Parenchyma is an ambiguate term for different forms of tissues in botany and zoology. (I’ve only taken BIOL422, so I’ll be updating this proper for botany next semester. Sorry!)
Organisms in the phylum Platyhelminthes possess a loose, sponge-like tissue which fills the unused pockets of the body that is referred to as parenchyma. It is referred to as a “filler” tissue which makes up the remainder of the platyhelminth.