Structural Variation among the Difflugian Rhizopods. 227 
the test in Difflugia of arenaceous particles cemented together by 
a chitinoid exudation from the animal, precisely after the same 
fashion as the chitinoid and arenaceous elements of the shell 
are cemented together in Litwola. Indeed in some specimens of 
the more common varieties of Difflugia, in which the. chitinoid 
matter assumes a sienna-tint, it is extremely difficult, if not im- 
possible, to say, on mere inspection of the broken-up wall of the 
test, whether we have under our eyes a portion of a Difflugian 
test or a Lituoline shell *. 
In Arcella, on the other hand, the test is almost invariably 
only chitinoid in its composition; and although we frequently 
meet with tests nearly devoid of mineral particles, and closely 
resembling that of Arcella in its hemispherical or depressed out- 
line, central aperture, and inversion of the lip, a very small degree 
of skill enables us to perceive that the object before our eyes does 
not present the characteristic symmetrical reticulation of that 
form, but is in reality an osculant variety from the side of 
Difflugia +. 
If we now turn to the figure and plan of growth of the tests, 
we shall, I think, perceive that these are analogous in the two 
forms. 
Commencing with the earliest state of the Arcelline test, (un- 
less I am much mistaken) Arcella hyalina (Khr.), Arcella 
patens (Clapar. and Lach.), and several varieties of Orthosiran 
or Melosiran discs have all been confounded more or less with 
it. That this should have been the case is in nowise surprising 
when we consider the minute and almost invisibly hyaline cha- 
racter of the test of Arcella at this period. Without, however, 
asserting positively that this has been the case, I may state that 
minute forms answering precisely to the published characters 
of the Rhizopods (not the Diatoms) mentioned above have been 
here a warrant for assuming; but, keeping in view the fact that the calca- 
reous or siliceous matters of which such structures as the shells of the 
Foraminifera and internal skeletons of the Polycystina are respectively 
formed are eliminated from the water in one condition, and somehow or 
other reproduced as an exudation from the animal in another, we certainly 
express the result by adopting the word, although the process by which it 
is brought about may be regarded as exceptional. 
* The appearances presented under the microscope by the broken-up 
test of one of these Diffugie, or a shell of Lituola, are very similar to those 
visible, by the unaided eye, in the beautifully constructed cylindrical tubes 
of Pectinaria; and they indicate a degree of adaptative skill which, however 
wonderful it may be thought in the articulate animal, is doubly wonderful 
in the Protozoan. 
+ Such a variety is referred to by Ehrenberg (Infusionsth. Taf. ix. 
figs. a,c) as Arcella aculeata, showing how very closely Diffugia Arcella 
(Pl. XV. fig. 1 y) and the plain form of D. globularis resemble each other 
in character. 
