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Phylum Platyhelmenthes (the flatworms) General characteristics

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Phylum Platyhelmenthes (the flatworms) General characteristics. Eumetazoa - animals with tissue Bilateria - have bilateral symmetry Head end-cephalization Tail end Right and left side Top (dorsal) and bottom (ventral) triploblastic -ectoderm, mesoderm, endoderm.

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Phylum Platyhelmenthes (the flatworms)

General characteristics

Eumetazoa - animals with tissue

  • Bilateria - have bilateral symmetry

Head end-cephalization

Tail end

Right and left side

Top (dorsal) and bottom (ventral)

triploblastic -ectoderm, mesoderm, endoderm.

Protostomia - a group of animals whose mouth develops from the blastopore, and the mesoderm forms from an area near the blastopore.

Acoelomata - They have a true mesoderm which fills the original blastocoel between the outer epidermis and digestive tract.

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Integumentary- Rhabdites and one cell layer epidermis in Turbellaria and usually ciliated; syncytial tegument in other classes.

Skeletal - hydrostatic

Muscle - longitudinal, transverse, and circular muscles are present.

Digestive - incomplete with intracellular and extracellular digestion; no system in Cestoda.

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Excretory - flame cells, or excretory tubes in Cestoda.

Respiratory - no system, diffusion

Circulatory - none, diffusion.

Nervous - anterior ganglia, ventral ladder-like system (two lateral cords with transverse cords).

a bilobed anterior ganglial mass (brain)

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Endocrine - hormones produced by nervous system

Reproductive - monoecious in most

Well developed reproductive organs, mostly internal fertilization.

Two of the parasitic classes have complex life cycles

Trematoda -

Cestoda -

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Class Turbellaria-

~5000 species, mostly free-living

~Dugesia- free living planarian

Branched gut

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Reproduction-

asexual- fission

sexual - monoecious

Diversity-land planarian- Bipalium

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Branched gut

eyespots

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Class Trematoda--flukes

~parasitic

endoparasitic

ectoparasitic

~tegument=body wall

~suckers

~feed on host cells, tissue fluids, mucus (yuk!) and blood

~life cycle

monogenic flukes

digenic flukes

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Class Cestoda = tapeworms

Proglottids-

Head-scolex

slide18
Scolex

proglottid

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Phylum Nemertina- ribbon worms

Proboscis can be extruded to capture food

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General characteristics-

~tube within a tube body plan- a complete digestive tract

~Acoelomate

~eversible proboscis, within a rhinocoel above the gut

~feeding-stylets and toxins

~closed circulator system, a single dorsal vessel with two lateral

vessels

~paired lateral longitudinal nerve cords

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~asexual reproduction by fragmentation

~sexual reproduction-

dioecious

free-swimming larva in some marine species

Advanced Features-

~anus

~closed circulatory system

~dorsal nerve cord in some

~complex excretory system in close association with circulatory system

~mesodermally derived blood vessels & mesodermally lined

rhinocoel may provide links to higher phyla

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