170 Transactions of the Boyal Microscopical Sociekj. 
Colm had demonstrated, what Ehrenherg had suspected, and we 
had readily confirmed, that its movements were produced and con- 
trolled by a pair of fine flagella, one at each end of its spiral body. 
This was extremely suggestive : while we had further the analogy 
of the minuter monads, many of them being only four or five times 
larger than the larger forms of B. termo — these also being endowed 
with one or more flagella, suggesting the probability that all these 
forms depended for movement on similar motile filaments. 
We therefore determined to endeavour to discover whether, by 
care and delicate manipulation, flagella could he demonstrated on 
B. termo itself. 
Fortunately at this time we were furnished with a ^‘new 
formula ” lens, which possessed precisely the qualities needed. 
Carrying out our method of admitting nothing, in our conjoint work, 
which both had not seen, I commenced the search ; and after many 
hours’ effort, and the use of a great variety of delicate appliances, I 
succeeded in clearly demonstrating a pair of flagella, one at each 
end of the body of B. termo, without the shadow of a question. 
Dr. Drysdale now, independently, commenced the search, simply 
employing the same methods, and ultimately demonstrated it as 
completely as could he desired. We were then able to study it 
together, seeing the delicate fibre as distinctly as tlm body of the 
B. termo itself, and I made careful drawings from this, which Dr. 
Drysdale accepted and confirmed.* 
Most of the forms at present grouped as Bacteria vary, very 
considerably, in size. Some, for example, that may be designated 
B. termo will be as large again as others ; and this is the case 
throughout, only that the variation will be still greater in those 
forms which, instead of dividing into two parts, divide into several. 
There is for instance very great variety in the length of S. volutans. 
But the average length of B. termo, resulting from a hundred 
measurements made from this form as taken from six different 
infusions, was the ro^^o oth of an inch. The average of one hun- 
dred measurements of S. volutans (taken only from one infusion) 
was the yoVoth of an inch, but these were large. In Fig. 2, 
Plate VIII., I give a drawing of a recent specimen of S. volutans 
magnified 2000 diameters ; and at Fig. 1 sl B. termo is placed 
beside it, as it was seen by Dr. Drysdale and myself, magnified 
4000 diameters. 
Since this account of the discovery of flagella on B. termo was 
published, several valuable monographs have appeared on the 
subject of Bacteria. From the point of view I am now specially 
considering one of the most interesting and valuable is by Dr. Eug. 
Warming.! In it he points out that the flagella are common to the 
* ‘ Monthly Microscopical Journal,’ vol. xiv. p. 105. 
t ‘ Om nogle vecl Daninarks Kyster levende Bakterier.’ Kjobenhavn, 1876. 
