The systematics of the trematoda.

The Trematoda Rudolphi, 1808 are a class of the phylum Platyhelminthes that comprises two subclasses, the Aspidogastrea Faust & Tang, 1936 and the Digenea Carus, 1863. The subclass Aspidogastrea is a small group (4 families, 12 genera considered valid, c . 80 species) parasitic in molluscs, fi shes and chelonians [ 1 , 2 ]. Aspidogastreans like the digeneans use molluscs as fi rst obligate hosts but are characterised by being external rather than internal parasites of these hosts, and by having a single-generation life-cycles lacking asexual reproduction and a stage comparable to the cercaria [ 2 – 4 ]. Key information on the aspects of morphology, life-cycles, taxonomy, systematics and phylogeny of the aspidogastreans can be found in Rohde [ 1 , 2 , 5 , 6 ], Gibson [ 3 ], Gibson and Chinabut [ 7 ] and Zamparo and Brooks [ 8 ]. The subclass Digenea comprises a large and diverse group ( c . 2,500 nominal genera, c . 18,000 nominal species; see [ 9 ]) of cosmopolitan platyhelminths that are obligatory parasitic in invertebrate intermediate and vertebrate defi nitive hosts. Digeneans are found in all vertebrate classes but are less diverse in agnathans and chondrichthyans [ 10 , 11 ]. The subclass is characterised by a number of autapomorphies, associated with the unique complex digenean life-cycle: (i) acquisition of a Chapter 2 The Systematics of the Trematoda