noun
Helobdella stagnalis (species of leech)
A specific species of freshwater leech. Typically encountered in biological or ecological contexts.
池でヌマビルを見つけた。
I found a Helobdella stagnalis leech in the pond.
General term for leech; ヌマビル is a specific species.
Compound of 沼 (numa, 'swamp') and 蛭 (hiru, 'leech'). The exact derivation is straightforward.