Sandbed Meiofauna Worms

Gastrotricha sp.

Hair Bellies

Likely Reef Tank Suitable

Likely Fish-Only Tank Suitable

More

There are many different very tiny crawling worms of interest that serve useful purposes in aquaria, some of which are described here for your edification in this grouping. They are listed here by Phylum, and their photo/sketch can be seen by selecting their label shown below.