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The aim of this article was to present the development of ideas about the provenience of parasitic helminths and the phylogenetical relationships within this taxon, since the publication of the "cercomer theory" just to nowadays. The following essentials of the Janicki theory are outlined: main differences between free-living Turbellaria and parasitic platyhelminths (ciliated epithelium in Turbellaria versus unciliated surface in the others); universality of the cercomer presence in Monogenea, Digenea and Cestoda; evolutionary changes in the morphology and function of the cercomer; homology of the caudal appendices of all parasitic helminths; the subsequent evolution of parasitic platyhelminthes from the ancestor to Monogena, Digenea and Cestoda; proposition to establish a new common taxon--Cercomerophora--for these three groups. In this background the evolution of evolutionary ideas is reviewed, divided into two periods: up to the eighties of the XX century, and up to date. The first period can be characterised by the criticism of some points of the "cercomer theory" and formulation of some new hypotheses; these are those of Fuhrmann, Bychovsky, Llewellyn, Price and Malmberg, which: questioned the homology of the cercarial tail with the caudal appendices of Monogenea and Cestoda; rejected Digenea from the common group; established the common taxon--Cercomeromorpha--comprising only Monogenea and Cestoda; opposed the idea of radial evolution of three main groups of Platyhelmithes (Turbellaria, Digenea and Cercomeromorpha) to the idea of subsequent evolution presented by Janicki. The differences between these last hypotheses are also underlined, arising mainly from the different ideas on the importance of particular features as the evolutionary indicators of affinities between and within the taxons. As to the hypotheses dealing with the evolution of particular groups of parasitic platyhelminths formulated at the same period, the publications of Freeman and Jarecka (Cestoda), Heynemann, Ginetsinskaja, Pearson, Cable, Rhode and Gibson (Trematoda), Bychovsky, Lambert and Malmberg (Monogenea) are referred, with special emphasises on the differences in the ideas presented by their authors. In the second period two points are underlined: a dynamic development of new techniques and methods (including molecular investigations) allowing to gather more and more different data on the parasites, and, as a consequence of this phenomenon, a new approach to the evolutionary problems--the birth of numeric and phylogenetic systematics. In this period "the cercomer theory", as well as supporting the group Cercomeromorphae generally are not accepted (exception: Brooks et al.). In contrast, the new taxon--Neodermata has been created by Ehlers. The importance of this publication for further evolutionary study is stressed. In this background some publications are quoted, especially those, presenting the results of searching for monophyletic groups and joining them in hieratic kladograms (Brooks and al., Rohde at al., Littlewood at al., and the others). It is stated, that in spite of some differences in the kladograms builded by various authors (resulting mainly from the set of features being analysed) the monophyly of big taxons of Neodermata (Trematoda, Monogenea, Cestoda) is fairly well documented. In conclusion several points connected with the "cercomer theory" are emphasized. It is now obvious, that the base of this theory--homology of caudal appendices of Janicki's Cercomerophora has to be rejected, as well as his concept of subsequent evolution of Platyhelminthes. But the base of his joining of Monogena, Trematoda (in his theory--Digenea) and Cestoda in one group (lack of cilia on the body surface), opposite to the Turbellaria (ciliary ephitelium) is maintained by the creation of Neodermata, undoubtedly documented better and in a different way. Also his idea (after many years of rejecting) on close affinity of Digenea and Cestoda seems to have returned due to the study of Lockyer at al., who write: "Among the Neodermata, the Cercomeromorphae (Cestoda + Monogenea) was not supported, whereas Cestoda + Trematoda was supported". |
