On the origin of Metazoa
...tazoa, while the others (Field et al., 1988; Christen et al., 1991; Philippe et al., 1994) favour polyphyletic origins. Nevertheless, recent molecular-genetic data on the Hox genes in various phyla of multicellular animals (including lower metazoans) (Slack et al., 1993; Valentine et al., 1996 and others) as well as the palaeontological data (Conway Morris, 1993; 1998b) seem to give some support to the idea of monophyletic entity ot the Metazoa. It is also appropriate to note that some important evidence concerning specific traits of the ancient ecosystems, in which the primary Metazoa originated and evolved, has been accumulated (Fedonkin, 1987; Chen, Erdtmann, 1991; Conway Morris, 1995, 1998c). In particular, we are now aware of extensive diversity of ancient protists and dynamic changes of the protistan world during Proterozoic (Knoll, 1994; Zavarzin, 1997). Although some recent publications favour the symbiotic theory of metazoan origin (Shostak, 1993) or the theory of cellularization of certain multinucleated protists, such as opalinids or infusoria (Raff, Kaufman, 1983; Denis, 1995; Gordon, 1999), the present paper deals with the theory of the origin of the Metazoa from colonial protistan ancestors, protists that we precisely do not know yet. This theory, supported during recent decades by numerous investigators (e. g., Ivanov, 1968; Salvini-Plawen, 1978; Malakhov, Nezlin, 1983; Pilato, 1992; Desnitski, 1993; Ivanova-Kazas, 1995, Kerszberg, Wolpert, 1998; Nielsen, 1998), claims that the hypothetical primary metazoons were (like the extant volvocine flagellates of the genus Volvox) free-swimming, spheroidal organisms consisting of several hundred or thousand of flagellated cells. According to different versions of the theory, the hypothetical blastula-like flagellate ancestor (blastaea) became two layered by multipolar ingression (Ivanov, 1968; Salvini-Plawen, 1978; Ivanova-Kazas,1995) or invagination (Wolpert, 1990; Nielsen, 1998) of cells executing a nutritive function. These diploblastic hypothetical metazoons have been called, respectively, parenchymella (phagocytella) and gastraea. In considering the problems of metazoan origins, two principles regarding the evolution of multicellularity obtained from studies of Volvox and its colonial relatives (Pandorina, Eudorina, Pleodorina) seem to be relevant: 1. significant diversity of structure and pathways of differentiation may exist even within a small taxonomic group of primitive organisms consisting of only two main cell types, i. e. the genus Volvox (Desnitski, 1992, 1995; Ransick, 1993; Kirk, 1998), 2. multilpe independent (and reversible) transitions to the truly multicellular state with a division of labour between two cell types may have occurred within a small taxonomic group, i. e. the family Volvocaceae (Larson et al., 1992; Schmi...