88 University of California Publications in Zoology. [Vol. 4 



as the sea urchin, as Proiiho (1890) and v. Uexkiill (1899) have 

 observed with some of the Mediterranean starfish. I placed a 

 small sea urchin about two and one-half centimeters in diameter, 

 in the aquarium near a large starfish. The sea urchin was a 

 specimen of Strongylocoitrotus purpuratus, with long, stout 

 spines. It soon came in contact with one side of a ray of the 

 starfish, near its middle. The rosettes of pedicellariae at once 

 rose to the attack all over this ray; they seized the spines and 

 tube feet, and other parts of the sea urchin, wherever they could 

 get hold. This ray and the one on the opposite side of the sea 

 urchin now began to curve inward and toward each other, so as 

 to surround the sea urchin and draw it close, while the disk of 

 the starfish began to rise, preparatory to the reception of the 

 prey at the mouth. Soon both rays of the starfish were against 

 the sea urchin; all the pedicellariae were attacking it, and the 

 tube feet of the two rays came forth and lay hold on the sea 

 urchin. But the sea urchin is not without defensive weapons; 

 indeed it is, of course, in many ways more strongly armed than 

 the starfish. Besides its long, stout, sharp spines, it has large 

 three-jawed pedicellariae, some of them poisonous, and each one 

 suspended at the tip of a long rod. All of these weapons at once 

 became active in defense. The spines bent over toward the re- 

 gions seized by the starfish and pressed their sharp points against 

 the latter; the starfish pedicellariae tried in vain to get a hold on 

 the hard, round spines. The large pedicellariae of the sea urchin 

 seized the attacking tube feet of the starfish and compelled them 

 to withdraw. The pedicellariae of the two animals seized one 

 another, interlocking jaws ; some of those belonging to the sea 

 urchin were thus pulled off. A spirited combat continued for 

 about five minutes, the starfish being throughout the aggressor. 

 But at the end of five minutes the starfish had had enough, and 

 began to uncurl its rays and withdraw. It took about five min- 

 utes for the animals to become disengaged, though both were 

 working to that end. They separated ; the sea urchin was not 

 eaten. 



Five minutes later the sea urchin was again placed in con- 

 tact with the starfish in the same place as before. The starfish 

 at once began to withdraw, the pedicellariae rising defensively ; 



