or THE PROTOZOA. RHIZOPODA. 219 



mals, because, as he supposed, the Polyihalamia are aggregated animals with 

 calcareous shells, and the ArcelUna solitaiy animals with a silicious testa. 

 Subsequent researches prove, on the contrary', that all these difiPerential cha- 

 racters are wanting. Each foraminiferous shell contains a solitary inmate ; 

 and although, as a rule, of a calcareous composition, yet a genus, Polymor- 

 phina, is pointed out by Schultze, which, as in the instance of Difflugia, has 

 its testa made up of coherent silicious particles (XXI. 38). Besides all this, 

 the shells of ArcelUna are not silicious, but of a chitinous nature, and the 

 basement membrane in which the earthy matter is deposited in Foraminifera 

 is the same. These circumstances, together with the homology in the animal 

 contents both of MonothaJamia and of Polythalamia, the absence of the h^-po- 

 thetical polygastric organization in the former, and of the imaginary internal 

 structures in the latter, render Ehrenberg's distinction of the two families as 

 separate classes imtenable. 



The ArcelUna of Ehrenberg, and the MonothaJamia of Schultze, do not en- 

 tirely accord in respect to the genera grouped under them. Ehrenberg in- 

 cluded in his family the genera Difflugia, Arcella, Cgphidium, and SpinlUna, 

 The last-named genus departed much from the others by ha\'ing a marine 

 habitat and a convoluted, sjnral, porous shell, — its only real relationship, it 

 would seem, being comprehended in the one assigned feature, its sihcious 

 lorica. On the other hand, Schultze (see tabular ^-iew of his system, p. 241), 

 by not emplopng the chemical constitution of the shells as a distinctive cha- 

 racter, includes among his MonotJialamia calcareous, membranous (chitinous), 

 and such silicious shells as are exemplified by Dijflugia. The essential cha- 

 racter employed is that of the imilocular chamber ; for the other nearly general 

 feature, ^iz. the presence of one considerable orifice, is departed from in the 

 instance of the porous shell of OrhuUna. 



The sheUs of Monothalarnia are of a more or less spherical figure ; some- 

 times they are ovoid (XXI. 11, 12, 16) or pjTiform (17), at others compressed 

 in one or other direction (XXI. 8), and even at times in opposite directions, so 

 that .-everal faces are produced. Thus in the genus Difflugia the spherical out- 

 line prevails (XXI. 10) : the sheUs are globose, or subglobose, or elongated in 

 a pear-shape (XXI. 17), or in a club-Hke (clavate) manner ; in Arcella they 

 are fi'equently compressed, and assume a more or less discoid figure, mostly 

 convex above and flat beneath (7, 8, 9). In G-romia, again, the ovoid or glo- 

 bular shape is diversified by the elongation of the portion about the mouth 

 of the shell into a sort of neck (16). In Lagijnis (Schultze) this tapering of 

 the oral end developes a retort-shaped sheU. In Squamulina (Schultze), again, 

 the testa resembles a plano-convex lens. An exceptional form is described 

 by Ehrenberg, under the name of Arcella disphcera, as oblong, almost divided 

 into two by a central constriction. The first impression would be that the 

 supposed species was no other than two animals coherent by the mouth of the 

 shell ; that such, however, is not the ease is indicated by the next clause of 

 the description — that one segment is nearly occupied by the large foramen. 

 Another example of a remarkably-formed shell is afforded by Cii2:)hidium 

 (XXII. 24-27), which Ehrenberg states to be cubical, with large protuber- 

 ances, giving it in some positions a four-sided or an irregular figure. Again, 

 in the genus Spirillina (Ehr.) (XL 37) and Cornuspira (Schultze) (XXI. 2b), 

 we have examples of spii-aUy-roUed equilateral shells, like those of Planorhis. 

 In consistence the shells of most ArcelUna are firm, mth a degree of flexibility 

 and elasticity, and are composed of a dense membrane proved by its chemical 

 properties to be of a chitinous nature. This shell not only resists the action 

 of boiling solutions of the caustic alkalies and of vinegar, but also concen- 

 trated nitric and chloric acids, and a mixture of the two, also chromic acid. 



