Body anteriorly cylindrical, posteriorly more flattened.
Prostomium conical, composed of ten rings, with palps and antennae forming a cross at the tip. Two small eyes on the basal ring.
Anterior 27-42 parapodia uniramous with one finger-like prechaetal lamella until segment 18-30, following uniramous segments with two prechaetal lamellae. Dorsal and ventral cirri finger-like. Posterior parapodia biramous with a small, triangular or elongate notopodium, one notopodial and two neuropodial postchaetal lamellae.
Notopodial chaetae numerous, finely denticulate capillaries. Neuropodial chaetae compound with long, denticulate blades. Papillae on pharynx are flat, heart-shaped and of equal size.
Macrognaths with 5-8 teeth. Upper half of proboscis with 4 small Y-shaped micrognaths, lower half with three somewhat bigger, X-shaped ones. 7-11 chevrons on each side (G. maculata-detail).
Up to 100 mm long for 200 segments.
Living animal bright green to lemon yellow, often with brown spots or bands. In alcohol brownish yellow, parapodia lighter with brown spots.
On all substrata, also in brackish water.
Whole North Sea, Skagerrak, Kattegat, Öresund, English Channel, Arctic, North Pacific, Atlantic and Mediterranean.