Medical Definition of Platyhelminthes
: a phylum of soft-bodied bilaterally symmetrical usually much flattened invertebrates comprising the planarians, flukes, tapeworms, and related worms, having the body unsegmented or composed of a series of proglottids formed by strobilation, built up of ectoderm, endoderm, and mesoderm, and without body cavity, the space between the body wall and the various organs being filled with parenchyma, and distinguished by an excretory system made up of tubules that permeate the body and usually communicate with the exterior and that end internally in flame cells
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