ABSTRACT
Within the free-living Platyhelminthes, the triclads or planarians are the best known group, partly as a result of being suitable for classroom studies but, largely, because they have been the subject of intensive research concerning the cellular bases of regeneration and pattern formation (for general reviews see Baguñà et al. 1994, and Baguñà 1998) and, most recently gene expression (Bayascas et al. 1997; Orii et al. 1999). The Tricladida Lang, 1884, which is best considered a suborder (Ehlers 1985a), forms, together with the suborder Proseriata Meixner, 1938, the Order Seriata. Autapomorphies for the Seriata are their backwards-directed tubiform and plicate pharynx and the division of testes and vitellaria into serially arranged follicles. Proseriata do not have obvious autapomorphies apart from their lack of lamellate rhabdites (Sopott-Ehlers 1985), whereas Tricladida are characterized by its three-branched intestine and its highly modified embryonic development with the presence of a transitory embryonic pharynx. A family of proseriates, the Bothrioplanidae Hofsten, 1907, was proposed by Sopott-Ehlers (1985) as the actual sister group of the Tricladida forming a taxon N.N. (Bothrioplanida + Tricladida) characterized by the presumed lack of epidermal collar-receptors, a tricladoid intestine, and a crossing-over of muscle layers at the root and the tip of the pharynx. A general phylogenetic scheme of Seriata is summarized in Figure 6.1.
