OBJECTS CABBIED BY THE SEA-UBCHIN. 447 



There was also a striking difference between the smaller 

 (10 and 15 mm.) and the larger (21 to 27 mm.) individuals in 

 the manner in which materials were picked up. The small ones 

 would proceed quickly to substitute other objects as often as 

 their materials were taken from them, but the larger ones either 

 neglected to do this or performed the operation in a less deter- 

 mined fashion. The records of three series of experiments out 

 of a number will serve to illustrate this point. 



1. The Sea-Urchin of 10 mm. was found one morning to be 

 almost hidden beneath the nine pebbles, weighing 1*0 gr., which 

 it carried. After these pebbles had been removed from the 

 animal and weighed, the Sea-Urchin was put back into the tank. 

 During the following fifteen minutes it covered itself with 

 twenty-one pebbles weighing 1*8 gr. Twenty-two minutes later 

 it had again completely covered itself with fifteen pebbles 

 weighing 1*9 gr. A third time it covered itself, with twelve 

 pebbles weighing 1*2 gr. It repeated the action a fourth time, 

 but the number and weight of the stones was not ascertained. 

 On the following morning the animal was found to be carrying 

 ten pebbles weighing 1*0 gr. Nineteen minutes after removal 

 of these it had picked up twelve stones and a piece of seaweed 

 weighing 2*4 gr. Nine minutes afterwards it had nearly covered 

 itself with nine pebbles weighing TO gr. Its subsequent actions 

 on this day were not noted. It should be pointed out that on 

 each of these occasions the animal was closely watched, and was 

 seen deliberately to pick up the pebbles, and with its spines 

 and tube-feet to move them slowly from place to place on its 

 body until it was hidden. Whenever the Sea-Urchin was 

 returned to the aquarium after examination, it was not merely 

 dropped into the water but was placed on the floor in its natural 

 position, mouth downwards, in order that there should be no 

 possibility of confusing the stones which were really picked up 

 with those to which the animal merely fastened* its tube-feet in 

 the act of turning over. 



2. Further experiments were made with the Sea-Urchin of 

 15 mm. Eight pebbles (weight not taken) were removed from 

 its body, and in fifteen minutes it had picked up seven pebbles. 

 It again covered itself with ten pebbles. Next day it was found 

 to be carrying six pebbles (weight not taken), and when these 



