92 S. O. MAST. 



DESCRIPTION. 



Didiniuiu uasittuiii is a ciliate protozoon considerably smaller 

 than Paramccium (Figs. 1-4). It can scarcely be seen with the 

 naked eye. It is approximately ellipsoidal in form and grayish 

 in color. One end, the anterior, is slightly flattened. From the 

 middle of this end there arises a conical protuberance at the 

 apex of which the month is situated. Two distinct bands of 

 cilia encircle the body, one near the middle and the other very 

 near the anterior end. Near the posterior end there is a contrac- 

 tile vacuole and near the central part of the body a relatively 

 large nucleus, which is frequently bent upon itself something like 

 a figure eight. The conical projection appears fibrous. It con- 

 tains numerous rod-like structures, many of which extend almost 

 to the center of the body. These rod-like structures can be 

 isolated by crushing the animal. They are insoluble in water and 

 appear to be quite rigid. Some of these structures are imbedded 

 in the protoplasm near the surface and appear to serve as support 

 for the conical protuberance, especially during the process of 

 swallowing. The rest are centrally located. Taken together, 

 they form a cylindrical structure referred to by Thon as " mit- 

 tlerer Strang." Didinium seizes and holds its prey by means of 

 this structure. I shall therefore call it the seizing organ. 



Didiniitin nasntitin is occasionally found in cultures containing 

 Paramecia; it usually appears after most of the Paramecia have 

 died out, but is seldom very abundant in such cultures. There 

 is, however, little difficulty in obtaining large numbers by judicious 

 feeding on Paramecia, which it captures and swallows entire. 



LOCOMOTION. 



Didinium is a very active creature. One seldom finds it at 

 rest. It swims about rapidly, constantly rotating clockwise on 

 its longitudinal axis and at the same time swerving toward a 

 given side. This causes it to proceed on a spiral course which 

 is usually not over 2 mm. wide. When the organism comes in 

 contact with anything, it ordinarily suddenly reverses the stroke 

 of the cilia, backs a short distance, turns toward one side, and 

 then proceeds on a new course at an angle with the old. That 



