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Plathehalminthes or flatworms are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates(having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both the ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.
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Dorso-ventrally flattened, distinct posterior and head (bilateral symmetry), and soft-bodied (no skeleton).

No body cavity (acoelomate condition) other than digestive (gut) and usually branched if present. Single opening to gut. Mouth is ventral.

A pair of fused cerebral ganglia form a rudimentary brain, with two interconnected parallel nerve cords running length of body. Ocelli (simple eyes) present in some.

respire by diffusion (no specialized respiratory organs).

No circulatory system.

excretory system consists of a protonephridial system consisting of ciliated flame cells.

Why must this system branch throughout the body?

A cautionary note about interpreting body plans that a more simple:
Traditionally considered the most primitive of the bilateria with the bilateria being grouped into acoelomates, psuedocoelomates, and coelomates.
Why were Platyhelminthes considered the primitive outgroup?

Now many zoologist are leaning toward three major branches of the bilateria where platyhelminthes and rotifers are closely related to lophotrochozoa and other acoelomates less related based primarily on molecular similarities.

The acoelomate condition may be secondary:

filled in as an adaptive response to reduce body size
neotonous from stage prior to coelom development. All three conditions may have occurred in the development sequence of the original bilaterian ancestor.

Are lack of systems and lack of coelom indications of primitive conditions or the result of constraints associated with selection for flatten morphology?
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