The crown, used by these animals for respiration and alimentation,[3] is the structure that is most commonly seen by scuba divers and other casual observers.

Worms of the genus Serpula are commonly found attached to submerged rocks, shells, and even boats in many coastal areas around the world.

[16][21] The prostomium bears a branchial "crown", a specialized mouth appendage which resembles a fan (for which the animals are given the name fanworms).

This crown, which can be extended for feeding and gas exchange, and rapidly retracted when threatened, consists of two bundles (one right and one left) of featherlike 'gills', known as branchiae or radioles.

Each of these bundles consists of a single row of radioles attached to a branchial stalk and curved into a semicircle.

[3][22][23] The radioles of different Serpula species are typically red, pink, or orange in color, with white transverse bands.

The operculum consists of a long, thick stalk with a cartilaginous, cone-shaped plug at the distal end.

[24] They are usually smooth with faint longitudinal ridges, curved but not strongly coiled, and are rarely more than 12 cm in length.

It is a broad, shallow, relatively indistinct trough running the length of the thorax and ending at the head.

[3] Worms of the genus Serpula have a distinct abdominal region, composed of up to 190 very short, wide segments.

Once in the abdominal fecal groove, further ciliary currents transport particulate matter (feces, gametes, etc.)

[28] When the crown is retracted inside the tube, the radioles and operculum cease to function as a respiratory organ.

Under these circumstances, respiratory exchange is probably carried out between the blood in these vessels and the surrounding water, which is kept moving through the tube by vigorous pumping movements of the abdomen and also by the activity of the ciliary tracts.

[28] The well-developed longitudinal muscles of the body wall of serpulids lack a special blood supply.

It seems probable that the outer body surface of serpulids serves as a respiratory membrane, supplying oxygen to the underlying muscles by diffusion.

[33][34][35] Its structure is very similar to erythrocruorin, each molecule being composed of more than a hundred interlinked 16-17kDa myoglobin-like subunits arranged in a giant complex with a total weight exceeding 3600kDa.

The nervous system consists of a central brain in the upper part of the head, which is relatively large compared with that of other annelids.

[38] When they reproduce, they simply shed their gametes straight into the water where the ova and spermatozoa become part of the zooplankton and are carried by the currents to new sites, where the juvenile worms settle into the substrate.

Length of the planktonic stage is unknown but comparison with other serpulids suggests it may be between six days and two months, although in other species the period has been shown to vary with season, salinity or food availability, and delayed settling may cause reduced discrimination of substrata during settling (see ten Hove, 1979 for additional references).