Nerves in the spine of a sea urchin: a neglected division of the echinoderm nervous system.

Electrical stimulation of the primary spines (greater than 10 cm long) of the tropical sea urchin Diadema antillarum elicits graded compound action potentials that are conducted at a constant speed of approximately equal to 27 cm/sec. Ion substitution experiments suggest that these are due to the summation of calcium spikes. Structural studies have revealed the presence of up to 21 regularly disposed nerves within the spine shaft, each nerve bundle including greater than 1000 neurites in the basal region, narrowing to slender groups of processes near the spine tip. The neurites in each nerve range in diameter from less than 0.1- approximately equal to 2 micron. Most appear to be distal processes of presumed sensory perikarya situated at the level of the tissue cone surrounding the spine base, or more proximally, although some neurites may arise from perikarya near the spine tip. These nerve tracts are thought to correspond to the nerve fibers described by Hamann [(1887) Jena Z. Naturwiss. 21, 114-176] almost a century ago in spines of Centrostephanus longispinus and, thus, to represent a long-neglected region of the echinoid nervous system.