OBJECTS CARRIED BY THE SEA-URCHIN. 451 



from fixing its everted stomach tightly upon the anus of the 

 echinoid. It maybe that whilst the Sea-Urchin is able gradually 

 to give up carrying a mass of protective or disguising materials, 

 because the larger it grows the less liable it is to attack, it is 

 still compelled to protect or hide one vulnerable spot. 



These and other considerations gave rise to some experi- 

 ments to decide (1) whether the larger Sea-Urchins would always 

 pass objects from outlying parts of their bodies on which the 

 objects had been laid to the anal region, and (2) the length of 

 time taken in this transference. Some facts which bear upon 

 these two points have already been incidentally mentioned 

 (p. 448). In one series of experiments, made in one morning, 

 six Sea-Urchins were used. An empty Nassa shell held against 

 the periphery of a horizontal Sea-Urchin of 23 mm., just above 

 the level of the ground, was retained by the animal and carried 

 to the anus in four minutes. A pebble held to the periphery of 

 a horizontal specimen of 21 mm. was grasped, carried across the 

 upper surface of the body, and let fall on the other side in three 

 minutes. A specimen of 23 mm., clinging to the vertical face 

 of a rock, took a stone at the same part, passed it around the 

 edge of its body, and dropped it, in three and a half minutes. 

 An empty shell of Trochus cinerariits placed directly in front of, 

 and about a quarter of an inch away from, a Sea-Urchin of 

 27 mm. advancing along a bare horizontal rock, was picked up 

 when the animal touched it and passed over to the anus. Six 

 and a half minutes elapsed from the moment of placing the shell 

 to the moment it reached the anus. A Sea-Urchin of 23 mm., 

 climbing up the vertical face of a rock, retained a pebble which 

 was given to it, and carried the stone to the anus, in ten 

 minutes. An empty shell of Nassa placed loosely upon the 

 uppermost part of a Sea-Urchin of 27 mm., which was climbing 

 up the vertical face of a rock, was allowed to fall. As a rule a 

 Sea-Urchin, even if it does not want to retain an object, will pass 

 it across its body in order to drop it, but in this instance the 

 animal raised the uppermost part of its body, let the shell pass 

 down between its oral surface and the face of the rock, and then, 

 lifting the lowermost part of its body, allowed the shell to slip 

 to the bottom of the tank. This operation took nearly two 

 minutes. Another series of experiments was made a few days 



