430 



THE BIOLOGY OF MARINE ANIMALS 



Removal of the radial nerve cord abolishes longitudinal conduction and 

 co-ordinated locomotory responses in the arm. The movements of the tube 

 feet are regulated by five centres, each of which lies at the junction of the 

 circumoral ring with a radial nerve (Fig. 10.9). From each centre fibres 

 enter the contiguous radial nerve, and also proceed in both directions 



Mot. A 



Fig. 10.9. Schematic Diagram of Nerve Centres and Tracts Involved 

 in the Innervation of the Tube Feet of Starfish 



I-V, radial cords of arms I-V ; centre I, one of the five nerve centres of the circumoral 

 ring; Mot. A, Mot. C, motor neurones of the first and second orders, respectively; 

 m, segment of the tube foot musculature; N.R., nerve ring; t.f., tube foot. Motor 

 neurones of the first order (Mot. A) lie in the radial perihaemal canal and their axons 

 invade the tube foot, where they make synaptic connexion with second-order motor 

 neurones (Mot. C) ; the latter supply the postural muscles of the foot. (From Smith (103).) 



around the ring to the other arms. During normal locomotion of a starfish 

 one of these centres becomes dominant, co-ordinates the direction of 

 stepping movements in all arms and causes the animal to move in one 

 particular direction. 



A similar innervation pattern has been discovered in the circumoral 

 ring and radial nerves of sea cucumbers (Cucumaria), where the responses 



