Figure 3.

The adult nervous system of S. purpuratus. Segmental radial nerves in adult echinoderms are known only from ophiouroids (Cobb, 1987). The adult sea urchin radial nerve appears to have a similar organization. A. Dissected radial nerve showing the segmental organization. Each segment spans the width of the nerve and a single nerve bundle projects from each segment. The segments are arranged so that alternating segments project nerve bundles to alternating sides of the nerve. A single segment is outlined (arrows). The patterned pigmentation is in the cellular layer. Bar = 1 cm B. Cross-section of a radial nerve showing that the cellular layer (c) is on the surface of the nerve and extensive neuropil (n), comprised of closely apposed neurites beneath the cellular layer. Bar = 500 μm C. The segmentation of the nervous system is revealed by whole mount of a nerve in which the lipophilic tracer DiI was injected into the lateral projections of two segments and neurites were allowed to back-fill overnight. The backfilling procedure identifies a similar cluster of nerve cell bodies in both segments (n) indicating the segments have identical organization and are repeated elements. Neurites also project between segments in longitudinal tracts that are predominantly on the lateral edges of the nerves. The darker pigmentation pattern (p) of the tissue provides orientation. Bar = 200 μm D. The adult nervous system arises in the developing adult rudiment on the side of the larval body. In this immunofluorescent preparation with anti-synaptotagmin the oral surface of the rudiment is uppermost and the larval nervous system (lns) is out of focus behind the specimen. Nerves appear in the developing appendages, spines (s) and tube feet (tf) simultaneously with the appearance of neurons in the radial nerves. Bar = 50 μm E. In confocal images of later rudiments, connecting neurites from the radial nerves to appendages appear (arrow heads). Bar = 25 μm F. The radial nerves (rn) first appear as a single segment and the neuronal cell bodies distinguish the radial nerves from the nerve ring (nr), which is simply a commissure of neurites. As the juvenile grows, additional segments are added to the radial nerves. Bar = 50 μm