As a child, you may have dug in the dirt and played around with

platyhelminthes: flatworms
nematode: roundworms
annelida: segmented worms
mollusca: mollusks

The answer is Annelida: segmented worms.

Segmented worms or Annelida are widespread animals. They live in water or on land. Burrowing segmented worms live in the soil where take place in decomposition of organic matter, so they can be easily found during digging in the dirt.
Flatworms (Platyhelminths) live in freshwater or as parasites inside the hosts. Roundworms (Nematode) are usually parasitic worms. Molluscs (Mollusca) inhabit almost all habitats in the world, however, usually can be found in marine environments, some terrestrial examples are slugs and snails.