noun
Marphysa sanguinea (a species of eunicid worm)
A marine polychaete worm, sometimes called 'rock worm' or 'blood worm' in English. Used in fishing bait and occasionally in cuisine.
釣り餌としてイワムシを使う人が多い。
Many people use Marphysa sanguinea as fishing bait.
イワムシは磯の岩の間に生息している。
Marphysa sanguinea lives among the rocks on the shore.
Compound of 岩 (iwa, 'rock') and 虫 (mushi, 'worm'), referring to its habitat among rocks. The exact derivation is straightforward.