210 
Is Pedalion a Rotifer ? 
two sentences are to be considered as constituting the definition of 
the term “ Rotifera,” then the definition will collapse as completely 
as the former one under the weight of various aberrant (but clearly 
rotiferous) forms. Let us see : rotifers are “ Vermes with a retrac- 
tile ciliated disk at the anterior extremity of their bodies which are 
ordinarily microscopic in size, though they may attain as large a 
size as ^th of an inch in length. They are usually more or less 
plainly annulated externally, but they never are divided internally 
into compartments by any transverse septa.” 
Now it is not long ago that I read an account of a discovery in 
Germany of a rotifer -without any ciliated disk at all ; this, if true, 
disposes of nearly the whole of the first sentence. As to the 
second sentence, the first part excludes the whole of the Brachionaea ; 
and the second excludes the Floscules. Here again, then, the defini- 
tion would shrink down into “ Rotifers are small worms ” ; a 
statement which I trust that Pedalion will effectually dispose of. 
Happily Pedalion, in all but its six limbs, too closely resembles 
such forms as Brachionus pal a and Triarthra longiseta, to compel us 
to discuss such a knotty question as that of how much of a rotifer’s 
form and structure an animal may lose and yet remain a rotifer. In 
Plate XXXIII., Fig. 1, 1 have taken the liberty of removing two of 
Pedalion’ s limbs as well as all its muscles, so as to show its internal 
structure. Its large trochal disk, though of a pattern peculiar to 
itself, is still constructed on the same general plan, and with pre- 
cisely the same object as those of Rotifer vulgaris and Melicerta. 
An imperforate oval disk bears on its edge large cilia, which by 
merely moving up and down, draw the water into currents setting 
right against the face of the disk. These currents on striking the 
disk are forced along the face of it outwards in all directions between 
the large cilia. Below the edge of the trochal disk lies a circular 
groove which surrounds the whole animal at its neck, and commences 
and terminates at its mouth. The outer edge of the groove is 
densely ciliated, and the cilia are very fine and set very closely 
together. When, then, the currents due to the larger cilia drive 
their floating atoms over the edge of the trochal disk, the smaller 
cilia lash the atoms back into the groove and hurry them along on 
both sides of the animal towards the mouth. This process can be 
readily watched in Melicerta or Limnias, and not quite so easily 
in (Ecistes , and has been often described : in Pedalion , too, it can 
be distinctly seen by those who have either the patience to watch 
for a favourable opportunity, or the good luck to catch the animal 
resting at the proper moment. 
Pedalion s trochal disk has a curious thin portion at each 
side (Fig. 2, a), bounded by branches of two longitudinal muscles 
(Fig. 2, b), and for the purpose I believe of permitting the disk to 
