Flatworm's features kept in convergent evolution with vertebrates?

Question

Flatworms part of the clade platyhelminthes are one the more primitive groups of bilateral animals, probably the most primitive bilaterial animals were very similar to flatworms.

I had the idea about megafaunal sizes creatures with some of main and more notorious flatworms' features, with a ventral mouth and I remember some even have a double cephalization (with sensorial organs in both extremes of the body), some species with strange an interesting reproductive methods.

As I said, the flatworms are comparable to ancient precursos of both lineage arthropods and vertebrates, so looked reasonable to think that if flatworims would have the oportunity to evolve and fill new niches would converge with the "more advanced" arthropods or vertebrates, the problem is, these animals neither have defined organs or limbs. That makes harder to determine if the evolved flatworms would keep their interesting features while they become more complex or they finally would get features more conventional to vertebrates.

With convergent with vertebrate I meant to develop a spine and maybe skeleton or in general the required structures to grow till big sizes.

I know this is hard to think because the evolution require hundreds of million years, but the question is practically reduced to consider if those current adaptations could be keep after all that time considering the possible advantages and disadvantages of that.

Answer 1

Evolutionary background

The idea that platyhelmints are basal/primitive bilaterians was based on morphological and developmental observations and has been disproven by phylogenetic analysis. There’s a smaller, flatworm-shaped (but not closely related) subphylum called Acoelomorpha which may really be basal, but your “typical” flatworms like Planaria and Turbellaria are quite comfortably embedded down the Protostome (invertebrate) branch, closest to things like snails. The current idea therefore is that their ““simplified”” anatomy is an evolutionary adaptation from a ““more complex”” ancestor (Scare quotes because evolution does not surge forwards to increasingly complex and superior species with, obviously, humans at their top. Flatworms are just as evolved as any other animal).

Developmental constraints

The defining feature of flatworms (and Acoels) is that their flatness is structural: because they lack a body cavity, their 3 developmental layers form a single sheet, folded over once. All the nice things you get from a coelom like a respiratory or circulatory system or just the ability to arrange your organs 3-dimensionally go out of the window. You’re also quite brittle due to your whole body being practically solid (no internal fluid for shock absorption) and constrained to a sheet or rope-like shape (all your gas and waste exchange has to diffuse through the skin), so a planarian megafauna is not really going to work, although frankly small critters are perfectly interesting and maybe you could consider whether it’s so important that your hypothetical creatures must be very large.

A lot of the features of flatworms that you are curious about are not necessarily that strictly associated with their evolutionary status, so I agree with @imsop’s comment that you should instead think of what features you want and how they may come about in a different evolutionary branch (i.e. snails have ventral-ish mouths, weird reproduction, and bonus a forehead anus despite not being acoels)