The worm is aptly named; Both its common and Latin names refer to the two, chromatically-hued spiral structures that are most commonly what is seen of the worm by divers. In actuality, these multicolored spirals are merely the worm's highly-derived respiratory structures.

S. giganteus appears like most tube-building polychaetes. It has a tubular, segmented body lined with chaeta, small appendages that aids the worm with its mobility. As it does not move outside its tube, this worm does not have any specialized appendages for movement or swimming.

The worms's most distinct features are the two "crowns" that are shaped like Christmas-trees. These "crowns" are actually highly modified prostomial palps which are specialized mouth appendages of the worm. Each spiral is actually composed of feather-like tentacles called radioles, which are heavily ciliated which allows any prey that are trapped in them to be transported straight towards the worm's mouth. While they are primarily feeding structures, S. giganteus also uses its radioles for respiration. It is because of this that the structures are commonly called "gills".

One of the major differences between christmas-tree worms and the closely-related sabellid fan worms is that the latter do not have any specialized body structures to plug the holes of their tubes with when they withdraw into them. S. giganteus, like the other members of its family possess a modified radiole, usually called the operculum, that it uses to secure its hole when withdrawn into its tube.