166 Transactions of the Boyal Microscopical Society. 
fication easy, but it will hardly be denied that after our knowledge 
of a natural group of animals has reached a certain point, their 
classification is made more difficult by a wider acquaintance with 
the species that the group comprises. For the more common 
forms, which are always the great majority, fall readily enough into 
divisions and subdivisions whose boundaries have that delightful 
sharpness which seems to make it so easy to study Nature in books, 
but which unluckily Nature herself never offers to the observer. 
Further research generally brings to light those rarer intermediate 
forms (of which Cejohalosijphon and possibly Ptygura are examples) 
which are at once the despair of the classifier and the delight of the 
naturalist; and, when the attempt is made to fit these into the 
original scheme, it is then found that Nature knows no sharp lines 
of demarcation and has no symmetrical system. 
The best classification of Nature's facts will always lack that 
precision and symmetry which is so dear to most men — who, if 
they had had the making of their own skeletons, would have 
substituted cylinders, spheres, and triangles for Nature's free curves 
and flowing surfaces. 
What had made it so difficult to include Cephalosijphon in any 
system of classification was this ; that while it appeared to be a 
tube-making rotifer of the Melicertan family, yet, unlike all other 
tube-makers, it had only one antenna instead of two, and that one 
on the wrong side of its body, namely on the side opposite to that 
of the mouth. 
Now the Philodines have only one antenna, and that antenna 
is on the side opposite to the mouth : moreover (as Mr. Cubitt had 
pointed out) some of the Philodines occasionally form tubes round 
themselves. I have several times met with Botifer macroceros so 
encased. Its long flexible antenna seemed to resemble that of 
Cej)halosiphon, as also did its habit of moving it about from side 
to side before emerging from its tube ; so that Mr. Cubitt 's sug- 
gestion, that Cephalosiphon was a temporarily encased Philodine 
and not a Melicertan at all, seemed worth entertaining ; though 
adopting it was to fall into another difficulty, as Mr. Slack's picture 
in ' Marvels of Pond Life ' showed that the creature had only one 
large wheel of cilia, and not two small ones. 
In fact Gephalosiphon had no right to exist if there was to be 
any comfort in classifying ; for its single antenna opposite to the 
mouth seemed to throw it out of the Melicertans, and its large 
single wheel to prevent its entering among the Philodines. 
Ptygura too was a difficulty ; for here was a creature whose 
form seemed to place it among the tube-makers, and yet which 
made no tube. Ehrenberg's description of it had made me think 
that it was some young Melicertan seen before it had begun to 
construct its tube. I had overlooked Mr. Gosse's statement (in the 
