Structural Variation among the Difflugian Uhizopods. 2^7 



the test in Diffiugia 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 Lituola. 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 

 onlg 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 f. 



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 (Ehr.), 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 Polyeystina are respectivelv 

 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 Difflugice, 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. 



t Such a variety is referred to by Ehrenberg (Infusionsth. Taf. ix. 

 figs, a-, c) as Arcella aculeata, showing how very closely Difflugia Arcella 

 (PI. XV. fig. 1 y) and the plain form of D. globularis resemble each other 

 in character. 



