Pterobranchs are small worm-like
filter feeders living on the ocean floor, often in relatively deep waters. Like their relatives, the
acorn worms, their body is divided into three parts: an anterior
proboscis, a collar, and a trunk. The proboscis is wide and flattened at the tip, and in most species contains glands that secrete a tube of organic material in which the pterobranch spends its adult life. The animals are mostly colonial, with several
zooids living together in a cluster of tubes. In some species, the individual zooids within the colony are connected by
stolons. The single member in the genus
Atubaria is unusual in lacking the tubes typical of other pterobranchs, living as a naked zooid on corals. Recently,
Atubaria has been regarded as a questionable species by Tassia et al. (2016) and is no longer considered valid. The collar bears a number of large arms, each of which includes a row of tentacles along one side. The number of arms varies between species, with anything from one to nine pairs. The tentacles are covered in cilia and aid in filtering food from the water. The trunk includes a simple tubular gut, and is curved over so that the
anus projects upwards, lying dorsal to the collar.
Cephalodiscus and
Atubaria have a single pair of gill slits in the
pharynx, although
Rhabdopleura has none. and from Bermuda (
Rhabdopleura normani). Both of these species are
dioecious, with the fertilised egg hatching to produce a free-swimming ciliated
larva. Despite the close relationship between the two groups, the larva does not resemble that of the acorn worms; they are "planula-like", and do not feed (lecithotrophic). Eventually, the larva settles onto the substrate and metamorphoses to an adult. Alternatively, they also reproduce
asexually by budding to create a new colony. ==Evolution==