PLAYHELMINTHES: FLATWORMS
the life cycle of a tapeworm
This phylum includes planarians, flukes, and tapeworms-basically, Platyhelminthes is just another name for flatworms. That probably says it all to you, but...
...while they may be disgusting, flatworms really are interesting creatures. As is to be supposed, flatworms are flattened dorsoventrally, meaning they are flattened from top to bottom. Unlike cnidarians, Platyhelminthes have three tissue layers instead of two and bilateral symmetry instead of radial symmetry. However, both have only one body opening for waste and food. Flatworms have cephalization, which means all of its sensory organs are near the top, or anterior end of the worm. Planarians, free-living flatworms, can reproduce sexually by fertilizing one another’s eggs or asexually by splitting in two and regenerating the missing parts of both. However, most flukes, a type of parasitic flatworm, reproduce only sexually. This occurs as the female lays eggs in the host. Tapeworms, perhaps the most disgusting type of flatworm, are parasitic worms that grow by producing new body segments. Each segment has male and female parts, and the eggs are fertilized by the sperm in its segment. When a segment has fertilized all of its eggs, the segment separates and passes out of its host. If another host eats one of those eggs, the egg will hatch into a young tapeworm. Interestingly, parasitic flatworms can go through many hosts throughout their lifetime. For example, flukes are born in one host, and then infect snails if in water. After leaving the snail, the fluke can then burrow into the skin of another host, perhaps a human. Tapeworms also are born in one host, hatch in another, and then relocate to a third (see image at left).