Acknowledgments

The following people must be thanked for their contributions to this key.  Ben Gunn (CSIRO Entomology), for his contribution of taxa and character images.  John Hawking (CRC Freshwater Ecology), for the loan of specimens of Turbellaria for photography.

General Information

The phylum Platyhelminthes (flatworms) is divided into three classes and a large number of orders and families.  The class Turbellaria contains free-living and commensal species, and the majority of aquatic flatworms are members of this class.  Classes Trematoda (= flukes) and Cestoda (= tapeworms), contain parasitic worms, but some Trematoda go through one or more free-living dispersive stages.  This key covers aquatic free-living turbellarians at family-group level, ectoparasitic trematodes (fish flukes, etc.) at order-level, and the final aquatic stage (cercaria) of the endoparasitic trematodes of some terrestrial vertebrates (family Fasciolidae: sheep liver fluke, etc.).  Other animals which could readily be mistaken for flatworms and are included in this key include Nemertea (ribbon worms), Hydridae (some Hydra-like Cnidaria), Tardigrada (tardigrades), and some families of segmented worms (Annelida).

Identification

Flatworms are bilaterally symmetric, soft-bodied animals showing a wide variety of body forms.  Most but not all are markedly flattened dorso-ventrally.  The known Australian aquatic species range in size from under 1mm to about 10mm when fully extended, but most species contract strongly when disturbed and size is not a good identificatory character.  Body-shape characters can also be problematic in preserved specimens unless care has been taken to kill and fix the animal in a relaxed posture.

Identification to phylum can be based on overall body shape, the possession of a ventral or terminal mouth and a blind gut without anus, an absence of appendages (other than tentacles in some families) and absence of a long eversible proboscis.

Misidentification

Species which move with a leechlike motion can be mistaken for (and in some cases were originally described as) leeches (Annelida: Oligochaeta: Hirudinea).  Confusion with leeches is especially possible for the commensal or ectoparasitic species in families Temnocephalida and Procerodidae.

Tardigrades (Phylum: Tardigrada) can look quite similar to some small flatworms, but are instantly distinguished by their short, stumpy legs with claws.  Leg-like body appendages without claws do occur in some flatworms from the family Temnocephalidae.

Some Temnocephalidae can be confused with cnidarians of the family Hydridae.  These flatworms (commensals on crustacea and other animals) have tentacles at the anterior end and wave these around while holding on to their host with a posterior adhesive disk.  The most obvious difference between these temnocephalids and Hydra is that Hydra have a simple gut cavity while these flatworms possess a muscular pharynx.

Annelid worms from families Aeolosomatidae (Class: Aphanononeura) and Histriobdellidae (Class: Polychaeta) also might be mistaken for flatworms.  Both are small (about 1mm long) worms with reduced segmentation, a sub-terminal mouth, a posterior anus and an internal coelomic cavity as in other annelid worms.  Aeolosomatids are free-living.  They move by means of ciliated areas, sometimes paired, in the head region.  Histriobdellids are ectoparasites in the gill chambers of some freshwater Decapoda (Crustacea).  They have five very short tentacles on a rounded head and 5-6 bundles of cirri (setae) each side, one on the head, 4-5 on the body, and the tail region is developed into two short 'limbs'.

Identification of flatworms to family sometimes can be achieved without preparing stained sections on microscope slides.  The most useful character systems are:

1 Overall body shape, especially the presence or absence of triangular lobes at the side of the head (these lobes are characteristic of the common triclad family Dugesiidae).

2 Position of the mouth (at the anterior tip, within the anterior 1/3 of the body, or mid-ventral).

3 Presence/absence of tentacles or of a short anterior proboscis.

4 Presence/absence of suckers and hooks, especially near the anterior tip, around the mouth, and at the posterior end of the body.

5 Form of the gut (a simple sac, lobed, bifurcated, or three-branched with one anterior and two posterior branches).

6 Form and orientation of the pharynx (a simple tube, muscular and spherical, muscular and barrel shaped, or plicate -- with an extensible, muscular tube on the inner surface).

7 Presence or absence of various secretory glands and ciliated pits near the body extremities or the mouth.

8 Form and positions of parts of the reproductive system.

9 Ability (or lack of ability) to reproduce asexually by transverse fission, and to form chains of partially-separated zooids.

Flatworms are hermaphroditic but the male and female systems are not always developed and may not mature simultaneously.  Maturity is not generally a permanent condition; the reproductive organs may be resorbed seasonally or in response to environmental conditions.  The gross configuration of reproductive parts can be seen in an appropriately stained wholemount slide preparation but more subtle reproductive characters require sectioned specimens.  In some cases identification to family level will not be possible unless specimens are observed in reproductive phase or else actively undergoing asexual fission.  Identification to below the family level is almost entirely based on details of the internal structure and usually requires a series of stained, histological sections.

All large (above 5mm extended) Australian aquatic free-living flatworms should key to the turbellarian orders Tricladida or Lecithoepitheliata (= Alloecoela).  Two triclad and one alloecoel family are known from Australian inland waters.  Smaller flatworms may key to any of about 10 families in the informal grouping Microturbellaria (orders Catenulida, Macrostomida, Lecithoepithheliata, Neorhabdocoela, Prolecithophora, and Proseriata).  At least some species in most of these families are ectocommensal or ectoparasitic.  The wholly ectocommensal Temnocephalida, sometimes treated as a separate class, is regarded here (following Parker, 1982) as a suborder of Neorhabdocoela.