The origins of cephalopod body plans: A geometrical and developmental basis for the evolution of vertebrate-like organ systems

The evolution of cephalopod body plans has been one of the most intriguing topics in zoology. Their body parts, particularly of coleoids (squids, cuttlefishes, and octopuses), are composed of multiple sets of components including vertebrate-like analogical systems as a result of convergence. However, in spite of the potential importance for understanding the evolution and diversity in bilaterians, the origins of cephalopods have been poorly understood. There is little consensus of opinion for morphological linkage among the plans of cephalopods, basal molluscs, and other bilaterians. Here, we provide a review and new interpretation with an emphasis on the topographic transition of the soft parts that is shaped by a shared concentric circle or ovoid pattern in the embryos and adults of extant or fossil molluscs. The purpose of this article is also to characterize the cephalopod body plans, set against those of the other bilaterians, in the light of recent data from paleontology, embryology, and molecular gene expression patterns.