134 J. A. DAWSON 



The behavior of the normal single animal is similar to that 

 described by Jennings ('06) for Oxytricha. On account of the 

 double character of the twin, its behavior is somewhat different 

 from that of the single animal. During creeping, the cii-ri of 

 only one of the components are in contact with the substratum 

 and the creeping movements are not acti\'e as those of the 

 single animal. In free-swimming, however, the twin has a 

 characteristic rapid rotation on its long axis which is in striking 

 contrast to the slow rotation of the single animal. 



Reproduction of the twin form takes place by transverse fis- 

 sion, just as in the single animal, with the notable difference 

 that, whereas the daughter single individuals are constricted 

 off sharply at the point of separation, the daughter twins usually 

 remain connected for some minutes by a thin strand of proto- 

 plasm which gradually pulls out into a finer and finer thread 

 and is finally broken. The unusual spectacle is thus presented of 

 the anterior twin swimming actively and rotating rapidly while 

 at the same time drawing after it the posterior cell in which the 

 rotation is not nearly so marked. The connecting strand, which is 

 invisible except under a magnification of about 600 diameters, 

 occasionally reaches a length fully three times that of the daughter 

 twins. The peculiar physical condition of the protoplasm, shown 

 by the tendency for individuals to remain connected for a longer 

 or shorter time by cytoplasmic strands, is highly significant, 

 since it indicates the continuance of the 'miscible' state of proto- 

 plasm which made possible the formation of twin animals. The 

 phenomenon just described has been noted throughout the life 

 of the twin race, which, in this series, lasted for over one hundred 

 generations. 



After twins had been under observation for a short time, it 

 was found that single animals sometimes w^ere given off during 

 reproduction by fission. This production of single animals oc- 

 curred most frequently in the following manner. A slightly 

 more marked separation of the anterior ends of the individuals 

 forming the twins (as show^n in fig. 17) gave, when fission oc- 

 curred (fig. 18), ii normal twin connected only in the posterior 

 region. Once separation in this way had begun, it invariably 

 resulted in the complete pulling apart of the twin animals. 



