Fig. 2

Scanning electron microscopic images of developmental stages of Sabellaria alveolata (a-d, g) and Idanthyrsus australiensis (e, f). Age of the larvae is given in days post fertilization (dpf) and the species identity is specified in the respective figure. In (a-c) anterior is left, in (e-f) anterior is up. d is a frontal view with the dorsal side up. a and b show a view from lateral. The inset in (b) shows a dorsal view with the anterior side up. c and (e-g) are dorsal views. The position of the larval median organ is marked by the white arrow and the position of the prominent ciliated tufts on the median organ is indicated by red dashed circles. a In pre-metamorphic larvae the prototroch (pt), telotroch (tt), the chaetae (ch) and the palps (pl) are well developed. The larval median organ is situated within the dorsal gap of the prototroch (pt), between the bases of the palps (pl). b A closer view reveals prominent ciliated tufts (red dashed ovals) and few additional cilia covering the surface of the larval median organ. The inset shows the same region in earlier larvae (~ 25 dpf) with palp (pl) buds already developing. Note the onset of the ciliation along the median organ. c A dorsal view reveals the position of the ciliated tufts at the base of the median organ. d In post-metamorphic specimens, the entire complex of the median organ is shifted from a dorsal towards a frontal position. The entire structure becomes flattened and incorporated in the formation of the juvenile anterior end. Nevertheless, the ciliated tufts (red dashed circles) are still detectable. e 26 dpf larvae of I. australiensis exhibit a median organ with a densely ciliated base. f The median organ is still well-developed even in larvae prior to metamorphosis. g In juveniles of S. alveolata the larval median organ is reduced in size, but still present as part of the anterior juvenile end. ch, chaetae; nh, nuchal hooks; op, opercular papillae; pa, palae; pl, palps; pt., prototroch; tt, telotroch. Scale bars: 20 μm (a-e), 10 μm (f) and 100 μm (g)