REACTIONS OF OPALINA RANARUM 327 



individuals are extremely active and swim rapidly as if in a 

 state of excitement. Unless stimulated, the mature Opalina 

 moves very smoothly and bends its body gracefully as it wanders 

 in and out among the debris. Finely ground India ink and 

 gelatin were used successfully in observing the movements. 

 Opalina was frequently found at rest either at the edge of a 

 drop of culture medium or against a bit of the faeces. It keeps 

 its cilia in active motion, however, at all times and apparently 

 does not attach itself in the same way that Chilomonas, Didi- 

 nium, and Paramoecium do. 



The action system is essentially like that of many free swim- 

 ming ciliates and flagellates. Opalina usually swims through 

 the water in a spiral course, but quite often one is seen swimming 

 without revolving on its long axis. Like many other ciliates, 

 such as some Hypotricha and Colpidium, it often swims forward 

 keeping one side against an object or in contact with the edge 

 of a drop of liquid. In making its screw-like revolutions, Opalina 

 turns over to the right. It was noted that Opalina does not 

 make as many revolutions for a given distance as Paramoecium. 

 Sometimes, it makes only half -re volutions at varying intervals, 

 and then turns over toward the left for a time. There is another 

 characteristic movement which does not seem to have been 

 noted in other ciliates. Frequently, after making half a revolu- 

 tion, an Opalina will turn back the same distance, and repeat 

 this movement several times in rapid succession. 



The spiral course is much like that of Paramoecium as de- 

 scribed by Jennings ('06). There are two factors which seem 

 to influence this particular type of movement in Opalina — the 

 forward movement of the animal, and the rotation on the long 

 axis to the right. Cilia extending from the left to the concave 

 edge on the right, beat directly backward and bring about the 

 forward movement. The rotation on the long axis is due to the 

 fact that the cilia on the surface of the body beat obliquely to 

 the right and backwards. The revolving to the right is prob- 

 ably partly due to the asymmetrical form and partly to the 

 cilia at the anterior end which beat obliquely forward. If all 

 the cilia beat directly backward the animal moves forward with- 

 out rotating. The cilia on the surface of the body beat in a 

 rhythmic wave-like manner. Objects, caught in the cilia at 

 the anterior end, were carried down the surface of the body in 



