Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/387

 FOBAMINIFEEA 373 present, find ever-increasing reason to believe that it lias exceeded that of all other marine animals taken collectively. A concise statement of the evidence in support of each of these positions will be embodied in the following sketch of the life-history, present and past, of this group ; which will be so drawn as rather to bring into view its great features of general interest, than those details with which the systematist only is concerned. Although the testaceous Foraminifcra are all marine, certain shell-less animals essentially similar in nature occur in fresh water; and these, of which Liebcrkuhnia (fig. 2) FIG. 2. Licbcrtiihniit, with reticular pseudopodia. is a characteristic example, afford the best means of study ing the life-history of tho &quot; reticulose&quot; type. In its inac tive state Lieberkiihnia is an ovate corpuscle of granular protoplasm about l-400th of an inch in diameter, not pre senting either any distinct differentia- tionof &quot;endosarc&quot; and &quot; ectosarc &quot; or &quot; contractile vesicle,&quot; or any evident &quot;nucleus,&quot; but including a largo number of vacuolcs filled with a homogeneous liquid. In its active state, on the other hand, the body puts forth a sort of stem, which soon branches ir regularly in to pseu- dopodia; and these subdivide and ramify with great minuteness to an extent two orthrcc times as great as that represented iu the figure, tho ramifications coa lescing whenever they come into cuntact with each other, so as to form an irregular network which. FIG- Z. Gromia, with extended psemlopodia. may be compared to an animated spider s-web. This net work serves to entangle either minute particles (such as chlo rophyll granules) or larger bodies (such as Infusoria) that come iu its way, the former being transmitted to the central body by a sort of circulation (closely resembling the cydosis of vegetable cells) which is constantly going on through the whole protoplasmic network, and the latter becoming enclosed in a kind of sheath formed by the blending of the neighbouring pseudopodia, which apply themselves to its surface and draw it into the central mass by their gradual contraction. Although destitute of any protective envelope, Lieberkiihnia does not put forth its pseudopodia indifferently from any part of its surface, their primary stem being en veloped in a transparent sheath, which may be traced as a thin pellicle over the whole body. And in Gromia (fig. 3), of which some forms inhabit fresh water (attaining a size that renders them visible to the naked eye), whilst others are marine, this pellicle is thickened into a distinct casing or &quot; test&quot; of ovoidal shape (probably composed of chitin 1 ), with a single round orifice of moderate size, through which the sarcodic body puts forth its pseudopodial extensions. When the animal is in a state of repose, these are entirely retracted into the test ; but when its activity recommences, they are put forth from its orifice, as from the stem of Lie bcrkuhnia, and form a sarcodic network which is in a state of incessant change new centres of radiation often arising where two or more pseudopodia coalesce, by the flowing of the protoplasm towards those points. It is specially noticeable in this type that the sarcodic body extends itself over the entire surface of the test, so as completely to enclose it, and that pseudopodia are put forth from every part of this extension, being especially numerous at the posterior end, where they probably serve to fix the test, and thus to enable the animal to put forth more power in seizing the larger creatures that serve as its food. For whilst it is partly nourished by the minute granules that adhere to its outspread network, it lays hold of the smaller Infusoria and Diatoms, the zoospores of Confer vac, &c., and draws them entire into the interior of the test, within which their indigestible remains may frequently be seen. Now, if the transparent pellicle of Lieberkuhnia were con solidated by calcareous instead of by chitinous deposit, a monothalamous (single-chambered) &quot; shell &quot; would be pro duced (9 in fig. 1), such as constitutes the permanent form of the simplest Foraminifera, and the primordial form of even the most complex. This shell, like the test of Gromia, may have but a single aperture, from which the pseudo podial extensions are put forth, and through which alone nutriment is received into the contained body. But in a large proportion of the polythalamous (many-chambered) Foraminifera (fig. 4), the shell-wall is perforated with minute pores over its entire surface, through which pseudo podia extend themselves in a radial direction ; and while the coalescence of these at their origin forms a continuous sarcodic layer that covers the exterior of the shell, the coalescence of their extremities forms new centres of radia tion at a distance from it. The aperture or mouth in these types is much smaller than in the preceding, and appears to serve rather for the projection of the stolon-process, by which new chambers are formed (in the manner to be pre sently described), than for the introduction of nutriment. And when it is considered that the diameter of the pores of the shell never exceeds l-5000th of an inch (being often much less than 1-1 0,000th), and that the chlorophyll- granules, zoospores, &amp;lt;fec., that nourish the fresh-water llhizopods are altogether wanting in the sea-depths inhabited by the higher types of Foraminifera, there seems additional ground for the doctrine already propounded, that the nutri tion of the animals of this group is mainly drawn rather from the organic matter that is dissolved in the medium they inhabit, than from solid particles suspended in it. 1 There seems reason for regarding this as representing the &quot; cellu lose wall &quot; of the vegetable cellj which, in the curious Chlamydomyxa of Archer, opens at a certain stage of its life, and allows its protoplas mic contents to project themselves into a &quot; rcticulose &quot; extension.