242 



H. P. KJERSKOG AGERSBORG. 



as the tide runs out. The rate of speed of such distance-move- 

 ments depends on the circumstances. For example, when insti- 

 gated to move by some artificial means, as by being turned on its 

 back, brought from deep water to shallow water, the rate was 

 2.5 feet per minute. When, however, during tidal recession it 

 would remain stationary on its temporary feeding ground, 

 selected during high-tide, until the water measured about 2.75 

 feet. Then it would begin to move down to deeper water, con- 

 tinuing moving until it had reached a depth of about 4.5 feet, 

 and at a rate of 3.5 feet per minute. This shows that when 

 instigated to move by a natural stimulation its rate of speed is 

 greater than when caused to move artificially. 



Irritability Limit. 



Another indication brought out in this connection is that 

 Pycnopodia is sufficiently sensitive to pressure so as to start 



FIG. 3. 



moving down to deeper water when the tide has receded to a 

 certain level. As a matter of fact, out of seventy-five specimens 

 that were examined during one low-tide on a given area (Fig. 

 3), only two were at the shallow depth of 2.5 feet. Specimens 

 brought from deeper water to shallower water, that is from any 



