DEVELOPMENTAL PATHWAYS, HOMOLOGY AND HOMONOMY IN METAMERIC ANIMALS

Following Wagner's (1989) distinction between historical and biological concepts of homology, we analyze homology problems of metameric animals in the light of a biological concept. In identifying homology, we refer to the common informational background which two structures share. Therefore, homology relationships are matters of degree; they are 'perfect' only when there is full identity of informational background between the structures under comparison. Homonomy (serial homology) is not fundamentally different from other kinds of homology. We regard the differences between epimorphically and anamorphically developed segments as minor; therefore, the two kinds of segments are largely homologous. The morphogenetic processes giving rise to segmental structures are regarded as not necessarily hierarchical. We contrast the phylogenetic pattern of hierarchically nested homologies with a largely non-hierarchical pattern of homologous structures within the individual organism. This topological difference adds to heterochrony in generating the widespread mismatch of ontogeny and phylogeny.