Invertebrates_I[1]

Invertebrates_I[1] - Diversification of the Bilataria...

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Diversification of the Bilataria
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• Bilaterally symmetric animals (unless secondarily changed) • Sister group to the Cnidaria, • All Bilateria are triploblastic , with a mesoderm layer • Bilateral symmetry and mesoderm (providing true muscle, etc.) allow greater activity Bilateria
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Traditional classifications, placed the Platyhelminthes as the sister group to the rest of the Bilateria. Perhaps, derived from planula-like ancestors Molecular studies indicate the old Platyhelminthes was polyphyletic blind gut no coelomic cavity between the gut and the body wall Triploblastic Respiration by diffusion no cell can be too far from the outside Platyhelminthes (flatworms) A c o e l a
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Planula larva of the jellyfish Gonionemus sp . The Planula larva of Cndarians resembles a simple flatworm
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Acoela • Traditionally placed in the Platyhelminthes • Small, simple planula-like flatworms • No gut, excretory system, etc • Simple nerve-net • Molecular studies suggest they are the earliest diverging members of the Bilateria • Now recognized as separate phylum from the Platyhelminthes?
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• Blind gut (with one opening) allows some extracellular digestion, but digestion is completed intra cellularly • Branches in gut carry food to tissues (no circulatory system, and no coelomic cavity) Platyhelminthes (flatworms)
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Flat body helps in locomotion and respiration, etc. Complex Muscle system allows good movement ability.
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• Major classes of Platyhelminthes Turbellaria : Free-living flatworms (mostly) – nearly all aquatic, and most live in the sea. Predators and scavengers • Many in sea are mimics of toxic molluscs called nudibranchs Platyhelminthes (flatworms) Nudibranch models Flatworm mimics
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Invertebrates_I[1] - Diversification of the Bilataria...

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