PROCEEDINGS OF THE SOCIETY. 
391 
the flea there was a somewhat kidney-shaped plate (drawn on the black- 
board) containing certain wheel-shaped markings, with a hair starting 
from each. The question was whether any part bearing similar hairs 
should also be called a pygidium, or whether it should only be given 
to an homologous part. 
Mr. Davis did not think that he was bound to prove the identity 
of the thing as being a pygidium ; it was for others to do that. What 
he had endeavoured to do was to show that in all these insects there 
were similar organs, and that they served similar purposes. 
Mr. Beck did not quite understand the question in dispute; it 
seemed to him to be whether the term was rightly applied to the 
object or not, or whether it only applied to that portion of the shell ” 
which in the flea had a certain structure upon it. They all knew that 
certain portions of an insect had certain names given to them, and 
Mr. Stewart stated that there was a certain portion of the flea called 
the pygidium which haj^pened to have upon it the very beautiful 
object which was ordinarily called by the same name by microscopists. 
The question was, were they right in calling this beautiful portion the 
pygidium ? If this was so, and it was a special organ, there might 
be a similar organ found in other insects. Mr. Davis had been 
endeavouring to trace a similar organ in others, and having found it 
there, he called it also a pygidium, whereas Mr. Stewart said the term 
applied to a position only. He thought the two things might very 
well be brought into harmony without much difficulty. 
The President said that the term pygidium would really apply to 
the position of the part rather than to its sj^ecial form or function. 
From its derivation the word would merely indicate something near 
the tail of the creature. 
Mr. Stewart said he had no doubt as to what was the pygidium of 
the flea ; what he had wanted to know was what Mr. Davis thought of 
the homology of the body which he had described. 
The President said that there might be two things which wore 
morphologically difierent, but which performed the same functions. 
Mr. Crisp said that it would be remembered that they had 
recently had a paper by Mr. F. A. Bedwell, on the mastax of 3Ieli- 
certa ringens ; since that paper appeared he had lent Mr. Bedwell 
the Zeiss oil-immersion (i), and he read extracts from a letter from 
Mr. Bedwell after having used the glass ; — “ It is the most magical 
addition to the instrument, and I feel certain that it will revolu- 
tionize a vast mass of information ; many papers will have to be 
rewritten, amongst others my last on the mastax of Melicerta ringens. 
Wherever organic texture is concerned, this glass makes the in- 
visible visible. As an example, I send you two slides of mastax of 
M. ringens, and one of Hotifer vulgaris, by Lord Sydney G. Osborne. If 
you look at them with an ordinary i, and then with the immersion, you 
will see that a part of the ramus in Melicerta which I have drawn in 
my picture as a plane surface, is exquisitely furrowed, and comes out 
like a revelation ; while the teeth and notches in Botifer vulgaris appear 
