Flatworms, roundworms and annelids

Platyhelminthes (flatworms) are flattened from top to bottom, like a ribbon. Most are parasites living inside other animals, including humans. They are bilateral, triploblastic, acoelomate, at the organ level. Parasitic ones have hooks and suckers to hold on, and simply absorb food through their body surface. Special flame cells remove waste. They are hermaphrodite. Planaria can regrow a whole body from a piece (regeneration). Taenia (tapeworm), Fasciola (liver fluke).

Aschelminthes (roundworms) are round in cross-section. They are at the organ-system level, bilateral, triploblastic, and pseudocoelomate. The gut is complete, with a muscular pharynx. Waste leaves through an excretory tube and pore. The sexes are separate (dioecious), and females are often longer than males. Ascaris (roundworm), Wuchereria (filarial worm), Ancylostoma (hookworm).

Annelida (from annulus, a little ring) are the ringed worms: the first animals with true metameric segmentation, and they are coelomates at the organ-system level. Long and circular muscles help them move; the sea worm Nereis also has paddle-like parapodia for swimming. Their blood stays in vessels (closed circulation). Nephridia remove waste. The nerve system is a pair of ganglia joined to a double nerve cord along the belly side.

Nereis has separate sexes. Earthworms (Pheretima) and leeches (Hirudinaria) are hermaphrodite (monoecious).