Nematodes have sac-like digestive systems with two openings unlike lower (simpler) organisms such as hydra or sponges. They are often dioecious (each species has separate sexes.) There are free-living nematodes (like C. elegans) as well as parasitic nematodes such as the infamous ascaris worm.
These organisms are always bilaterally symmetrical, have no circulatory system, reproduce sexually, and usually live just about anywhere and feed on just about everything. They are often parasites as well.