Fig. 5

Three proposals for the source of the dorsal organ (do) and its relationship to other axial structures that Pikaia may possess. A Option 1: that it forms prior to neurulation, which may then not occur, and from mesoderm in the same fashion as the notochord, or it is a notochord, so that its contents, whether cells or secretions, would be mesodermal in origin. Any neural component would lie above it, so the only access deeper structures would have to innervation would be by means of muscle extensions (arrow), in the same way that neuromuscular connections are made in amphioxus. B Option 2: that it forms after neurulation as a secondary invagination positioned above the nerve cord (*). It is shown here (in blue) as ectodermal in origin, so its contents would be ectodermal secretions, or it might have a cuticular lining. A variant of this option would be to have neurulation followed by a dorsal expansion of the coelomic system to produce a similar chamber of mesodermal origin, so it would be pink in the figure, and the contents would be connective tissue and matrix. C Option 3: the dorsal organ is the neural tube, much expanded in size, which then innervates the somites while contributing to body support either because of its intrinsic structural strength, or because hydrostatic forces make it turgid, or a combination of the two. This is the least conventional of the three options but, as discussed in the text, there are reasons for giving it serious consideration