Planarians are free-living (nonparasitic) flatworms that belong to class Turbellaria in the phylum Platyhelminthes. They are often used in classroom experiments because they have the ability to regenerate amputated parts of their body. When a planarian is divided in half, their neoblasts (adult stem cells) will begin to multiply – eventually differentiating into the appropriate cell types required to regenerate the missing parts. In nature, planarians also use this ability as a means of asexual (one individual) reproduction, where one individual can split into front and rear halves that will both regenerate, thus resulting in two planarians. The two-headed example in the image is not at all typical and has only been induced experimentally in microgravity experiments – but it’s still cool!
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