REACTIONS OF OPALINA RANARUM 329 



will rest against a glass rod, a needle, or bit of filter paper, as 

 readily as against faecal debris. Particles which might contain 

 food are brought to the resting protozoan by the vigorous stroke 

 of the cilia. 



When the anterior end of Opalina is touched by a fine needle 

 or a glass rod, the animal usually responds with the avoiding 

 reaction. If the same stimulus is applied to the side of the 

 body, there is usually no reaction, although there is occasion- 

 ally a forward movement. Opalina will allow itself to be pushed 

 along with a needle without attempting to move away. 



REACTIONS TO CHEMICAL STIMULI 



In studying the reactions of Opalina to chemical stimuli the 

 fluid contents of the frog's rectum served as a medium in order 

 to have conditions as normal as possible. This was usually 

 alkaline, but sometimes slightly acid, and though such varia- 

 tions may have caused discrepancies they were probably neglible. 

 Dale ('01) has carefully worked out the chemotaxis of Opalina 

 in alkaline, acid, and neutral cultures. Opalina showed posi- 

 tive reactions to acids and negative to alkalies; but in an acid 

 solution was negative to stronger acid and positive to alkali. 



Although Opalina resembles Paramoecium in its responses to 

 chemicals, it is usually slow in reacting to stimuli. It often- 

 times swims quite a distance into a strong solution before react- 

 ing. The chemical, if injurious, proves fatal before the animal 

 can make its escape. Usually, however, an Opalina will swim 

 up to the border of a chemical solution and turn directly to the 

 right. Like Paramoecium, Opalina sometimes enters and swims 

 directly across a drop of a solution without response until it 

 comes in contact with the original liquid on the other side, 

 where it gives the avoiding reactions. As Dale ('01) pointed out, 

 Opalinas are occasionally seen in clusters which are probably 

 due to the presence of carbonic acid. The writer noted that 

 Opalinas were frequently grouped together when taken from the 

 rectum. As Mast ('12) noted i.n the case of Peranema, there 

 was no evidence of orientation with regard to the direction of 

 diffusion of the stimulus. Chemicals sometimes caused Opalina 

 to swim in circles without revolving on the long axis. 



When Opalina is dropped into distilled water, it swims hur- 

 riedly about for a time. Tap water placed next to a drop of 



