Fig. 1

A Pikaia gracilens, from the Middle Cambrian Burgess Shale, a schematic drawing showing the main features discussed here: the small head and associated appendages, possibly gill-related, the series of putative somites, and the dorsal organ (shaded). The animal is portrayed with a pronounced bend in the body as if a wave of muscle contraction were propagating along it, but whether the body could flex in this way is a matter of conjecture (see [6] for a discussion). Figure 2 shows the head region in more detail. B An overview of the enteropneust nervous system, from [26], showing fibers from the extensive proboscis plexus projecting caudally through the collar cord (cc). The junctions (J1 and 2) between the three subdivisions of the body correspond in molecular terms with landmarks in the vertebrate brain, J1 with the zona limitans and J2 with the mid-hindbrain boundary [22], which means the neurogenic domain between these (in blue) is equivalent to the dien-mesencephalon of chordates as defined in molecular terms [12] while the neurons occupying the region immediately forward of J1 (in purple) correspond to types localized to the vertebrate hypothalamus. The trunk marks the beginning of the zone expressing Hox genes as shown. C The front of an amphioxus larva for comparison showing what is effectively the brain, i.e., the cerebral vesicle (cv), whose anterior and posterior parts map as shown to the vertebrate hypothalamus (hyp) and the basal dien-mesencephalon (tegmentum), which are colored to match their hemichordate counterparts in B. In amphioxus, the junction between the anterior and posterior cv occurs at the infundibular organ (red), which marks a major change in organization [28], implying very different evolutionary histories for the regions forward of this point and caudal to it. In contrast, the transition from the posterior cv to the rest of the nerve cord is more gradual, with Hox expression beginning at about the midpoint of somite 2