as Infusoria flagellata. 261 
which I haveto speak consists of two excessively faint, very long, 
and quite large, bristle-like filaments (s/, s/) ,which project from 
each end of the body. ‘The straight one (s/) always precedes 
when the creature is in motion; and the curved one (s/!) 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 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 which 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. : 
“1 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 (nm), 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., & VIL. 
The corresponding parts in the figures are lettered alike, excepting 
when otherwise stated in the description of any particular illustration. 
a, anus: b, membranous collar; 6!, edge of 0; 8°, base of b: bk, the 
beaks of the valve of Dysteria: c, calyx; c', aperture of c; c*, lower half 
of ¢: cel, cl', vibratile cilia: cv, contractile vesicle: d, digestive vacuole, 
or ingested food: e, furrow in fissigemmation ; e', anterior end of e; 2, 
prolongation of e, e': f, broad sulcus (in Heteromastix) : fl, flagellum ; 
