![]() ![]() Their bodies are basket-weaves of muscle and connective tissue, with no internal cavities full of soft organs. There are thousands of species of planarians, and they’re all very different from more familiar worms like earthworms. As the naturalist John Graham Dalyell wrote in 1814, planarians could “almost be called immortal under the edge of the knife.” And if Collins needs more animals quickly, she can do with a scalpel what the worms do with their own muscles. Breeding them is a cinch: Given enough food, planarians will repeatedly double themselves by halving themselves. “It’s just mind-blowing,” Eva-Maria Collins of Swarthmore College, who studies these animals, told me. And even more miraculously, the tail regrows its head. Within days, the head piece grows a tail. After a few minutes of stretching and ripping, it separates into two halves-a head and a tail. The planarian begins as a small, flattened, sluglike creature with a spade-shaped head and two googly eyes. Others, more straightforwardly, tear themselves in two. Most of the free- living species live in aquatic habitats, but some live in moist soil.When planarian flatworms want to reproduce, some have sex. They eat other small invertebrates and decaying animals. At the very top of the head is a “crown” of hooks called a scolex. The head of a tapeworm has several suckers. Instead, they absorb nutrients directly from the host’s digestive system with their suckers. Unlike other invertebrates, tapeworms lack a mouth and digestive system. Tapeworms also have a scolex, a ring of hooks on their head to attach themselves to the host (see Figure below). Tapeworms and flukes have suckers and other structures for feeding on a host. How could such a complicated way of life evolve? The sheep liver fluke has a complicated life cycle with two hosts. If you follow the life cycle, you can see how each host becomes infected so the fluke can continue its life cycle. As an adult, the fluke has a vertebrate host. Look at the life cycle of the liver fluke in Figure below. Usually, more than one type of host is required to complete the parasite’s life cycle. Tapeworms live in the host’s digestive system. Flukes live in the host’s circulatory system or liver. \)īoth flukes and tapeworms are parasites with vertebrate hosts, including human hosts. ![]()
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