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Flatworm

Platyhelminthes.
unsegmented, soft-bodied
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.
- need to keep the concentration of dissolved substances in their body fluids at a fairly constant level.
no internal body cavity, so are described as acoelomates.
Mostly - The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.
- some long species have an anus and some with complex, branched guts have more than one anus
The space between the skin and gut is filled with mesenchyme, also known as parenchyma, a connective tissue made of cells and reinforced by collagen fibers that act as a type of skeleton, providing attachment points for muscles.
Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments, such as leaf litter.

Tapeworms

Cestodes

Flukes

Trematoda

Monogenea

external parasites infesting aquatic animals, and their larvae metamorphose into the adult form after attaching to a suitable host.

Planaria

Planaria have much of what we have: eyes; auricles have mechano/chemo -sensors: like both nose & ears, a mouth, brain, sex; their circulatory & digestive system are sort of united. But if you cut their brain off they can regenerate one & apparently still retain their old memories. - MT