Australian Biological Resources Study

Australian Faunal Directory


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C. Burdon-Jones, Queensland Museum, Brisbane, Queensland, Australia


The living members of the order Cephalodiscida Fowler, 1892 form sedentary colonies or aggregations of small zooids which are 1–12 mm long, with 1–9 pairs of tentaculate arms, and a single pair of branchial pores. They are devoid of any organic linkage, and live in individual coenoeces or share a common protective housing which they construct with secretions produced and moulded by their proboscides.

Each zooid has a long muscular stalk with an adhesive terminal portion which enables the animal to anchor itself on a marginal filament (see Fig. 3a) while feeding at an ostium or on the surface of the coenoecium (Lester 1985). The distal part of the stalk is capable of budding new zooids. Zooids, with numerous buds attached, move freely throughout the coenoecium, and over its outer surface. Detached buds are presumed to remain within the parent colony (Fig. 3b), and contribute to the growth and repair of the coenoecium. Usually zooids and their buds are the same sex, but Ridewood (1907) recorded a male zooid with a female bud in Cephalodiscus (Cephalodiscus) hodgsoni. Colonies are often dominantly one sex.

Sexes are separate, but hermaphrodite zooids have been recorded. Gonads are paired, ova are large and heavily yolked. According to Masterman (1900), Andersson (1903, 1907), Harmer (1905), Schepotieff (1909) and Gilchrist (1915), development into a ciliated planula larva takes place within the coenoecium. John (1931) described post-planula stages, but dispersal, settlement and the formation of a new colony by budding have not been fully described.

The structure of the coenoecia varies considerably between species and forms the basis for the grouping of the various species into several subgenera (Andersson 1907; Ridewood 1907; John 1931). In the Cephalodiscus sub-genus, Cephalodiscus, the colony is branching, each ostium leading into a cavity which is continuous throughout the colony, and is occupied by the zooids and their buds. In the subgenus Orthoecus Andersson, the colony consists of a cluster or cake-like mass of individual tubular coenoecia each attached to its neighbour for part or almost its entire length by common coenoecial material. The coenoecia may or may not include sand grains, sponge spicules, echinoderm spines, sponge spicules and shell fragments. The coenoecia may be closely set and parallel or irregularly bent and straggling (see Fig. 4a). Colonies vary in size and shape (Ridewood 1907, 1918; John 1931; Hyman 1959), from a diminutive, brittle disc-like structure c. 10 mm in diameter and a few millimetres high, to a bulkier mass, some 30 cm diameter and 10 cm high. Tropical species of the subgenus Orthoecus are very much smaller than those species found in the Antarctic.

By contrast, the colonies of the subgenus Idiothecia Ridewood, have a branching structure, each branch studded with short projecting ostia, leading to individual tubular coenoecia, with blind ends connecting up at the centre. In the irregularly branched and more elaborately structured housing of the nominate subgenus Cephalodiscus (Fig. 4b), each ostium leads into a common coenoecial cavity, continuous throughout the colony, and occupied in common by all the zooids and their buds. Such colonies may attain a height of 25–30 cm and be 15–20 cm in diameter, and can form a fairly dense carpet on the sea-bed.

In the sub-genus Acoelothecia John, the colony has two forms—one small and tuft-like, the other larger and branching. Internally, they both consist of a meshwork of bars and spines without any true coenoecial cavities. The zooids and buds occupy the irregular and continuous spaces between the sponge-like meshwork of the housing.

The order dates from the Upper Cambrian and contains two families—Eocephalodiscidae Kozlowski, 1949, all of which are extinct (Kozlowski 1949; Bulman 1970), and the Cephalodiscidae Harmer, 1905 which includes both living and fossil forms (Chapman et al. 1995).

A third family, the monospecific Atubaridae Benito, 1982, has been included in some classifications (e.g. see Benito 1982) but omitted from others (Dawydoff 1948; Van der Horst & Helmcke 1956; Hyman 1959; Bulman 1970; Markham 1971; Nishikawa 1986). In the Catalogue, the genus Atubaria Sato has been accorded equal rank with Cephalodiscus M'Intosh in the family Cephalodiscidae.


General References

Andersson, K.A. 1903. Wiederentdeckung von Cephalodiscus. Zoologischer Anzeiger 26: 368-369

Andersson, K.A. 1907. Die Pterobranchier der Schwedischen Südpolar expedition 1901–1903. Nebst Bemerkungen über Rhabdopleura normani Allman. Wissenschaftliche Ergebnisse der Schwedischen Südpolar-Expedition 1901–1903 5: 1-122

Benito, J. 1982. Hemichordata. pp. 819-821 in Parker, S.P. (ed.). Synopsis and Classification of Living Organisms. New York : McGraw-Hill Vol. 2.

Bulman, O.M.B. 1970. Graptolithina, with sections on Enteropneusta and Pterobranchia. pp. V1-V163 in Teichart, C. (ed.). Treatise on Invertebrate Paleontology. Part 5. Kansas : Geological Society of America, University of Kansas Press.

Chapman, A.J., Durman, P.N. & Rickards, R.B. 1995. Rhabdopleuran hemichordates: new fossil forms and review. Proceedings of the Geologists' Association 106: 293-303

Dawydoff, C. 1948. Embranchement des Stomocordés. pp. 367-449 in Grassé, P.P. (ed.). Traité de Zoologie. Echinodermes, Stomocordés, Protochordés. Paris : Masson et Cie Vol. 11.

Fowler, G.H. 1892. The morphology of Rhabdopleura Allman. Fetschr. z. 70ten Geburtstage R. Leuckarts. 293-297 pp.

Gilchrist, J.D.F. 1915. Observations on the Cape Cephalodiscus (C. gilchristi Ridewood) and some of its early stages. Annals and Magazine of Natural History 8 16(94): 233-243 [with appendix by S.F. Harmer pp. 243–246]

Harmer, S.F. 1905. The Pterobranchia of the Siboga Expedition with an account of other species. Siboga-Expéditie Report 24: 1-132

Hyman, L.H. 1959. The Enterocoelous Coelomates—Phylum Hemichordata. Ch. 17. pp. 72–207 in, The Invertebrates. Smaller Coelomate Groups. New York : McGraw Hill.

John, C.C. 1931. Cephalodiscus. Discovery Reports 3: 223-260 pls 33-38

Kozlowski, R. 1949. Les Graptolithes et quelques nouveaux groupes d'animaux du Tremadoc de la Pologne (Graptolity i pore nowych grup zwierzat z tremadoku Polski). Palaeontologia Polonica 3: i-xii 1-235 pls I-XLII

Lester, S.M. 1985. Cephalodiscus sp. (Hemichordata, Pterobranchia) observations of functional morphology, behaviour and occurrence in shallow water around Bermuda. Marine Biology, Berlin 85: 263-268

Markham, J.C. 1971. The species of Cephalodiscus collected during operation Deep Freeze, 1956–1959. Biology of the Antarctic Seas. Antarctic Research Series 17: 83-110

Masterman, A. 1900. On the further anatomy and the budding processes of Cephalodiscus. Transactions of the Royal Society of Edinburgh 39(3): 507-527

Nishikawa, T. 1986. Hemichordata pp. 1–110 in, Systematic Zoology (Dobutsu Keitobunruigaku). Vol. 8(3). Tokyo : Nakayama Shoten.

Ridewood, W.G. 1907. Pterobranchia. Cephalodiscus. pp. 1–67 pls 1–7 in, National Antarctic Expedition 1901–1904. Natural History. London : British Museum.

Ridewood, W.G. 1918. Cephalodiscus. British Antarctic 'Terra Nova' Expedition. Zoology 4(2): 11-82 pls I-VI

Schepotieff, A. 1909. Die Pterobranchier der Indischen Ozeans. Zoologische Jahrbücher. Abteilung für Systematik 28: 429-448

van der Horst, C.J. & Helmcke, J.G. 1956. Cephalodiscidae. Vol.3(2,8). pp. 33-66-50 figs in Kükenthal, W. & Krumbach, T. (eds). Handbuch der Zoologie. Berlin : Walter de Gruyter & Co.


History of changes

Note that this list may be incomplete for dates prior to September 2013.
Published As part of group Action Date Action Type Compiler(s)
12-Feb-2010 (import)