Like acoelomorphs, members of the phylum Platyhelminthes are dorsoventrally flattened and so are commonly called “flatworms.” In fact, that’s what “platyhelminthes” means
– “platy” means “flat” and “helminthes” means “worms.”
To say that an animal’s body is dorsoventrally flattened
means that it is compressed from the top and bottom, not from the sides; in other words, it’s wider than it is tall. A laterally-compressed
animal, in contrast, is taller than it is wide.
As you might imagine, acoelomorphs and platyhelminths have a great deal in common. In fact, acoelomorphs were originally classified as members of the Platyhelminthes, before it was discovered that they have a number of important molecular and developmental dissimilarities.
Platyhelminths range from only a millimeter or so in length up to 10 meters or more in length for some tapeworms. Most are only a few centimeters in length, however. Many species are free-living, but quite a lot of modern species are highly-specialized parasites, and these are the ones most people think of when they think of platyhelminths.
“What is a ‘parasite’?” I hear you ask. Well, a parasite
is an organism that lives on or inside another organism (called the host
) and feeds on its host’s tissues. A free-living
organism is one that does not
live on or inside another organism.