The main function of animal nervous systems is to collect, process, and appropriately respond to sensory inputs from the body and the external environment. The composition, structure, and thus overall architecture of nervous systems, however, vary greatly between animals, and despite enormous advances in our understanding of the physiology and the development of these organs, their evolutionary history remains highly contentious. The main unanswered questions are when and how often during evolution centralized (also called cephalized) nervous systems arose. Two opposing scenarios dominate the current scientific discourse. The first proposes that a centralized nervous system first appeared in a last common ancestor and that centralization was subsequently lost in different animal lineages. The second proposes that nervous system centralization took place independently in different animal lineages. The only consensus accepted by both schools of thought is that this issue can only be resolved by characterizing the nervous systems of a wide variety of different animals, especially from so-called “minor/non-classical” animal groups, which is the focus of the EvoInSiDe team.