Absence of a coelom in the phylum Platyhelminthes

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Platyhelminthes, or flatworms, lack a coelom, which is considered a primitive trait. This absence limits organ specialization, as organs are not suspended in a fluid-filled cavity that could allow for more complex arrangements. The absence of a coelom means there is no perivisceral cavity (a body cavity surrounding the internal organs), and there is no body cavity between the mesoderm and the endoderm layers, which is what defines a true coelom.

Absence of a Coelom in the Phylum Platyhelminthes