The two forms could hardly be less alike. The female's body is a green sac a few centimetres across with a proboscis that can extend far beyond it, forked at the tip and used to gather organic particles. The male lacks a gut and a proboscis, is roughly the size of a large ciliate, and lives in the female's reproductive tract, fertilising eggs as they pass.
The switch is environmental, not genetic. Free larvae are undifferentiated; contact with female tissue triggers masculinisation. Bonellin, the green pigment in the female's skin, has been implicated in the effect, and the pigment is separately known to be toxic and photoactive.
How far the mechanism is understood should not be overstated. The masculinising influence of female contact is well documented, but the precise signalling pathway, whether bonellin alone is responsible, and how firmly the decision is fixed are not fully settled. The genus itself sits within the echiurans, which molecular work has placed inside the annelids, so spoon worms are segmented worms that lost their segments.