ZOOL.— Vol. III.] STEVENS— CILIATE INFUSORIA. 21 



sections it often stands with its base crowded in between 

 folds of the inner epithelium of the membrane. 



The organism is much elongated and subcylindrical in 

 form, tapering slightly from oral to aboral end. Living 

 specimens measure, without cilia, 54 /w- to 81 /* in length, 

 18 At to 21 /u. in diameter at the oral end, and 9 /u. to 18 /x at 

 the aboral end ( fig. 32, also fig. 36, a-e). 



The infusorian is very resistant to pressure and to mac- 

 erating agents; osmic fixation preserves the form and size 

 perfectly. Ordinarily its form is definite and comparatively 

 rigid, but when in close quarters it exhibits surprising flex- 

 ibility of body, doubling itself up and squeezing through 

 the narrowest opening with the ease of amoeboid forms. 



The cilia are of two distinct types — long coarse peris- 

 tomal cilia, and short fine body cilia. The oral cilia average 

 one-half the length of the body, the body ciha one-eighth 

 to one-sixth, those on the aboral end being somewhat 

 shorter. The oral cilia are disposed in a double dexiotropic 

 spiral of about one and one-half turns, with closed ends, 

 the inner end widening out to enclose the mouth-opening 

 (fig. 49). The inner peristomal cilia are somewhat shorter 

 than the outer and more constantly active in feeding, those 

 on opposite sides of the mouth bending toward one an- 

 other, the outer very constantly and rapidly to carry food 

 into the mouth, the inner slowly, and standing curved over 

 the mouth, evidently to prevent food particles from escap- 

 ing. The outer row alternately opens widely, rotates 

 rapidly toward the mouth, and closes up until the tips of 

 the ciha are interwoven, the latter phase being much longer 

 than the former. 



The general body cilia are arranged in from 20 to 27 

 slightly oblique longitudinal rows extending from the outer 

 turn of the oral spire nearly to the center of the opposite 

 end, where they leave a small circular spot free from ciha 

 (fig. 47). These cilia are in constant rapid motion, always 

 beating strongly toward the peristome, producing a rapid 

 current in that direction, and at the same time driving the 

 body rapidly backward in swimming, or holding it firmly 



