Comparative Aspects of Superficial Conduction Systems in Echinoids and Asteroids.

SYNOPSIS. The spine-convergence response of sea urchins and the papula (deimal branchia) retraction response of starfish persist after removal of the central nervous system, but are blocked by scratching through the skin. They are mediated therefore by diffuse superficial conduction systems capable of limited spread—to a few centimeters. The spine response does not bend around the ends of a cut, and therefore its conduction system is not a nerve net, though probably nervous. Every sensory site is connected to every effector site, within centimeters, but only by straight lines. This curious and unprecedented arrangement is not yet understood or anatomically based. Some plausible connectivities cannot be ruled out geometrically as being too extravagant. The response is not a chain reflex requiring spine movement or stretch on its far side. Facilitation of successive responses is not evident. The papula retraction response of starfish spreads around the ends of cuts. Its conducting system therefore acts like a true nerve net, the first such system to be demonstrated in echinoderms. It is capable of limited spread and, like the sea urchin spines, central reflexes are normally superimposed to mediate widespread coordination.