-25- 



Jt has been Been that an isolated tube foot will not 

 contract or extend quite normally. Certain conditions then may 

 be said to exist in the nerve net at the base of the stimulated 

 tube foot, which affect the muscles of the pedicel and ampulla and 

 cause the normal withdrawal (or extension) of the tube foot. Now 

 in accord with the well known laws of transmission of excitation 

 in a nerve net (Parker 191^ these conditions may spread in any 

 direction (within the ambulacjal nvrvous system) and cause the 

 retraction or extension of other tube feet. We shall see, 

 elsewhere that no such simple condition will account for the 

 physiological orientation of the tube feet and their coordination 

 in locomotion. 



Coordination ig. gills. 



The physiology of movement in the gills is quite similar to 

 that of the tube feet in the rigid starfish. Although there is 



lateral movement in each there is no orientation of these lateral 



(fa 4-W^u*- 

 movements in any particular direction in the gills. A- stimulus 



will cause the contraction of one group of the (dorsal) gills, 

 will be communicated to others near these and cause 



their retraction (Jennings 1907). In this region the nerve 

 net is quite diffuse, so that the spread of the contraction may 



be in any direction. The wave of r -extension usually takes Adm^cn 



-t* 

 opposite atii-tfutiun rrum^that of contraction. It is centripetal 



rather than centrifugal. If the wave of retraction is sufficiently 

 strong it may be communicated to the tube feet and involve their 

 retraction as well. The retraction of the tube feet does not 

 involve the retraction of the (ambulaoral) gills (De Moor & Chapeaux 

 1691) an evidence of polarity in the nerve net which suggests 

 something in the nature of a synapse. That part of the nerve net 

 whicfe extends up the sides of the long ambulacfal gills in 

 Pisaster also shows evidences of polar^ation similar/ to the 



