THE WORLD OF LIVING ORGANISMS 167 



which set the limit for each individual, decide whether attacker 

 or attacked will emerge victorious from the fight. 



We can watch a fight of this kind, when the star-fish 

 Asterias attacks the sea-urchin Strongylocentrotus. The star- 

 fish is provided with powerful tube-feet, which are covered 

 with a poisonous mucus. They are suited for gripping the 

 firm shell of the sea-urchin and paralysing the muscles of the 

 spines, while the five arms wind themselves round the spherical 

 body of the opponent. Then the membranous stomach bulges 

 forth from the mouth, presses itself against the surface of the , 

 prey, and begins its digestive activity outside the assailant's 

 own body. 



The behaviour of the sea-urchin is very remarkable. 

 For general purposes, a close-set rampart of spines is sufficient 

 to ward off hostile approach. But the short spines cannot 

 keep off the long tube-feet of this foe. For this quite a 

 different sort of defensive apparatus comes to the rescue. 

 As soon as the tube-feet of the star-fish approach the sea- 

 urchin, the spines lie down, and three-bladed pincers, swollen 

 with poison, rise up, which hitherto have been hanging down 

 flaccid. The pincers grip into the tube-feet of Asterias, and, 

 in so doing, break away from their base, so that, permanently 

 lodged in the enemy, they can transfer to it their full load 

 of poison. 



If the sea-urchin is in good form and has not lost too 

 many of its poison-pincers, it succeeds in driving off the star- 

 fish. But if not, the star-fish is the victor. 



The adjustment is as undeniable in the case of the sea- 

 urchin as in that of the star-fish, and the limits set to both 

 organisms are such that no one can say beforehand which 

 will conquer. The limit is determined by the condition of 

 the animal's health at the moment. The sea-urchin that is 

 in bad condition will be conquered ; while the strong and 

 healthy one, in full possession of its weapons, can count on 



