What organ systems first appeared in flatworms?

The flatworm species is related to three-layered animals. They have a mesoderm that is located between the endoderm and ectoderm. This type of worm belongs to a free-living parasitic species. these are very numerous animals, numbering about 12,500 species. They do not have a body cavity, however, for the first time, organs and their systems appear. Instead of a body cavity, there is a parenchyma. The anterior end of the body is crowned with a head; there is also a clearly defined posterior end. Also, for the first time, they have such sides of the body as the abdominal and dorsal, differing in bilateral symmetry. There are three classes of worms.

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