Phylum: Nematoda

Roundworms

Nematodes have a tubular body (unlike the flatworms), and a digestive tract with two openings, but lack the partitions and segments of the segmented worms. Some nematodes burrow harmlessly through the soil, but many are terrible parasites. For example, hookworms live in the soil, but use their sharp teeth and hooks to burrow into their host through unprotected skin, finding their way into the bloodstream, and eventually into the intestines. As much as one quarter of the world's population is infected by hookworms.

NOAA
Marine roundworms.