as Infusoria flagellata. 255 
surface with scarcely a perceptible effort, and at another pro- 
essing with a laborious hitching gait, and lashing its guber- 
naclum (fl?) about, and swinging its body from side to side, 
with frequent jerks, in its efforts to pass over some obstacle. 
The body is colourless and enclosed in an uncontractile, 
smooth integument. It has an asymmetrically ovate shape, 
rounded behind, and rapidly narrowed anteriorly into an ob- 
lique, truncate, conical front. Dorsally it is convex (figs. 67, 
68); but ventrally, ¢. e. on the reptant side, it is concave on the 
right and in the middle, and so strongly incurved on the left 
that its sharp edge (¢) reaches nearly to the median line. Be- 
neath this inrolled border the enclosed space (fig. 68, ¢!) pro- 
jects into the left side like a longitudinal covered way. In 
front it is very deep, but from that point going backward it 
narrows gradually, and finally, with the inrolled edge, fades 
out at the posterior third of the body. 
The two flagella (fl, fl?) ave as widely diverse in character 
and function as any two similar organs in the whole group of 
Protozoa. The anterior one (ff) is, strictly speaking, the pre- 
hensile organ, as well as the main propulsory agent. It is 
quite delicate, and tapers gradually, from its subterminal base 
within the longitudinal covered way, to an extremely fine tip. 
In point of length it varies from one-half to two-thirds longer 
than the body. It is always carried in an extended position 
in front, and vibrates very actively, especially during repta- 
tion. 
The posterior flagellar organ, or gubernaclum (fl?), is from 
three to four times the length of the body, and arises far from 
the front, in the deepest part of the covered way (fig. 68, ¢!), 
and immediately beneath the contractile vesicle (cv). It is 
therefore attached quite near to the left margin of the body, 
and between the anterior and middle thirds. Its base, which 
is applied very obliquely to its point of attachment, is quite 
broad; but it narrows rapidly into a uniformly but scarcely 
tapering lash, which always projects forward more or less, and 
then curves backward and extends to a long distance behind. 
During reptation over smooth surfaces, it lies along the abdo- 
minal scsi line, and trails behind in long gentle undula- 
tions. Although it never vibrates, it frequently lashes about, 
and applies itself against obstacles on the right and left, or 
even in front, and acts as a prop upon which the body is 
thrown to one side or the other, according to varying cir- 
cumstances. ‘That it is contractile would seem incontestable 
on observing the sudden jerk with which it sometimes draws 
the body back toward its distal end; but I am pretty well 
convinced, from a careful study of this movement, that, al- 
though this organ may be to a slight degree resilient, it is not 
