These have fascinating specialities, but need not concern us here. Apart from the ciliated whorly bit which catches food they are a varied group, varying from wormlike to flowerlike, in being attached at one end. But all forms are still basically bilaterally symmetrical. Well a through gut for a start, running from a mouth near the front end to an anus, a new development near the rear, which takes output from digestive, reproductive and excretory systems. The Round worms.
Flatworms are flat, roundworms are round.
They are a very uniform group, many parasitic, many living in restricted environments which vary from soil to beermats. Some live in dogs, some in humans. This means that food has to be pumped into the gut, which it is by a muscular pharynx, and leave via a muscular sphincter. When a roundworm opens this sphincter a jet may shoot half a metre. The nervous system is a nerve ring around the pharynx and dorsal and ventral nerve cords.