Pterobranchia is a clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of small, worm-shaped animals. They belong to the hemichordata

Hemichordata

Hemichordata is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include two main classes: Enteropneusta , and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a...

, and live in secreted

Secretion

Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

 tubes on the ocean floor. Pterobranchia feed by filtering plankton

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

  out of the water with the help of cilia attached to tentacle

Tentacle

A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s. There are about 30 known living species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 in the group.

The class Pterobranchia was established by Ray Lankester

Ray Lankester

Sir E. Ray Lankester KCB, FRS was a British zoologist, born in London.An invertebrate zoologist and evolutionary biologist, he held chairs at University College London and Oxford University. He was the third Director of the Natural History Museum, and was awarded the Copley Medal of the Royal...

 in 1877. It contained, at that time, the single genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Rhabdopleura. Rhabdopleura was at first regarded as an aberrant polyzoon, but with the publication of the Challenger report

Challenger expedition

The Challenger expedition of 1872–76 was a scientific exercise that made many discoveries to lay the foundation of oceanography. The expedition was named after the mother vessel, HMS Challenger....

 on Cephalodiscus

Cephalodiscida

Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, living in clear water andsecrete tubes on the ocean floor.-Taxonomy:...

in 1887, it became clear that Cephalodiscus, the second genus now included in the order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

, had affinities in the direction of the Enteropneusta.

Studies under an electron microscope

Electron microscope

An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

 have suggested that pterobranchs belong to the same clade as the extinct

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 graptolite

Graptolite

Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...

s.

Biology

Pterobranchs are small, worm-like animals, living on the ocean floor, often in relatively deep waters. Like their relatives, the acorn worm

Acorn worm

The Acorn worms or Enteropneusta are a hemichordate class of invertebrates, closely related to the echinoderms. There are about 90 species of acorn worm in the world, the main species for research being Saccoglossus kowaleskii....

s, 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 zooid

Zooid

A zooid or zoöid is a single animal that is part of a colonial animal. The zooids can either be directly connected by tissue or share a common exoskeleton...

s living together in a cluster of tubes. In some species, the individual zooids within the colony are connected by stolon

Stolon

In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...

s. The genus Atubaria is unusual in lacking the tubes typical of other pterobranchs.

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

Anus

The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

 projects upwards, lying dorsal to the collar. Cephalodiscus has a single pair of gill slits in the pharynx

Pharynx

The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

, although Rhabdopleura has none.

Pterobranchs are dioecious

Dioecious

Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...

, with the fertilised egg hatching to produce a free-swimming ciliated larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

. 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

Asexual reproduction

Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 by budding to create a new colony.

Taxonomy

The class is a small one, with only three known families, each containing a single genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

.

Class Pterobranchia

• Order Cephalodiscida
  Cephalodiscida
  Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, living in clear water andsecrete tubes on the ocean floor.-Taxonomy:...
  
  
  • Family Atubaridae
    • Genus Atubaria
      • Species Atubaria heterolopha Sato, 1935
  • Family Cephalodiscidae
    • Genus Cephalodiscus
      • Cephalodiscus agglutinans Harmer & Ridewood, 1914
      • Cephalodiscus atlanticus Bayer, 1962
      • Cephalodiscus australiensis author unknown
      • Cephalodiscus calciformis Emig, 1977
      • Cephalodiscus densus Andersson 1907
      • Cephalodiscus dodecalophus McIntosh 1882
      • Cephalodiscus evansi Ridewood
      • Cephalodiscus fumosus John, 1932
      • Cephalodiscus gilchristi Ridewood, 1908
      • Cephalodiscus gracilis Harmer 1905
      • Cephalodiscus graptolitoides Dilly 1993
      • Cephalodiscus hodgsoni Ridewood, 1907
      • Cephalodiscus indicus Schepotieff 1909
      • Cephalodiscus kempi John, 1932
      • Cephalodiscus levinsoni Harmer
      • Cephalodiscus nigrescens Lankester, 1905
      • Cephalodiscus sibogae Harmer 1905
      • Cephalodiscus solidus Andersson, 1907
• Order Rhabdopleurida
  Rhabdopleurida
  Rhabdopleurida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, living in clear water and secrete tubes on the ocean floor.-Taxonomy:...
  
  
  • Family Rhabdopleuridae
    • Genus Rhabdopleura
      • Rhabdopleura annulata Norman, 1921
      • Rhabdopleura compacta Hincks, 1880
      • Rhabdopleura grimaldi Julien
      • Rhabdopleura manubialis Julien 1903
      • Rhabdopleura normani
        Rhabdopleura normani
        Rhabdopleura normani is a small, marine species of worm-shaped animal. It is a sessile suspension feeder, lives in clear water, and secretes tubes on the ocean floor.-Description:This species grows in colonies...
        
        Allmann, 1869
      • Rhabdopleura striata Schepotieff, 1909

Evolution

A 2011 paper in Current Biology

Current Biology

Current Biology is a scientific journal that covers all areas of biology, especially molecular biology, cell biology, genetics, neurobiology, ecology and evolutionary biology. The journal is published twice a month and includes peer-reviewed research articles, various types of review articles, as...

describes an early pterobranch. The 525-million-year-old fossil from China is named Galeaplumosus abilus.