MONOGR.\PH OF THE EXISTING CRINOIDS. 699 



yet regain these more or less completely after an interval of half an hour to an 

 hour. If, however, sufficient care has been taken to destroy entirely the central 

 capsule, the loss of power is absolute and permanent. 



Experiment 9. — The centrodorsal plate of an active specimen was removed 

 and the central capsule entirely destroyed ; the cavity was also very freely painted 

 with nitric acid, so as to expose and destroy the pentagonal commissure connecting 

 the axial cords at their roots. After being left at rest for an hour tlie arms were 

 irritated one by one. Each arm responded readily and extensively to the stimu- 

 lation, but the movement was limited to the arm directly irritated, none of the 

 other arms sharing in it, except sometimes the other arm of the pair to which the 

 irritated arm belonged. 



Expenment 10. — Various parts of the surface, both of the disk and of the 

 arms, of active uninjured specimens were irritated, both mechanically and chem- 

 ically, in order to determine the normal distribution of sensation. All parts of 

 the surface were found to be sensitive, but in very unequal degrees. Irritation of 

 the dorsal surface of the calyx caused only slight movements of the arms, unless 

 the irritation were severe or prolonged. 



Irritation of the dorsal or lateral surfaces of the arms, where the layer of 

 integiunent is very thin, caused flexion of the arms, with extension of pinnules 

 close to the irritated spot. The response was usually ready, but the movement 

 only slight. Prolonged or more violent irritation caused exaggeration of the 

 movement, together with approximation of the adjacent arms toward the irritated 

 arm, as though to remove the source of irritation, and in some cases active move- 

 ment of the whole animal in a direction away from the irritated arm. Irritation 

 of a pinnule causes, according to the degree and duration of the stimulation, 

 movement of the pinnule, movement of the whole arm, approximation of the 

 adjacent arms to the affected one, or active movement of the whole animal away 

 from the source of irritation. Irritation of the oral pinnules causes, as already 

 noticed, immediate and very active flexion of all the arms, so as to close in over 

 the disk. 



The epithelium of the ambulacral grooves is extremely sensitive, and the 

 results of the stimulation are very definite. The slightest irritation causes instan- 

 taneous movement of the four or five pairs of pinnules immediately adjacent to 

 the irritated spot, the pinnules being folded alongside the ambulacral groove so 

 as to close it in and grasp the needle or other source of irritation. If the stimu- 

 lation be continued the arm is actively flexed and the adjacent arms applied 

 to it and rubbed along the affected part, as though to remove the source of 

 irritation. Finally, irritation of the ventral surface of the disk between the 

 ambulacral grooves causes movements of the arms, but not nearly so active as 

 when the oral pinnules are touched. 



Experiment 11. — An active specimen was eviscerated and left for half an 

 hour. The calyx, arms, and pinnules were then successively stimulated, as in 

 the preceding experiment. The results were exactly the same. 



Experiment 12. — An active specimen was taken and all the soft parts scraped 

 away with a knife from the ventral surface of one of the arms, the scraped 



