REACTIONS OF DIDINIUM NASUTUM. 95 



different parts of normal specimens are duplicated, the bands 

 of cilia, contractile vacuoles, nuclei, seizing organs, etc. When 

 two individuals are fused together, the anterior ends usually point 

 in opposite directions, as represented in Fig. 4, but a few speci- 

 mens were found in which they pointed in the same direction. 

 Only one specimen composed of three individuals was found. In 

 this the individuals were symmetrically arranged, forming a 

 structure something like a three-pointed star. All these abnormal 

 creatures appear to swim equally well with any one of the an- 

 terior ends ahead. They feed just as normal animals do, but 

 I have never found any that reproduced, although at various 

 times isolated specimens were under observation for several days. 

 One specimen, however, was found with two additional bands of 

 cilia indicating that the process of fission had begun (see Fig. 4). 

 I am unable to account for the origin of these monsters. One 

 might naturally assume it to be a case of imperfect fission, but 

 this is improbable, for in fission an anterior end and a posterior 

 end form at the plane of separation, while in these creatures 

 posterior ends are always in contact. 



VARIATION IN SIZE, FISSION AND ENCYSTMENT. 



The results of the study of the feeding habits of Didinium 

 depend largely upon the relations in size between the Didiiiia 

 under observation that the Paramecia upon which they are feeding. 

 Both of these forms vary much more in size than is ordinarily 

 supposed. The smallest Didinium obtained (see Fig. i) measured 

 0.058 mm. in length and 0.034 mm. in width. The largest normal 

 specimens (Fig. 2) with but two bands of cilia, that is, specimens 

 which were not about to divide, measured 0.130 mm. in length 

 and 0.085 mm - m width. They were therefore more than twice 

 as long and nearly three times as wide as the smaller specimens, 

 which means that the larger are about six times as great as the 

 smaller. Some of the specimens (Fig. 3) about to divide 

 measured 0.166 mm. by o.ioi mm. and one of the largest abnormal 

 specimens (Fig. 4) was 0.207 mm. in length and 0.126 mm. in width, 

 nearly four times as long and four times as wide as the smallest 

 specimens, and about fourteen times as large. Paramecia vary 

 nearly if not quite as much in size as Didinium. One of the 



