ON ACTINIANS 



59 



FORCED BY GRAVITATION TO PUSH 

 ITSELF THROUGH THE NET THREE 

 TIMES (a, b, AND c). See text. 



incision be made in the middle of a Cerianthus which 

 almost but not quite separates the two halves, and the 

 animal be placed immediately after the operation on 

 a wire net, the foot works 

 itself into one of the 

 meshes up as far as the 

 incision and assumes a 

 vertical position. The 

 oral piece, on the con- 

 trary, from the place of 

 incision to the head, usu- 

 ally remains lying hori- FIG. 15. ACTINIAN THAT HAS BEEN 



zontally on the net. This 

 shows that the foot pos- 

 sesses geotropic irritabil- 

 ity. But if the Actinian be divided transversely we 

 see that the head-piece as well as the tail-piece pushes 

 itself through the meshes, although not so frequently. 

 While an Actinian that is suspended vertically in a 

 test-tube or in a mesh of a wire net seldom retains this 

 position longer than two days, it remains indefinitely 

 in the sand after burrowing. In addition to gravita- 

 tion, some other stimulus must hold it there. I be- 

 lieve that it is the contact-stimulus of the sand. I 

 called this kind of irritability stereotropism, and have 

 shown that in a series of animals it determines their 

 habits. Positive geotropism and positive stereotrop- 

 ism cause the Cerianthi to burrow in the sand vert- 

 ically, and the positive stereotropism keeps them 

 permanently in the burrow. 



