Worms
Three unrelated phyla sharing one common name.
“Worm” is not a scientific grouping. It covers at least three separate phyla: annelids (segmented, including earthworms and leeches), platyhelminths (flatworms), and nematodes (roundworms).
Earthworms and soil
Earthworms ingest soil and organic matter and excrete castings. Their burrows aerate the soil and improve drainage, and their movement mixes organic material downward.
Nematodes
Roundworms are among the most numerous animals on earth and are mostly microscopic. Studying them requires magnification, which is why they appear early in most microscopy sequences.
Nematodes are mostly microscopic, so studying them is a microscopy exercise from the start. A wet mount at 100x is usually enough; see stage and focus.