382 University of California Publications in Zoology [^'ol. 20 



the length is as a rule somewhat greater, contraction undoubtedlj' 

 occurring as individuals slow down, and in fixation. The length of 

 the exposed tip of the ax;ostyle was 2 to 4 microns. The diameter of 

 the nucleus ranged from 2 to 4 microns. The transdiameter in the 

 plane of the iindulating membrane was greater by about one-tenth 

 than that at right angles. The optical cross-section was broadly pyri- 

 form with the narrower end at the undulating membrane. 



REPRODUCTION 



Binary fission (pi. 37, figs. 8-10) preceded by mitosis occurs 

 abundantly in stools and in cultures. We have repeatedly seen 

 multiple fission or somatella formation in cultures. A two-, four-, 

 and eight-nucleate somatella of spheroidal shape only slightly larger 

 than the unicellular phase with a neuromotor system attached to each 

 nucleus is formed. This presumably undergoes plasmotomy and forms 

 eight unicellular flagellates. We have never seen the least trace of 

 encystment in either fresh stools or cultures. Efforts to induce 

 encystment by slow desiccation of stools and cultures were also 

 fruitless. 



BEHAVIOR 



Observations on living Pentatrichomonas in liquid stools and 

 especially in the marginal zone between normal saline and iodine- 

 eosin stain, where movement is progressively slowed down, has enabled 

 us to analyze the movements of certain elements of the neuromotor 

 system and to trace certain stereotyped features of the behavior of 

 the animal, and especially to determine the differential movements 

 of the single independent, and the clustered anterior, flagella. 



Locomotion in Pentatrichomonas is continuous in normal indi- 

 viduals. It is accompanied by an anticlockwise rotation, which at 

 times appears to be interrupted by a very brief interval of a clock- 

 wise direction. Progression may be in a straight line for short dis- 

 tances, but is generally curved and frequently interrupted by brief 

 intervals of circling movement in short arcs of shorter radius in an 

 anticlockwise direction. 



A quick spasmodic contraction precedes these interruptions in 

 speed and direction. This appears to be analogous to the motor or 



