Flatworms skip lungs and hearts, so they stay thin
Platyhelminthes—flatworms—are soft, unsegmented bilaterian invertebrates whose Greek name means flat parasitic worm. Traditional texts split mostly free-living Turbellaria such as planarians from wholly parasitic Cestoda, Trematoda, and Monogenea. Without body cavities they were long cast as primitive bilaterians.
Like other bilaterians they are built from three cell layers, unlike two-layered jellyfish and comb jellies, and their left and right halves mirror each other. With no heart or lungs, every cell must be reached by diffusion, so many species are microscopic and big ones are ribbons or leaves; large kinds compensate with guts branched throughout the body. Collagen-stiffened mesenchyme between skin and gut anchors muscles and holds the organs. Most have no anus and spit leftovers back out of the mouth, though some long species have one or several, and symbiotic Paracatenula has neither mouth nor gut.
The old four-way division into Turbellaria, Trematoda, Monogenea and Cestoda was always artificial. In 1985 Ehlers broke the polyphyletic turbellarians into about a dozen orders and united the three parasitic groups as Neodermata, and since the mid-1980s the Acoelomorpha have been moved out as basal bilaterians.
Free-living turbellarians number about 4,500 species, from 1 to 600 millimetres long, mostly predators or scavengers; land species creep out at night from leaf litter and rotting wood. Many feed through a pharynx that turns inside out. Pigment-cup eyes, usually one pair but sometimes clusters on tentacles, only sense which way light comes from. Planarians regrow from fragments, a new head forming fastest on pieces cut nearest the old one, which hints at a chemical gradient. Most are hermaphrodites, and some large aquatic species duel by penis fencing, the loser raising the eggs.
Trematodes take their name from Greek for hole, after the sucker-like holdfasts that anchor them in hosts, and their skin is a single shared membrane. A planarian released against the giant African snail in the Maldives worked only briefly, and it has since spread across the tropics.
Source: Flatworm
