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Figure 2 | BMC Evolutionary Biology

Figure 2

From: Immunolocalization of serotonin in Onychophora argues against segmental ganglia being an ancestral feature of arthropods

Figure 2

Two competing hypotheses on phylogenetic position of arthropods and their bearing on the evolution of segmental ganglia. (A) Articulata hypothesis: Arthropoda and Annelida are sistergroups. Segmental ganglia represent one of the synapomorphies uniting these taxa, but must have been reduced in Onychophora and modified in some annelids. (B) Ecdysozoa hypothesis: Arthropoda and Cycloneuralia (nematodes, priapulids, and allies) are sistergroups. Segmental ganglia evolved within the Arthropoda after the separation of Onychophora from the remaining arthropod groups.

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