94 



Dr. J. F. Gemmill. 



[Jan. 17, 



but not always, an effective starter. It would seem as if a certain amount of 

 physiological inertia had first to be overcome before the nerve stimuli could 

 be released on which co-ordinated movements of the lantern depend. Habit 

 also counts for something. The most persuasive method of starting the 

 movements is to set tbe urchin upside down and apply irritation to one side. 

 Once the movements have begun, you may expect with confidence that the 

 urchin will travel freely if it be restored to the mouth-downward position, 

 even though prior to inversion it had refused to budge at all. Similarly, if 

 it be next immersed in sea water so that it can travel by help of spines and 

 sucker feet alone, it may continue to exhibit an ineffective, but still rhythmic 

 and regular lantern activity. Inertia has been replaced by momentum. 



Urchins which one can recognise as being in an unhealthy condition fail 

 altogether to travel out of water, though under water they may still be able 

 to move about by using their sucker feet. 



Distance Travelled and Time Taken. — The furthest I have noted urchins to 

 travel out of water is 12 to 16 inches in 20 minutes. Usually the distances 

 are shorter, and, in over 100 experiments, just 45 percent, of the urchins went 

 upwards of 4 inches in the time named. The direction of progress cannot be 

 predicted. The same urchins, similarly oriented, in successive experiments at 

 intervals of an hour or so, often started off in quite different directions from 

 those which they had previously adopted, although the greatest care was 

 taken to prevent surface irritation during the experiments, as well as during 

 the intervals when the urchins were replaced in the tanks. The lines of 

 progression may be divided into three groups, as follows : — 



(«) Straight, or showing only slight irregularities, usually wavy in 

 character, from side to side. 



(&) Curving more or less uniformly and quickly to one side. 



(c) Showing one or more distinct changes of direction. The tracks 

 intervening between the changes fall naturally under a or b. 



The proportion of instances in which the above lines of progression were 

 adopted in my experiments and the relation to rotation are shown in the 

 table under the next heading (p. 96). 



Rotation. — Out of water urchins usually exhibit a certain amount of 

 rotation as well as progression. The two movements are associated together 

 in the sense that both are caused by the action of the lantern and of the 

 spines, but, so far as I can judge, they do not depend on one another in any 

 intimate or causal sense. There may be rotation without progression, and 

 progression without rotation, and when they occur together there is no fixed 

 ratio between them. For example, an urchin may travel 12 inches and yet 

 have practically the same orientation at the end as at the beginning, or it may 



