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Early forms of jellyfish seem to have evolved into various different kinds of worms, one of which was the ancestor of flatworms. They, in turn, probably evolved into higher animals. Ancestral flatworms may have first appeared around 570 million years ago. Here we see the basic plan laid out for the structure of the body which would be used and modified by all later groups of animals.
Tissues (first seen in jellyfish) evolved into organs (parts of the body devoted to doing a specialized job). So, for example, they evolved a stomach. The stomach of the flatworms was just a sac. The worms took food in and send out waste through their mouths. This was not as efficient as the more advanced system we are used to, where the digestive system forms a tube with an outlet separate from the inlet! However it was a big step forward from the jellyfish. The cells of the stomach specialized in secreting digestive enzymes, making processing of food more efficient.
Flatworms crawled forwards and so evolved sensors for smell and touch at their front end. An organ, the brain, developed at the front to process this information, with nerves going back to control the flatworm's body.
Their bodies evolved so the two sides were the same (bilaterally symmetrical). This was quite different from the radially symmetrical bodies of the jellyfish. This has been the basic design of most the higher animals. It was probably more efficient for the embryo of the worm to grow in this way rather than any other way.
Approximately 13,000 species of flatworms are found on Earth today. They are grouped in five classes:
Among other diseases, the flukes cause schistosomiasis in man.
- flukes (Trematoda) - mostly parasitic
- tapeworms (Cestoda) - mostly parasitic
- planarians (Turbellaria) - mostly free-living
- Monogenea - mostly parasitic
- Aspidocotylea or Aspidobothria - mostly parasitic
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