228 Dr. G.C. Wallich on the Extent and Causes of 
repeatedly observed by me, in which it was only by dint of ex- 
treme care in manipulation of the light, the focus, and the spe- 
cimens’ themselves, aided by the employment of high magnify- 
ing power, that it became possible to perceive that, instead of 
the test being a simple cup-shaped disc, it was shaped like the 
mature Arcella—the plane surface being present, and at its 
centre the minute aperture through which the pseudopodial 
sarcode was protruded. 
In somewhat older specimens, the figure of the Arcelline test 
admits of no doubt ; but in these the convexity is generally very 
trifling (probably, as before stated, in consequence of the still 
extremely delicate texture), and the inversion of the margi of 
the aperture is but slight. At this stage, barring the presence of 
mineral particles in the test of the ordmary Difflugia, and the ab- 
sence of obliquity in that of Arcella, there is nothing to distin- 
guish one test from the other. The inversion of the lip is but 
a repetition of what has already been shown to take place in 
Difflugie; so is the depression of the test; whilst, lastly, the 
variety of Arcella to which the name of A. angulata has been 
given is nothing more than the common form of the mature 
Arcelline test pulled inwards at various points of its convex 
aspect by the action of the stolons, which are constantly seen ex- 
tending from the posterior portion of the sarcode-body, and 
enable the creature to carry its test on its back, just as a snail 
carries its shell (Pl. XVI. fig. 36). 
Under these circumstances, however convenient it might be 
to retain the two generic appellations, simply because we have 
become familiar with them, if we regard classification in its 
only legitimate light, namely, as a guide in the interpretation 
of the physiological differences prevailing through the organic 
world, we must either consent to forego convenience by break- 
ing down the fictitious generic boundary-line which has hitherto 
been assumed to exist between Arcella and Difflugia or per- 
petuate a very serious error. 
With regard to the means whereby the composite tests are 
built up, it may be recollected that, in the ‘Annals’ for last 
January (p. 78), I suggested the probability that in Difflugia the 
external portion of the test receives fresh additions either of chi- 
tinoid or mineral matter through an expansion of the sarcode- 
substance reflected back from the main aperture, or formed by the 
coalescence of sarcode-stolons which escape through one or 
more pores distributed here and there over its surface. This 
view appears to be substantiated in a great measure by the 
fact that in the ordinary forms, whatever may be the mineral 
composition of the superficial layer of the test, there is gene- 
rally to be seen below this, and resting immediately upon the 
