The phylum Annelida includes earthworms, sandworms and leeches. Characterised by elongated, segmented bodies, they have well developed respiratory, vascular and nervous systems.
Only one species of earthworm was included in the Prodromus: McCoy's original description of Megascolides australis, the Giant Gippsland Earthworm. The longest earthworm in the world, this was a significant and surprising discovery.
Other earthworms, along with several species of leech, were drawn in the laboratory by Arthur Bartholomew, McCoy's lecture room assistant. Unlike the fragile Giant Gippsland Earthworm, these smaller animals could be kept alive in an aquarium within the lab for much closer observation. This allowed Bartholomew to capture the sinuous contraction and expansion of the leech's movement over several images.