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Ciliated animals, usually no more than 0.5mm long, found associated with Protozoa and Rotifera at the bottom of shallow, still waters (eg. ponds and puddles). Head usually distinct, without the circles of cilia which characterise rotifers. Body somewhat flattened with bands of cilia, especially on the ventral surface. Mouth terminal. Intestine of the form typically seen in roundworms (Phylum: Nematoda). Posterior end of body generally divided (forked), with the anus opening above the base of two furca or 'toes'. The body cuticle is thin and generally is patterned, and may carry spines or other protrusions.
All freshwater species are parthenogenetic; males are unknown. All freshwater species are referred to order Chaetonotoidea, and most genera are cosmopolitan. The Australian fauna is not well known but it has been suggested that most freshwater genera are cosmopolitan, and on this basis four families (Chaetonotidae, Dasydytidae, Dichaeturidae, and Neogosseidae) might be expected. Williams (1980) reports only Chaetonotus s as having been positively identified from Australia. The illustration shows a representative member from each of the four families most probably occurring in Australia.
Identification to Gastrotricha can be based on size and general body structure. Gastrotrichs have a more or less distinct head region, an oval body without the telescopic 'tail' typical of bdelloid rotifers, and always two terminal 'toes'. The mouth is apical, the pharynx is strong and tubular, of a type seen otherwise in many Nematoda (roundworms). The integument carries tufts or bands of cilia and may also bear superficial annulations or spines and scales of various kinds.
Animals with which Gastrotricha can be confused include Rotifera and the juvenile stages of some small flatworms (Platyhelminthes). An absence of wheel-like cilia around the head distinguishes them from rotifers. The universal presence of two 'toes' serves to distinguish them from flatworms.
Williams, W.D. (1980) Australian Freshwater Life: The Invertebrates of Australian Inland Waters. The Macmillan Company of Australia, Melbourne.