NOTES AND MEMORANDA. 
189 
stigmata, but unfortunately tlio incessant movements of the animals 
rendered the observation dijBficult, and I have been able to provoke on 
each of them only the hernia of a single vesicle. 
The aquarium having subsequently frozen, the plants and or- 
ganisms were destroyed. Since then I have never been able to find 
tbe rotifer in question. Perhaps I have had to do with the larval 
state of a sj^ecies better known at an adult age. I have not the least 
doubt that my observation was exact — it conformed to what is known 
of the respiration of certain classes of worms.” 
A 3IetJiod of Preserving the Botatoria, Infusoria, dc., with their 
Organs Extended. — Eeferring to the preceding observations. Dr. 
Pelletan writes : — “ The result showed me the necessity of resuming 
them hereafter, but upon animals rendered immovable at the momeut 
of complete activity, and in all the positions that they are capable of 
taking. In fact, their extreme mobility and continual changes of 
form, due to their contractility, are a serious obstacle to their study, 
and it is only by a long course of fatiguing observations that the 
same animal can be seen in its difierent states of extension and under 
its different aspects so as to obtain a fairly complete idea of it. I 
therefore endeavoured to find a method which would enable them to 
be fixed in all attitudes, to preserve them in that state so as to study 
them in the same way as histological preparations and with high 
powers, which is ordinarily very difficult with living animals. Under 
the influence of all the reagents, even with narcotic or anaesthetic 
agents, the rotifers immediately contract and become only a small 
globule, in which all the organs, crowded one on the other, show 
nothing distinctly. It was necessary therefore to find a fixing agent 
which would enable an absolutely instantaneous effect to be produced. 
This reagent is osmic acid. It has always furnished me with 
excellent results, and I am not aware that it has been previously 
applied to the preparation of the Eotatoria and Infusoria. 
Everybody knows the property which osmic acid has of fixing the 
histological elements instantaneously in their actual form, but it is not 
sufficiently known that to act with this instantaneousness it is not 
enough that it should be concentrated, but it must be employed in a 
way that its action should not be too much dissipated. Thus if a 
drop of a solution of I per cent, is placed on a tissue, the exact point 
where the drop has been placed is almost immediately fixed, but the 
neighbouring parts, over which the acid is diffused and acts only, so to 
say, at second hand, are subjected to a very much feebler action. If a 
more concentrated solution is employed there is not much difference in 
the effect — the action of the acid is exhausted upon the point imme- 
diately affected, and does not extend to any distance. It is thus that 
M. Eanvier has shown that the arms of the hydra may be fixed instan- 
taneously whilst extended, notwithstanding the exceeding rapidity 
with which they retract them, but the drop of acid must be placed 
directly upon the little polyp, which can be best done by the ordinary 
dipping tube. 
It is in an analogous manner that I operate on the rotifers and 
tlie contractile infusoria. I put about half a cubic centimetre of the 
