Responding to an environment is a demand placed on all organisms and necessitates the ability to sense external stimuli. For organisms that have complex interactions with their environment, such as vertebrates, this ability has been evolutionary optimized to ensure precise methods of discriminating sensory information. Such optimization has been driven by coordinated development of a centralized processing center that is connected to peripheral sensory organs by way of peripheral neurons. Many studies have investigated the origins of the nervous system (Layden, 2019). Current evidence suggests a common ancestor for cnidarians and bilaterian nervous systems (Galliot et al., 2009). In chordates, the central nervous system (CNS) develops from the dorsal ectoderm. However, the urbilaterian ancestor likely had a similar organization to protostomes with a ventrally positioned central nervous system (Gerhart, 2000). The evolution of deuterostomes saw an inversion of that body plan. Multiple ideas have been proposed to explain these differing dorso-ventral developmental inversion schemes and originated primarily from comparative data on neuron, heart, and mouth location (Arendt and Nübler-Jung, 1997; Fritzsch et al., 2017; Gee, 2007). Among protostomes, the central nervous system is located ventrally, whereas the heart is found dorsally in the body (Gerhart, 2000). In contrast, among deuterostomes, the central nervous system is located dorsally and the heart is ventral (Gerhart, 2000). The ancestral deuterostome mouth is hypothesized to have originally formed dorsally following the inversion of the body plan, suggesting the mouth had to migrate ventrally (Lacalli, 2008). Closer examination showed that this inversion of body plan may have happened in different steps, as shown in the lateral positioning of the mouth in developing lancelets, which migrates ventrally during development (Kaji et al., 2016; Lacalli, 2008). The tunicates, which are the sister group to vertebrates, form their mouth directly from the neuropore, the opening at the rostral end of the neural tube (Veeman and Reeves, 2015), confirming the independent connection of the neuropores with the opening of the gut (Veeman et al., 2010) to forming a new ventral opening of vertebrates (Figure 1.1). Movement of the deuterostome mouth ventrally allows for the entire gut to remain ventrally, whereas in protostomes, the gut passes from the ventral mouth through an opening in the brain to then run dorsally in the organism (Gerhart, 2000). Elimination of the gut passing through the brain in deuterostomes would have allowed for the evolution of larger brains in vertebrates and thus acquisition of more complex processing centers. Gaining these new and more complex sensory processing occurred following duplication and diversification of gene regulatory networks (Fritzsch and Elliott, 2017).