Flatworms (phylum Platyhelminthes) are a primitive type of worm. More than 20,000 species of Platyhelminthes exist, ranging from free-living to parasitic forms that infect up to 200 million humans (Homo sapiens) and a variety of other animals worldwide. Unlike more advanced forms of life, a flatworm doesn't have lungs. Instead, it absorbs oxygen directly through its skin.
How Flatworms Breathe
Flatworms, as indicated by the name, are tiny, flat worms that absorb oxygen directly through the skin into their bodies' cells. This mechanism is known as cutaneous respiration, skin breathing or diffusion. Because they have no lungs or circulatory system, all their cells must be close to the skin to receive oxygen from the surrounding water or fluids. Flatworms also absorb nutrients from a rudimentary digestive tract directly into the cells, using their mouths to suck in the food and then pushing out any undigested bits.
The Rest of the Worm
The flatworm has a nervous system, consisting of a group of nerves in its head, called a cerebral ganglia, and a nerve cord on each side of its body. Small nerves branch off from the nerve cords to the rest of the body. The eyes are actually simple, light-sensing groups of nerves called eye spots. Flatworms are hermaphrodites, with both male and female organs, so two flatworms can fertilize each other at the same time, and then lay eggs. A flatworm can also reproduce asexually, by dividing itself into two identical flatworms.
With more than 3,000 species, free-living flatworms (class Turbellaria) are found in moist land environment, fresh water and marine waters around the world. They swim by moving their bodies in a wave-like motion, or move on surfaces by using cilia, which are tiny, hair-like projections from their skin cells. While the tropical marine flatworms (Polycladida spp.) grow up to 3.9 inches or 10 centimeters long, most free-living flatworms are less than a centimeter in length.
The parasitic flatworms include flukes (classes Monogenea and Trematoda) and tapeworms (class Cestoda). There are more than 4,000 species of Monogenea, 9,000 of Trematoda and 5,000 of Cestoda. Most species of flukes and tapeworms use one or two intermediate species, such as snails or other animals for the larval stages, before infesting humans or other animals. The longest human tapeworm found was a Diphyllobothrium latum, at 40 feet long. Tapeworms (Hexagonoporus spp.) that infest whales are known to grow 100 feet or more long.
- PBS.org: Phylum Platyhelminthes
- Nebraska Wesleyan University Natural Science Division: Flatworms Lab -- Dugesia
- Animal Diversity Web: Platyhelminthes -- Flatworms
- Animal Diversity Web: Turbellaria
- Museum Victoria: Marine Flatworms
- University of Washington: Key -- Phylum Platyhelminthes, Class Turbellaria, Order Polycladida
- Animal Diversity Web: Trematoda
- Animal Diversity Web: Cestoda
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