MONOGRAPH OF THE EXISTING CBINOIDS. 695 



stronger is the reaction. When the animal is not fixed, or when the attachment 

 is precarious, the reaction is always feeble, and slow. Bohn believes that this has 

 to do with variations of sensibility. The tactile sensations of the disk may be 

 inhibited more or less by certain sensations of the cirri. 



It may happen that, the comatulid being attached by its cirri, certain arms 

 rest upon a solid surface, while the others float freely in the water. In this case the 

 latter react much more strongly when the disk is stimulated. 



Bohn often observed that a series of stimulations at short intervals on the 

 same spot end by having quite a different effect than one or two stimulations on the 

 same place. 



Repeated stimulation on the body or on the arms of a comatulid results in 

 undulatory movements of all the arms, not synchronous, until finally the animal 

 swims away. 



If a comatulid from which the two arms of the same ray, or two adjacent 

 arms on neighboring rays, have been removed lies upon a smooth bottom, pro- 

 gression usually takes place in the direction indicated by the missing arms. 



If the arms be numbered 1 and 1', 2 and 2', 3 and 3', 4 and 4', and 5 and 5', 

 according to the movement of the hands of the clock, and arm 1 be stimulated, 

 the animal generally tends to move in the direction 3' ; but if arms 4 and 4' rest 

 on a rough surface progression may be in the direction 4-4' ; if 4' and 5 rest on a 

 rough surface progression may be in the direction 4'-5, or at least in a direction 

 between 3' and 5. The importance the condition of the surface has in determining 

 the direction of progress is evident. 



Speaking of the comatulids which he found at Maer Island, Torres Strait, 

 Dr. H. L. Clark says that mechanical stimuli met with ordinary response under 

 normal conditions. The disk and the pinnules at the bases of the arms seem to be 

 more sensitive to touch than either the middle or terminal part of the arms. The 

 cirri and the whole aboral surface are slow to respond to mechanical stimulation. 

 The response to touching the disk is a closing of the arms over it, and the response 

 to touching the oral surface of an arm is the gradual infolding of that arm. 



Mechanical stimulation alone, even a severe pinch of an anterior arm with 

 forceps, seldom caused any change in the direction of locomotion. 



The net result of all Doctor Clark's observations was the impression that 

 comatulids are surprisingly callous to mechanical stimuli, and the response is never 



sudden or hurried. 



GEOTROPISM. 



Bohn has found that on an inclined bottom Antedon mediterranea when 

 stimulated tends to move along the line of greatest declivity in the direction 

 opposite to that of the pull of gravity. 



EFFECT OF THE RAIN OF CINDERS IN THE BAY OF NAPLES IN APRIL, 1906. 



Antedon mediterranea, which in the Bay of Naples is abundant on the bottoms 

 of detritus and in the patches of Posidonia so abundant, indeed, that thousands 

 ordinarily could be captured with the dredge almost completely disappeared 

 as a result of the eruption of Vesuvius in April, 1906. 

 142140 21 Bull. 82 46 



