Articulata hypothesis
The Articulata hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda. This theory states that these groups are descended from a common segmented ancestor. The Articulata hypothesis is an alternative to the hypothesis that ecdysis (the shedding of outer cuticle) is a primitive characteristic – this would place Panarthropoda in the group Ecdysozoa. The Articulata hypothesis originates from the phylogenetic analyses of Georges Cuvier in his 1817 published work Le Règne animal, distribué après son organization. In this work, Cuvier theorized that all organisms exist as a functional whole, meaning that all of the physiological structures of an organism are important for survival. By studying these physiological structures, Cuvier was able to group the known animal kingdom according to structural similarities resulting from what he referred to as special "ground-plans", which are analogous to blueprints. Each of these ground-plans, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these ground-plans, Cuvier separated the known animal kingdom into four branches or embranchements: Vertebrata, Articulata, Mollusca and Radiata. From this phylogenetic grouping, the Articulata hypothesis was born. The Articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum Annelida (which includes polychaetes, oligochaetes, and leeches) together with the phylum Arthropoda (arachnids, insects and crustaceans) into the common taxon Articulata. Cuvier grouped these diverse phyla together according to the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin. Since its original formulation in 1817, there have been significant challenges and modifications to the Articulata hypothesis as new theories have been accepted (Charles Darwin's theory of evolution) and new technologies have become available (confocal microscopy, DNA sequencing, genomics).
