Phylum Platyhelminthes (Flatworms)
General Characteristics:
- Commonly known as flatworms.
- Body is soft, unsegmented, and dorsoventrally compressed (flattened from top to bottom).
- Bilateral symmetry: distinct left/right and dorsal/ventral sides.
- Triploblastic: body tissues arise from three embryonic layers — ectoderm, mesoderm, and endoderm.
- Acoelomate: no true body cavity (coelom). Instead, the space is filled with parenchyma (a type of loose connective tissue).
Habitat and Examples:
- Free-living species: e.g., Planaria.
Parasitic species: live inside host bodies.
Examples:
Liver fluke- Tapeworm
- Blood-fluke
Body Systems:
-
Respiratory System:
- Absent.
- Exchange of gases occurs through diffusion.
- Absent.
- Nutrients are distributed by diffusion.
Excretory System:
- A network of tubules called protonephridia.
- Each tubule ends in a flame cell (bulb-like structure with cilia).
- Cilia beat to draw fluid into the tubules.
- Waste is filtered and excreted through nephridiopores (tiny openings on body surface).
Nervous System:
- Composed of a network of neurons.
- Anterior end has cerebral ganglia (simple brain-like structure).
- Two longitudinal nerve cords connected by transverse branches.
- Eyespots present in free-living forms to detect light.
Reproduction:
Asexual reproduction:
- By fission — the body splits into two parts, and each regenerates the missing part.
Sexual reproduction:
- Most are hermaphrodites (bisexual) — have both male and female reproductive organs.
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