Nervous Systems 809 



duction is reflex in nature; i.e., that contracting muscles re-excite conduct- 

 ing fibers. Reciprocal contraction of circular and radial muscles does occur. ^'* 

 The possibility of such reflex arcs in nerve net systems seems not to have 

 been explored. 



PERIPHERAL VERSUS CENTRAL CONTROL OF LOCOMOTION 



A subepidermal nerve network exists in many animal groups— echinoderms, 

 balanoglossids, annelids, and molluscs. Jordan-'^" has called those animals in 

 which behavior is under peripheral control the "hohlorganartige Tiere." Jor- 

 dan and his students think of the peripheral network in invertebrates as anal- 

 ogous to autonomic ganglia, centers of distribution of certain centrally orig- 

 inating messages. To what extent can the peripheral system act independ- 

 ently? It is appropriate to compare locomotion in many invertebrates with 

 movement of the mammalian intestine, in which the nerve network is es- 

 sential for coordinated movements, even though the muscle may be capable 

 of spontaneous movement. The subepidermal nervous system and the cen- 

 tral nervous system may be in competition with respect to locomotor control. 

 The competition between peripheral and central nervous components is of 

 phylogenetic interest. In general, in more sluggish animals, the peripheral 

 system is more important, and in more active forms central reflexes take over 

 locomotor reactions. Even in animals where locomotion is entirely centrally 

 controlled, visceral plexi remain autonomous, e.g., the myenteric plexus of 

 the vertebrate intestine and the stomatogastric system of annelids and arthro- 

 pods. 



The peripheral portion of the nervous system is more important for loco- 

 motion in echinoderms and in balanoglossids than in any of the worms, mol- 

 luscs, or arthropods. The nervous system of echinoderms consists of a central 

 oral ring from which the radiating ambulacral nerves originate. These radial 

 nerve trunks contain cell bodies and connect by branches to the peripheral 

 network which innervates tube feet, spines, etc.^^^ The peripheral network 

 contains ganglion cells, in Echini in rings at the base of large spines. The 

 peripheral nerve network can conduct impulses independently of the ring 

 and radial nerves. Jennings^-"* cut through a radial nerve of a starfish and 

 found that a prick at the tip of the injured ray caused pedicellariae of other 

 rays to rise. In a piece of the dorsal body wall of a starfish, stimuli in any part 

 excite all of the pedicellariae on the piece. If one arm of an ophiuroid is 

 placed under tension, excitation of the ring goes to the stretched arm rather 

 than to other arms.''^® Similar observations were made with fragments of sea 

 urchin shells, in which spine movement in response to NaCl stimulation 

 was studied.-*^ A piece of paper soaked in NaCl caused a nearby spine to 

 bend toward the stimulus; if a cut was made between the paper and the 

 spine, there was no response. If one spine was pulled, a nearby one was 

 stimulated to bend; if a cut was then made between the two, the excitation 

 still spread from one to the other, provided a third spine was at the end of 

 the cut. If a spine was held so that it could not move, waves were not con- 

 ducted past it. These results indicate that the ganglion cells at the base of 

 each spine constitute local reflex centers, and that the peripheral system is 

 not a continuous network. 



The starfish central ring has a directive function. A starfish with radial 



