1. The body of these animals is soft and compressed from center.
2. There are about 20000 species of flat worms living in marine, freshwater and damp terrestrial habitats.
3. They are triploblastic acoelomate. There is development of a third layer, the mesoderm, which separates the ectoderm and endoderm.
4. Flatworms exhibit bilateral symmetry, and body is unsegmented.
5. They range in size from few millimeters to several meters.
6. Much of the body space is taken up by a branching sac type digestive system. The digestive system is poorly developed in some species or may be absent.
7. The excretory system consists of branching tubes ending in bulb like cells, the flame cells.
8. A well-developed nervous system is present in flatworms.
9. Respiratory and circulatory systems are absent.
10. The parasitic species absorb nutrients from the hosts. The free-living species feed on small animals and bodies of dead and decaying animals.
11. The free-living forms are motile. They move by cilia present on their undersides.
12. They reproduce by both sexual and asexual methods.
2. There are about 20000 species of flat worms living in marine, freshwater and damp terrestrial habitats.
3. They are triploblastic acoelomate. There is development of a third layer, the mesoderm, which separates the ectoderm and endoderm.
4. Flatworms exhibit bilateral symmetry, and body is unsegmented.
5. They range in size from few millimeters to several meters.
6. Much of the body space is taken up by a branching sac type digestive system. The digestive system is poorly developed in some species or may be absent.
7. The excretory system consists of branching tubes ending in bulb like cells, the flame cells.
8. A well-developed nervous system is present in flatworms.
9. Respiratory and circulatory systems are absent.
10. The parasitic species absorb nutrients from the hosts. The free-living species feed on small animals and bodies of dead and decaying animals.
11. The free-living forms are motile. They move by cilia present on their undersides.
12. They reproduce by both sexual and asexual methods.
