as Infusoria flagellata. 261 



which I have to speak consists of two excessively faint, very long, 

 and quite large, bristle-like filaments {si, sZ*), which project from 

 each end of the body. The straight one (si) always precedes 

 when the creature is in motion ,• and the curved one (sP) 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 disajjpear suddenly, and probably are bent under the 

 body. What their office is I cannot say, but conjecture, from 

 their resemblance to what are called the saltatory 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 Avliich the creature 

 has fairly come to a standstill. 



" 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 contracts every ten seconds, and 

 with more vigour than any other that I know of. It is very 

 conspicuous, as it is two-thirds of the time in an expanded 

 state ; and it disappears and reappears like the sudden closing 

 and opening of a large eye. 



" I have already indicated the position of the mouth (m) 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 (d). 



"The presence of a reproductive organ («), which we find 

 here in the form of a clear, colourless, globular body, when 

 added to all the other systems which I have mentioned, puts 

 this animal in the condition of a fully organized ciliated infu- 

 sorian, and would seem to give us full warrant for believing it to 

 be the culmination of a progressive development whose ten- 

 dency is to pass through such forms of animate organization 

 as we have just been tracing in the successively more and 

 more complicated creatures whose images are before us." 



EXPLANATION OF PLATES V., VI., & VII. 



The con-esponding parts in the figures are lettered alike, excepting 

 when otherwise stated in the description of any particular illustration. 



a, anus : b, membranous collar ; b^, edge of b ; W, base of b : bk, the 

 beaks of the valve of Dysteria : c, calyx ; c^, aperture of c ; c^, lower half 

 of c: cl,cl'^, vibratile cilia: cv, contractile vesicle: cl, digestive vacuole, 

 or ingested food : e, furrow in fissigemmation ; e\ anterior end of e ; e^, 

 prolongation of e, e^ : /, broad sulcus (in Heteromastix) : _/?, flagellum ; 



