Phylum Characteristics

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Kingdom:   Animal

Phylum:   Platyhelminthes

Class:  Turbella

Class:  Trematoda

Class:  Cestoda



Phylum Characteristics:


General Characteristics

aka.  Flatworms


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)



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


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



Asexually :  Planaria - can re-grow body parts or an entire new body (regeneration)



Life Cycle

Sessile species usually parasites: obtain nutrients from inside host




Platyhelminthes Classes

Class:   Tubellaria (Planarians)

General Characteristics


  • Mostly aquatic

    • marine or freshwater, bottom dwellers (live in algae masses or under stones)

    • few terrestrial live in humid forests

Body Structure

    • Elongated, with triangular shaped head

    • Two anterior eye spots

    • 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

o       Oval and flattened

o       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)


Class:   Cestoda (tapeworms)

General Characteristics

All are endoparasites

o       Adult worms live in the intestines of vertebrate hosts

o       Larvae live in tissues of alternate hosts

Body Structure

o       Mostly slender and elongates with a flat body of many short segments (proglottids)

o       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


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