| Pterobranchia | |
|---|---|
| Rhabdopleura normani | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Hemichordata |
| Class: | Pterobranchia Lankester 1877 |
| Orders | |
Pterobranchia is a clade of small, worm-shaped animals. They belong to the hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group.
The class Pterobranchia was established by Ray Lankester in 1877. It contained, at that time, the single genus Rhabdopleura. Rhabdopleura was at first regarded as an aberrant polyzoon, but with the publication of the Challenger report on Cephalodiscus in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta.
Studies under an electron microscope have suggested that pterobranchs belong to the same clade as the extinct graptolites.[1]
Contents |
Pterobranchs are small, worm-like animals, 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 within which the pterobranch spends its adult life. The animals are 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 genus Atubaria is unusual in lacking the tubes typical of other pterobranchs.[2]
The collar bears a number of large arms, each of which includes a row tentacles along one side. The number of arms varies between species, with anything from one to nine pairs being present. 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 has a single pair of gill slits in the pharynx, although Rhabdopleura has none.[2]
Pterobranchs 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. Eventually, the larva settles onto the substrate, and reproduces asexually by budding to create a new colony.[2]
The class is a small one, with only three known families, each containing a single genus.
Class Pterobranchia
A 2011 paper in Current Biology describes an early pterobranch. The 525-million-year-old fossil from China is named Galeaplumosus abilus.[3][4]