Schematic representation of VGLUT, GAD + VGAT, VGAT, VAChT, SERT/TpH and TH expression in the central and peripheral nervous systems of the ascidian tunicate tadpole and the amphioxus larva. For both the ascidian tunicate tadpole and the amphioxus larva the regions of the central nervous system homologous to the vertebrate forebrain (FB), midbrain (MB), hindbrain (HB) and spinal cord (SC) are indicated. The central nervous system of the ascidian tunicate tadpole larva is divided into a sensory vesicle (SV), a visceral ganglion (VG), and a caudal nerve cord . A slender neck (NE) region is present between the SV and VG. The anterior part of SV contains two sensory organs: the otolith (OT), used for the perception of gravity, and the ocellus (OC), for light reception. The left side of SV contains other sensory cells named coronet cells (CO). The VG consists of at least 8 motoneurons and some interneurons, while the anterior nerve cord contains the anterior caudal inhibitory neurons (ACIN). The ascidian tunicate tadpole peripheral nervous system consists of an adhesive organ with papillar neurons (PN), ectodermal neurons in the head (RTEN, rostral trunk epidermal neurons), trunk (ATEN, apical trunk epidermal neurons) and tail (DCEN and VCEN, dorsocaudal and ventrocaudal epidermal neurons, respectively), and bipolar interneurons (BP). The CNS of an amphioxus larva includes an anterior cerebral vesicle (CV), which, anteriorly, contains a frontal eye complex (FE) and, posteriorly, the infundibular organ (IO) as well as the lamellar body (LB). The primary motor center (PMC) is located at the posterior end of the CV and thus significantly anterior to the first pigment spot (PS). S, somite; N, notochord; M, mouth.