238 Trochosjflisera aequatorialis. By Herr Semjper, 
boanga. In tlie montlis of October and November, 1859, it was 
not rare there. Of a perfectly spherical shape (Figs. 1, 2, and 11), 
without any perceptible front or back end, this animal, in 
diameter, moves with a continuous rolling and circular motion ; no 
fixed axis of direction, however, can be distinguished. It is never 
at rest, and never fastens itself on any stone or plant, for it possesses 
no organ whatever, which serving such a purpose might be regarded 
as a homologue to the foot of rotifers. In fact, motion is caused 
simply by its strangely altered trochal disk. Just as the animal 
itself forms a perfect sphere, so with mathematical symmetry the 
rotatory organ surrounds the sphere in the shape of a fringed 
equator, and thus divides the creature into two hemispheres exactly 
equal in size. The remaining organs certainly somewhat disturb 
this symmetry. The greater part of the inner organs lie in the 
one hemisphere, whilst the other contains hardly any. The perfect 
transparency of the skin renders it possible to perceive their relative 
positions. In the following description I shall give the name of 
"oral" to the hemisphere containing the mouth and anus, of 
" aboral " to the other. The surface of both hemispheres is per- 
fectly smooth, yet exhibits certain cavities leading to the mouth 
and anus (Figs. 1,2 a and g) as well as certain places of junction 
of other organs, which, on more minute investigation, are found 
to be organs of sense and muscles. As almost aU the internal 
organs are situated in the oral half, their orifices and places of 
junction of course belong to the same hemisphere. The equatorial 
line of fringe is not quite complete. If we call a meridian touching 
the mouth (Figs. 1, 2 a), which lies close by the equator, the first, 
then one diametrically opposite to it would mark the centre of a 
small interruption in the fringe, and also give the position of an 
organ (Figs. 1, 2 o) to which a nerve is joined (1, 2 p) that may be 
traced straight to the brain, and runs along the aboral hemisphere 
in a great circle. The anus, its circular edge slightly swollen 
(1, 2^), is situated almost exactly at the oral pole; and the two 
simple eyes (1, 2 m) are so placed on the circular equatorial fringe 
that a great circle passing through them and the anus cuts the 
first meridian connecting the mouth and the problematical sense- 
organ at right angles. Thus the mouth and the eyes exactly 
divide the equatorial fringe into four quadrants. 
The remaining organs, distinguished by their points of junction 
with the outer skin, are so situated that the mathematical symmetry 
becomes considerably disturbed by them. First of all on either 
side above the eye, and at the same time rather farther towards 
the mouth, we have two organs (1, 2n), which both by reason of 
their connection with nerves as well as in form and appearance 
correspond exactly to the already mentioned problematical sense- 
organ. Farther on towards the mouth is a wide, ribbon-like 
