Neuronal form in the central nervous system of the tadpole larva of the ascidian Ciona intestinalis

The dorsal tubular central nervous system (CNS) of the ascidian tadpole lar va is a diagnostic feature by which the chordate affinities of this group, as a whole, are recognized. We have used two methods to identify larval neu rons of Ciona intestinalis. The first is serial electron microscopy (EM) as part of a dedicated study of the visceral ganglion (1), and the second is the transient transfection of neural plate progeny with green fluorescent p rotein (GFP) (2), to visualize the soma and its neurites of individual neur ons in whole-mounted larvae of C. intestinalis. Our observations reveal tha t ascidian larval neurons are simple inform, with a single axonal neurite a rising from a soma that is either monopolar or has only very few, relativel y simple neurites arising from it, as part of a presumed dendritic arbor. S omata in the visceral ganglion giving rise to axons descending in the cauda l nerve cord are presumed to be those of motor neurons.