Hi-res art file & notes: Helminthes! (Patreon)

I was once commissioned by a biologist to realistically illustrate both a planarian and a mated pair of blood flukes as examples of Platyhelminthes (flatworms), which they used on a banner for a school event!

As direct illustrations of real creatures I don't have "creative notes" but here are various facts about these animals:

-Platyhelminthes are some of the simplest of all bilaterally symmetrical organisms, and one of the only animal groups with no internal body cavity. They are basically solid flesh consisting of several different cell types, with at most a thin network of fine digestive channels throughout.

-When a flatworm is cut apart, every piece can usually grow back into a whole animal. These pieces will actually consume some of their own tissues to form any anatomy they're missing, restructuring into a more complete but smaller worm.

-Many planarians have a pharynx, a tube or saclike structure that secretes digestive enzymes and can be pushed out of the body like an "external stomach."

-Some planarians live on land and prey on other organisms, especially insects, snails or earthworms, dissolving them in acid within seconds.

-Blood flukes are one of the most common and debilitating diseases in the world, though there are many species in the group that don't affect humans at all.

-The larger blood fluke is male, while the thinner one is female and spends her adult life protected by the male's body. He folds lengthwise like a taco shell to keep her safe!

-Sometimes several females will share a male, and sometimes a male blood fluke ends up pairing with another male, producing no eggs but still guarding each other.

-Sometimes a blood fluke pair will split up, but we don't actually know exactly why.

-Tapeworms are also Platyhelminthes, but even weirder in structure. Flatworms aren't normally segmented, but tapeworms sort of are because their long bodies have repetitive "seams" separating the body into segments called proglottids. These seams exist to let proglottids easily break off, like toilet paper, so they can pass out of the host.

-Flatworms get their most colorful in the ocean, as well as their largest. This includes tapeworms, the largest of which are of course mostly found in whales.

-Some flatworms swim by rippling their bodies, but all flatworms can glide around thanks to the millions of microscopic cilia on their surface.