Dr. H. Fol on the Family Tintinnodea. 75 
under a high power, itis of great importance to have a process 
which enables them to be fixed instantaneously in their natural 
attitude before they have had time to withdraw into their 
test, and which preserves faithfully the details of their 
structure. i 
I have tried the various reagents most in vogue without 
attaining my purpose. With weak osmic acid I did not suc- 
ceed in preserving the cilia of the peristome; and with a 
stronger dose the body became absolutely opaque : in both cases 
there was always a strong contraction. Acetic acid, chromic 
acid, and picrosulphuric acid only gave me a fixation which 
was too slow, so that the animal died contracted in the bottom 
of its test. Finally I succeeded with a reagent which is not 
employed in histology, perchloride of iron; by its means I 
have obtained a considerable number of specimens of various 
species fixed in a state of full expansion. These subjects, 
washed with alcohol, and treated with gallic acid, present a 
brown coloration which is especially localized upon the 
nuclei and renders them very visible; the other parts of the 
animal acquire a light brown tint, which renders them easy 
to see. 
The specimens thus treated may be mounted in Canada 
balsam, which produces permanent preparations; but they 
are much more distinct and more instructive if simply placed 
in glycerine. 
By treating in the manner just indicated the whole pro- 
duce of a capture we can afterwards, on returning home, seek 
at leisure for the Infusoria, a more or less considerable number 
of which will be fixed in a state of full extension of the body 
and peristome, with the cilia and the vibratile palettes pre- 
served in perfection. 
Anatomy.—The structure of the test is more complete in 
certain forms, and presents more difference between the forms 
than I previously supposed. ‘Tests slightly tinged with gallic 
acid and mounted in balsam or in glycerine are especially 
instructive. By compressing the test a little we obtain at the 
margins perfectly distinct optical sections. 
Examined under these conditions with a good homogeneous 
immersion-lens, the tests of the Z%nténni prove to be composed 
of two very distinct layers, as I previously indicated (XII. 
p- 12 [242]); but what I did not see in fresh preparations is 
that the layers are symmetrically placed, one at the inner, the 
other at the outer surface, and separated by a vacant space 
(Pl. IV. fig. 7). The substance of the test, browned by 
gallic acid, shows very clearly these two parallel layers, appa- 
rently of the same thickness throughout their whole extent. 
a 
