THE 
MONTHLY MICROSCOPICAL JOURNAL. 
NOVEMBER 1, 1872. 
I. — Is Pedalion a Botifer ? By C. T. Hudson, LL.D. 
Plate XXXIII. 
Is Pedalion a Rotifer ? To this I may reply, “ "What is a Rotifer? ” 
a question as difficult to answer as Sir Robert Peel’s famous query, 
“What is a pound?” Few things are so difficult to construct as 
a good definition, and there are few better ways of tripping up a 
troublesome antagonist than that of suddenly calling upon him to 
define some term which he is using. He must be a very capable 
man who can at a moment’s notice so reply, that his definition may 
not be torn to bits almost before it has left his lips. 
It is easy enough to define the typical form of a natural group 
of animals, or even to include in the definition forms that must be 
placed pretty near the central one ; but, in the ambitious search 
for a definition which shall include many families, as we get further 
away from the typical form, we find that one by one all the positive 
statements are disappearing from our definition, till at last we have 
nothing left but the mere shuck of a proposition, shelled of every- 
thing worth the stating. Take the definition of one of the 
zoological manuals : — “ Rotifers are microscopic animals, contractile, 
with vibratile cilia at the anterior part of the body, which by their 
motion often resemble a wheel moving rapidly. Intestine distinct ; 
terminated at one extremity by a mouth, at another by an anus ; 
generation oviparous, sometimes viviparous.” 
Now there are rotifers that have no “ vibratile cilia at the 
anterior part of their bodies,” others that have no intestine or anus, 
so that including these the above definition must shrink down into 
“ Rotifers are very small animals that lay eggs.” 
That it is not an easy matter to answer off-hand the question 
“ What is a rotifer ? ” may be readily seen by turning to one of our 
latest text-books, where no less than two handsome octavo pages 
are taken up in describing the characteristics of the class. I take 
for granted that the author considers the whole of this necessary 
in replying to the question, “What is a rotifer?” for if the first 
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