1. Flattened Body:
- The most outstanding feature of flatworms is their flattened body, which gives them a ribbon-like or leaf-shaped appearance. This structural adaptation enables them to navigate narrow spaces within their hosts or substrates, such as soil or water.
2. Acoelomates:
- Flatworms lack a coelom, which is a body cavity enclosed by mesoderm. Instead, they possess a solid mass of mesodermal tissue, known as parenchyma, that fills the space between their gut and body wall.
3. Bilateral Symmetry:
- Flatworms exhibit bilateral symmetry, meaning their bodies can be divided into mirror-image left and right halves along a central axis.
4. Absence of Segmented Body:
- Unlike some other worm phyla, flatworms do not have a segmented body. Their bodies are continuous and unsegmented, with no clear divisions into distinct segments.
5. External Cilia:
- Many flatworms, particularly those that are free-living, possess cilia on their body surface. These cilia aid in locomotion, gliding, and moving food particles toward the mouth.
6. Gastrovascular Cavity:
- Platyhelminthes have a unique digestive system characterized by a gastrovascular cavity, which functions as both a digestive cavity and a circulatory system.
7. Protonephridia:
- Flatworms utilize a primitive excretory system known as protonephridia. Protonephridia are flame-cell-based structures that assist in regulating water balance and removing waste products.
8. Cephalization:
- Some flatworm species exhibit cephalization, which involves the concentration of sensory structures and nervous tissues at the anterior end of the body, forming a rudimentary head.
9. Reproduction:
- Flatworms have a diverse array of reproductive strategies, including both asexual and sexual reproduction. Asexual reproduction occurs through fragmentation or budding, while sexual reproduction involves the formation of gametes (eggs and sperm) for fertilization.
10. Parasitic and Free-Living Species:
- Flatworms encompass a wide range of parasitic and free-living species. Parasitic flukes, tapeworms, and parasitic worms are examples of flatworm parasites, while planarians and marine flatworms represent free-living species.
These characteristics define the phylum Platyhelminthes and distinguish flatworms from other invertebrate groups.