Body PlanFlatworms exhibit cephalization, which means they have a distinct head and tail region. They have bilateral symmetry. Platyhelminthes are known as acoelomates, which means "without a coelom". A coelom is the fluid filled body cavity lined with tissue.
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Feeding/DigestionCan be carnivores that feed on tiny aquatic animals or they can be scavengers. Flatworms only have one opening through which food and waste pass. They possess a muscular tube called a pharynx near the mouth which pumps food into the digestive cavity. The nutrients move through the animal via simple diffusion. Parasitic flatworms feed on blood, tissue fluids, or pieces of cells inside the host's body. These worms do not need a complex digestive system because the host does all the work.
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ExcretionSome flatworms have flame cells that help in excretion. These cells are specialized to remove excess water and filter ammonia and urea.
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Life Cycle/ReproductionSexual: Most free-living flatworms are hermaphrodites (have both male and female reproductive organs) that reproduce sexually. During mating, the flatworms pair up and exchange sperm, fertilizing each other's eggs. Parasitic flatworms have a complex reproductive system.
Asexual: Asexual reproduction is also common, and takes place by fission, in which an animal splits in two. In some organisms, the worm simply falls apart, with each piece growing into a new worm. Response to StimulusIn the head of non-parasitic flatworms, there are several groups of nerve cells called ganglia. They control a simple nervous system.
Support/SkeletonA flatworm has no skeleton, but has tiny bristles called cilia that help it move at least two layers of muscles under its skin. It has three cell layers called the endoderm, the mesoderm and the ectoderm.
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MovementFree-living flatworms can move in two ways:
Respiration/CirculationBecause of their flat, thin bodies, many flatworms do not need a circulatory system - they use simple diffusion.
HabitatTurbellaria are found in the oceans, in fresh water, and in moist terrestrial habitats. Trematoda live in the blood or organs of their hosts. Cestoda live in the intestines of most vertebrates.
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