NERVOUS SYSTEM AND BEHAVIOUR 



429 



contract. Longitudinal cuts, below the stimulated region but lateral to the 

 motor centres, abolish the response. 



Some of the motor neurones are connected directly with the peripheral 

 plexus. These include motor neurones of the pedicellariae, spines, papulae, 

 muscles of the body wall and podia. These neurones receive impulses from 

 the diffuse plexus or through-conduction tracts and can be excited through 

 local circuits without the intervention of the radial nerve cords. 



The central co-ordinating action of the circumoral ring and radial nerve 

 cords is revealed by analyses of locomotory activities. During locomotion 



Papula. 



Lateral 

 muscle 

 of arm 



Motor 

 axons 



Association neurone 

 Sensory neurone 



Longitudinal muscle 

 of podium 



Radial nerve 

 cord 



Fig. 10.8. Nervous Elements in the Arm of a Starfish 



A, distributive neurones of medial nerve cord. B, C, motor neurones of foot complex. 

 D, lateral motor system. (From Smith (103, 104).) 



the animal progresses with one arm leading and co-ordinated stepping 

 movements of the tube feet. Movements of the latter are brought about by 

 contraction of longitudinal muscles in the foot which drive water into the 

 relaxed ampulla, causing retraction; and contraction of the ampullary 

 muscles, which drive water into the relaxed podium, causing protraction. 

 Ampullary muscles and longitudinal muscles of the foot are thus antagonis- 

 tic in action and operate reciprocally. In addition to these movements, 

 stepping involves swinging the foot through an arc in line with the direction 

 of progression, through the activity of special postural muscles. Motor 

 neurones supplying the podial and ampullary muscles lie at the base of 

 the podium in the foot complex (Fig. 10.8, B, C), and are connected with 

 the radial nerve tracts by segmental distributive neurones (A). 



