On the other hand, our previous idea is in stark contrast to the traditional "acoeloid planuloid" hypothesis favoring evolution of Bilateria from a simple body organization toward more complex forms with a last common ancestor resembling a flatworm without segmentation and coelomic cavities [ 60, 79– 82].
Bilaterians, whose LCAs arose after Cnidaria divergence, are characterized by a triploblastic body organization along two axes, anterior posterior and dorsal ventral, and a centralized nervous system.
Within Annelida sensu lato, it appears that a segmented body organization has been lost independently multiple times, but that it has been at least partly retained in the larval nervous system of some taxa that show no external signs of metamerism in the adult stage, such as echiurans [ 34, 35, 39] and sipunculans [ 47, 51].
It has been suggested that rapid embryogenesis and a simplification in body organization are at the basis of the breakdown of central Hox genes in these organisms.
This idea is completely corroborated with numerous data that the last common ancestor of Deuterostomia, Ecdysozoa and Spiralia had a segmented and coelomate body organization and that morphologically more simply organized taxa evolved by secondary reductions [ 74– 74].
The group of bilaterians that most strikingly matches an idealized definition of metameric body organization from an adult morphology point of view are the annelids [ 14].
Within this category we found a gene coding for oleosin OLE-2 protein (VIT_17s0000g09000), with a putative role in oil body organization and biogenesis as well as in reproduction and seed development.
In this context, holometabolous insects clearly represent an intriguing model because the overall body organization of the larva changes completely during metamorphosis: most of the organs undergo deep remodeling or even completely degenerate at that stage, and proliferation and differentiation processes are required to form the new body structure typical of the adult insect [ 1].
These pre-cladistics analyses emphasised as shared features: multiciliated (with multiple cilia per cell), glandular epidermis; rod-shaped secretory bodies or rhabdites; frontal glands or organs; protonephridia; and acoelomate body organization.
The highest level of body organization, then, is that of the organ system.
