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About Polychaetes
Worms that are not polychaetes

Platyhelminthes  -  flatworms


Platyhelminthes: Planocridae [photo: Leslie Newman & Andrew Flowers]

Platyhelminthes are acoelomate bilaterally symmetrical worms which are typically strongly dorso-ventrally flattened; they include both free living and parasitic forms. The mouth is ventral, the gut is sack-like and there is no anus (waste is regurgitated). Parasitic Platyhelminthes, though they are widespread in marine vertebrates, are beyond the scope of this publication. However, free living flatworms belonging to the Polycladida and Tricladida are occur widely in marine environments.

Polyclads are mostly marine worms, strongly flattened and either circular or elongate and reaching at least 15 cm in length. A pair of triangular appendages may be present on the anterior margin, or even more ear-like dorsal appendages may be present. The pharynx often results in a mid-dorsal ridge; the mouth is located at the anterior end of this ridge on the ventral surface. Various kinds of eyes are normally present, either in the head region, or around the margins. Many polyclads are nondescript or translucent, however others, especially tropical species, often have conspicuous and distinctive colour patterns.

Marine triclads belong to the Maricola and range from interstitial forms about 1 mm long up to a maximum of about 25 mm; most are at the small end of this range and are typically elongate in shape. Preserved specimens contract to even smaller sizes.


Polyclads and triclads are apparently all predators and are mostly benthic, although a few pelagic, interstitial and commensal forms are known.

Polyclads include some 1,000 species world wide. The Australian fauna is too poorly known to estimate diversity. At least 20 species are known in southern Australia.

About 60 species of Maricola (marine triclads) are known world wide; Sluys (1989) is a comprehensive guide to the world fauna. There are a very few records from Australia and New Zealand, but this certianly reflects the lack of study of this difficult group rather than low abundance or diversity.

Acoela, and other kinds of Platyhelminthes also occur in marine environments (Cannon, 1985) and the Australian fauna is beginning to attract systematic study (Winsor, 1988, 1990).

Identification of polyclads and triclads relies on use of appropriate narcotics and fixatives, dissection histology and microscopic examination of serial sections. Study methods and identification guides are provided by Cannon (1986), Prudhoe (1982a, 1982b, 1985) and Sluys (1988). Prudhoe (1982a) is a simple introduction to the fauna of southern Australia and Edgar (2000) has photographs of 4 species from the same region. Winsor (1990) has a key to free living genera of Acoela known from Australian waters. Ecologists whose only need is to distinguish species (without identifying them) may be able to achieve this by careful observation of morphology and colour of living specimens.


Anderson, DT. 1977. The embryonic and larval development of the Turbellarian Notoplana australia (Schmarda, 1859) (Polycladida: Leptoplanidae). Australian Journal of Marine and Freshwater research 28, 303-310.

Ball, IR. 1977. A monograph of the genus Spathula. Australian Journal of Zoology 47, 1-43.

Cannon, LRG. 1986. Turbellaria of the World. A Guide to Families and Genera. Queensland Museum, Brisbane.

Edgar, GJ. 2000. Australian Marine Life. Reed, Kew, Victoria.

Haswell, WA. 1907. Observations on Australian polyclads. Transactions of the Linnean Society of London, Zoology 9, 465-485, 3 plates.

Hilbig, B and Blake, JA. 1997. Phylum Platyhelminthes (Class Turbellaria). In Blake, JA and Scott, PH (eds), Taxonomic Atlas of the Benthic Fauna of the Santa Maria Basin and the Western Santa Barbara Channel. Introduction, Benthic Ecology, Oceanography, Platyhelminthes and Nemertea. Santa Barbara Museum of Natural History, Santa Barbara, California, pp. 65-82.

Hyman, LH. 1959. Some Australian polyclads. Records of the Australian Museum 25, 1-17.

Noldt, U and Wehrenberg, C. 1985. Quantitative extraction of living Platyhelminthes from marine sands. Marine Ecology Progress Series 20, 193-201.

Prudhoe, S. 1978. Some polyclad turbellarians new to the fauna of the Australian coasts. Records of the Australian Museum 31, 586-604.

Prudhoe, S. 1982. Polyclad flatworms (Phylum Platyhelminthes). In Shepherd, SA and Thomas, IM (eds). Marine Invertebrates of Southern Australia Part 1. Handbooks Committee of the South Australian Government, Adelaide, pp. 220-227.

Prudhoe, S. 1982. British Polyclad Turbellarians. Cambridge University Press, Cambridge.

Prudhoe, S. 1982. Polyclad Turbellarians of the southern coasts of Australia. Records of the South Australian Museum 18, 361-384.

Prudhoe, S. 1985. A Monograph on Polyclad Turbellaria. British Museum (Natural History) and Oxford University Press, London and Oxford.

Sluys, R. 1989. A Monograph of the Marine Triclads. A.A. Balkema, Rotterdam.

Winsor, L. 1988. A new acoel (Convolutidae) from the north Queensland coast, Australia. In Ax, P, Ehlers and Sopott-Ehlers, B (eds). Free-living and Symbiotic Platyhelminthes. Fortschritte der Zoologie Vol 36. Gustav Fischer Verlag, Stuttgart, pp. 391-394.

Winsor, L. 1990. Marine Turbellaria (Acoela) from North Queensland. Memoirs of the Queensland Museum 28, 785-800.