Starfish and sea urchins(echinoderms)share a basic body structure comprising a pentaradial configured nervous system and motor organs. Lacking a central nervous system to integrate their radially distributed body parts, the circumoral nerve ring is essential to mediate activities of the body parts and facilitate organized behavior in these individuals. Their tube feet, arranged in parallel, flanking each of the radial nerves, work as both sensors and motor organs. As sensors, the tube feet of starfishes and sea urchins exhibit aspects of chemical, light and tactile sensors, contributing to adaptive behaviors such as feeding, defense and homing. Spatial orientation in these animals was previously considered to be based mainly on chemical sensors, however, recent studies suggest vision-based navigation may play a more integral role. These ventral, tube-like projections also control coordinated movements of the echinoderm body parts in the absence of a central nervous system. Obstacle avoidance and righting behaviors are fascinating examples of such coordinated movements, and have long attracted researchers' interests concerning the question of learning. Attempts to test theories that these animals can learn coordinated movements through repeated trials have generally been unsuccessful, although in some studies improvements(reductions)in time required to complete such behaviors were reported. Nevertheless, results remain inconclusive given inherent difficulties in interpreting the learning capacity of these animals, and may be, in some part, due to the ambiguity of the rewards and penalties established in the published experiments.
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