: Dugesia tigrina (Planaria)
Class: Trematoda (flukes)
General Characteristics
All are parasitic (mostly endoparasites)
o
Most hosts are vertebrate (sheep), but immature stages
are often harbored in invertebrates (slug)
Body Structure
Oval and flattened
Mouth at anterior end & may have adhesive suckers around the
mouth
Example: Fasciola hepatica ( Sheep liver fluke), Fasciola buski
(Human liver fluke), Clonorchis sinensis (Chinese liver fluke)
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Class: Cestoda (tapeworms)
General Characteristics
All are endoparasites
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Adult worms live in the intestines of vertebrate hosts
Larvae live in tissues of alternate hosts
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Mostly slender and elongates with a flat body of many short
segments (proglottids)
Body Structure
No mouth or digestive track; food is absorbed from host directly
through body wall
o Scolex : anterior segment, knob-like, covered in hooks
o Stroblia : remainder of tapeworm, composed of proglottids
(flattened segments)
Example: Teania pisiformis (dog and cat tapeworms), Teania
saginatum (sheep tape worm), Teania solium (human tape
worm)
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