CILIARY CURRENT IN PARAMECIA 



291 



a region containing ink no cone is observed until after the ante- 

 rior end has actually reached the original boundary of the ink and 

 has come in contact with the particles in it. When this occurs 

 the animal usually responds with a weak avoiding reaction, and 

 in turning a cone is drawn out just as in the case of Paramecium. 

 The cone is produced by a modification in the usual motion of 

 the cilia on the disk, such that those on the aboral side strike 

 forward, while those on the opposite side strike backward. This 

 causes the animal to turn toward the aboral side and at the same 



Fig. 6 Diagram illustrating the avoiding reaction in Spirostoinum. 1, 2, S, 

 4, successive positions assumed during the reaction. 



time produces a cone-shaped current which proceeds from some 

 distance in front of the disk toward its outer oral edge and back 

 along the side of the body. 



In free-swimming Spirostomum there is no observable cone if 

 the body is straight, but if it is curved so as to offer consider- 

 able resistance there is a decided cone. This, however, was ob- 

 served only under a cover-glass, where it was impossible to 

 eliminate the probable effect of contact. When the animal 

 comes in contact with a cloud of ink it responds usually with a 

 momentary cessation of ciliary activity and a marked contrac- 

 tion of the body which immediately expands again, taking a 

 somewhat curved form, after which a large feeding cone is pro- 



THE JOURNAL OF EXPEKIMENTAL ZOOLOGY, VOL. 21, NO. 



