Fine structure of the brain and brain nerves ofOikopleura dioica (Urochordata,Appendicularia)

Summary Each second brain nerve consists of only one single fibre terminating at two different types of touch receptors in the oral region. The two nerves are the dendrites of two perikarya in the forebrain and are the master neurons for ciliary reversal in the stigmata, which is a two-neuron reflex. By axoaxonal synapses they control one motor neuron in the midbrain, i.e. the command neuron for ciliary reversal in both rings. This cell sends one axon branch in each third nerve to the cilia cells. In the left nerve this fibre is closely associated with a coarsely granulated accessory fibre, which apparently regulates the ciliary beat. The third nerves also contain one fibre each from another motor neuron in the hindbrain. These fibres make synaptic contacts at some specialized epidermal cells in the lateral trunk behind the ciliary rings. A few previously unknown nerves in the dorsal forebrain innervate epidermal cells. It is likely that the complicated epidermal motor innervation regulates the secretory activity of the oikoplasts or of the epidermal cells in constructing a new house, including the necessary complicated filters and food trapping mechanisms.