Fig. 3

Development of the neuronal innervation (α-tub-LIR) of the larval median organ in S. alveolata (a) and I. australiensis (b-d). Confocal maximum projections. Anterior is up in all images. All views are dorsal, except of (d) which shows a view from apical. The position of the distinct ciliated tufts of the median organ is indicated by dashed circles and prominent perikarya at the tip of the median organ are marked with arrowheads. Age of the larvae is given in days post fertilization (dpf) and the species identity is specified in the respective figure. a The prominent prototroch (pt) as well as the innervating prototroch nerve (pn) are present in pre-metamorphic larvae of S. alveolata. The larval brain (br) consists of a ventral (vr) and dorsal root (dr) of the circumesophageal connective. Prominent neurite bundles innervating the median organ (nm) are detectable in the dorsal part of the brain. The latter bundles branch of from the dorsal root (dr) and run towards the median organ tip, where they terminate in distinct immunoreactive perikarya (arrowheads). At the base of the median organ distinct ciliated tufts (dashed circles) are present. b In pre-metamorphic stages of I. australiensis a comparable situation can be found. In these larvae the neurite innervating the ciliated tufts (dashed circles) branches of from the neurite bundles innervating the median organ (nm). c 26 dpf the bifurcation between (nm) and the neurite running towards the ciliated tuft is obvious. d The neurite bundles innervating the median organ branch off in a region closely related to the dorsal root of the circumesophageal connective (dr) and run towards the median organ tip. A prominent bifurcation (arrowhead) connects the ciliated tufts (dashed circles) with the later bundles. e In post-metamorphic stages the prominent neurite bundles innervating the median organ (nm), the apical immunoreactive perikaya as well as the ciliated tufts (dashed circles) are still present. Note that the entire median organ complex has changed its position during metamorphosis – from a dorsal towards a frontal orientation. br, brain; cc, circumesophageal connective; dr, dorsal root of the circumesophageal connective; nm, neurite bundle innervating the median organ; pl, palps; pn, palp nerve; pt., prototroch; vr, ventral root of the circumesophageal connective. Scale bars = 80 μm (a), 60 μm (b, c) and 30 μm (d, e)