228 Dr. G. C. Wallicli on the Extent and Causes of 



repeatedly observed by me, in whicb 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 Arcellinc 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 max'gin of 

 the aperture is but slight. At this stage, barring the presence of 

 mineral particles in the test of the ordinary 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 

 Difflugice; 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 (PI. 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 



