150 THE CRITERION OF DIVERSITY 



jerk, and disappears in the oblique furrow ; probably acting 

 there in concert with the other cilia in the introduction of food 

 into the mouth. The fourth and last kind of cilia which I have 

 to speak of are two excessively faint, very long, and quite large, 

 bristle-like filaments ($/, sl l ) which project from each end of the 

 body. The straight one (sl l ) always precedes when the creature is 

 in motion, and the curved one (si) is attached a little to the left 

 of the posterior end of the body. Both are always rigid when 

 the animal is not in motion. but yet there can be no doubt that 

 they are flexible, for at times they disappear suddenly, and prob- 

 ably are bent under the body. What their office is I cannot say, 

 but conjecture, from their resemblance to what are called the sal- 

 tatory bristles of other infusorians, that they are used as accessory 

 means of sudden propulsion, or leaping, a habit which seems 

 to be the most frequent mode of leaving any point at which the 

 creature has fairly come to a stand-still. The contractile vesicle 

 (cv) lies close to the forward end of the body, and corresponds 

 in activity to the vivacity of the motions of the latter. It con- 

 tracts every ten seconds, and with more vigor than any other 

 that I know of. It is very conspicuous, as it is two thirds of the 

 time in an expanded state ; and disappears and reappears like 

 the sudden closing and opening of a large eye. I have already 

 indicated the position of the mouth as being near the broader 

 anterior end of the oblique furrow, but again speak of it here in 

 order to make the description of the digestive system complete. 

 From the mouth (m) the food passes directly into the general 

 cavity without going through any throat, and most frequently 

 combines in large masses (st). The presence of a reproductive 

 organ, which we find here (n) in the form of a clear, colorless, 

 globular body, when added to all the other systems which I have 

 mentioned, puts this animal in the condition of a fully organized, 

 ciliated infusorian; and would seem to give us full warrant for 

 believing it to be the culmination of a progressive development, 

 whose tendency is to pass through such forms of animate organ- 

 ization as we have just been tracing in the successively more 

 and more complicated creatures whose images are before us. 



