-17- 



reflex with as much readiness and regularity (Pisaster) as 

 similar stimulus at tho place 1. It must be remembered 

 however that in normal locomotion the diso is often placed 

 flat on the substrate, tid that when the tube feet are exposed 

 to the air the ourfaoo tension fil.-i may be expected to oontnot 

 with equal pressure on all sides of the disc, and thus to 

 stimulate them all equally* We ha TO to count then upon the 

 greater excitability of the muscles on the side 1-3 in the 

 post-contact phase of the step reflex* This is comparable to 

 the increased tension of the muscles on the aide 2-4 in the 

 pro-contact stage of the step reflex* The oscillations of the 

 tube feet may be explained in terms of Von Uexkiuflfs law of 

 "tonus" or may be left unexplained* the fact is, of course 

 that they move back and forth in the step-reflex with con- 

 siderable regularity and precise orientation* The factors thai 

 control the orient ition of the animal will be taken up in 

 connection with an analysis of coordination aoong the tube feet* 



Status pjf foe attaohjlnf reflex during the stop reflex* 



The strength of attachment during the atop reflex differs 

 as we shall see with the different species and with the amount 

 of resistance there is to the accomplishment of the step* 



In general wo may assume from observations n ordinary 

 locomotion that the tendency to attach is strongest, during the 

 progress of the step reflex. Just after the contact* The 

 tube foot usually remains attached during the first half of the 

 backward oscillation, but the likelihood of release (or slipping) 

 is found gradually to onorease during tho last phase of the 

 atep reflex* 



A largo grain of sand was placed on one of the ambulacra! 

 disos of an active Pyonooodia. The step reflex which resulted 

 was so violent that tho grain of sand was thrown as from a 



