328 ELSA SHADALL 



jerks. The cilia on a pair of conjugating Opalinas beat in har- 

 mony until the animals are ready to separate and then each set 

 of cilia beats so as to part the pair. 



AVOIDING REACTIONS 



Opalina reacts to stimuli by using avoiding reactions similar 

 to those of Paramoecium, Chilomonas (Jennings, '06) and 

 Didinium (Mast, '09). It backs for a short distance without 

 revolving on its long axis, and after turning to the right, swims 

 forward at an angle to the original course. Sometimes the 

 angle may be as much as 90.° An Opalina may make " tests ' 

 in several directions, moving forward or to the side, trying the 

 conditions until they prove to be satisfactory. When reacting 

 to some stimuli, Opalina swims in a circle without revolving and 

 keeps the left side away from the center of the circle. Certain 

 stimulating agents cause it to turn " somersaults " by bending 

 up the anterior end and going over and over, but this type of 

 reaction was not common. 



In several instances it was possible to be very certain of the 

 exact position of the body during rapid movements on account 

 of a little blister or some other peculiarity. Individuals which 

 came in contact with objects did not always back away but 

 sometimes turned directly to the right. The locomotion of a 

 conjugating pair is similar to that of single individuals. 



REACTIONS TO MECHANICAL STIMULI 



While swimming in a normal medium, Opalina frequently 

 comes in contact with various objects and responds by the 

 avoiding reaction already described. It may not retreat at all, 

 however, but become fixed against a bit of faecal matter and 

 remain in contact with it for some time; the behavior resembling 

 Paramoecium against a bacterial zoogloea. If bits of filter paper 

 are put into the medium, Opalina responds when it touches 

 them with the avoiding reaction, or merely rests against them, 

 beating its cilia as it does when standing against faecal debris. 

 Occasionally, it moves along the edge, keeping the body close 

 against the paper. 



The ability to select food is not as evident in Opalina as in 

 Didinium or Lacrymaria (Mast, '09, '11). Opalina does not 

 readily discriminate between organic or inorganic matter for it 



