Joseph Leidy (1879) Freshwater Rhizopods of North America. U.S. Geological Survey of the Territories, Vol. XII, pp. 277-295.
The Foraminifera, though constituting the most extensive and important order of the Rhizopods, are almost exclusively marine. A single well-known genus, Gromia, is represented by several species, inhabiting salt and fresh water.
Greek, grumaia a small bag.
Animal spherical or oval, composed of granular protoplasm, with a large central nucleus, and invested with a homogeneous chitinoid membrane. Mouth situated at one pole of the body, and more or less copiously emitting streams of protoplasm, which flow around the body and extend into numerous pseudopodal rays, freely branching and anastomosing, so as to form an intricate net, which exhibits an incessant flow of granules along the filaments, both outward and inward.
PLATE XLVII, figs. 1-4..
Gromia terricola. Leidy: Proc. Ac. Nat. Sc. Phila. 1874, 88.
Body spherical or oval, pale yellowish or cream-colored, and more or less translucent. Investing membrane or shell chitinoid, homogeneous, thin, transparent, colorless, or pale yellowish, smooth, or with more or less adherent sand and dirt. Interior protoplasm white by reflected, pale yellow, by transmitted light, composed of a pale granular basis with fine oil molecules, usually a few clear vacuoles of variable size, and a large, clear or pale granular nucleus. Mouth obscure, emitting an abundance of finely granular protoplasm, which ordinarily flows around the body, and then breaks up into a multitude of diverging streams or filamentous pseudopodal rays, that frequently form and anastomose so as to produce an intricate net. An incessant circulation of granules outward and inward along the course of the pseudopodal filaments.
Size. --From 0.112 mm. to 0.12 mm. in diameter; the oval variety
0.112 mm. long by 0. 1 mm. broad.
The genus is of special interest, because it is a representative, in
the simplest condition, of that great order of Rhizopods, the Foraminifera,
which are exclusively marine, with the exception of the present one.
On several occasions, having observed half a dozen individuals, I was led to believe it was common, but I have since failed to find it after frequent search in the same and similar localities. Nor have I been so fortunate to find any other Gromia, a circumstance I have greatly regretted, from the feeling that, with the experience gained in the investigation of rhizopods, after several years I would have been better able to resolve its characters.
Gromia terricola, represented in figs. 1-3, pl. XLVII, has a spherical or slightly oval, translucent body. By reflected light it is white or cream colored; by transmitted light of a pale yellowish color. The investing membrane or shell is thin, transparent, homogeneous, and colorless or feebly yellowish. It is smooth, but usually has more or less adherent dirt consisting of fine granules and coarser particles of quartz-sand. See figs. 1, 2.
In the individuals observed, the mouth was obscure, and its exact character
I neglected to determine.
The food contents observed in Gromia terricola consisted of minute diatoms, fragments of Lyngbya, and a globular, green algae, together with sand and other materials.
In the emission of the pseudopodal filaments of Gromia terricola, the protoplasm pours from the mouth of the shell in a slow manner, and gradually envelopes the body, as represented in figs. 1-3. From the protoplasmic envelope delicate streams extend outwardly, at first emanating from the front; 'they more or less rapidly multiply and radiate in all directions. Gradually extending, they fork into branches of the utmost tenuity. Contiguous branches freely join or anastomose with one another, and thus establish an intricate net, which in its full extent covers an area Upward of four times the diameter of that of the body of the Gromia. The pseudopodal net incessantly changes, --putting forth new branches in any position, while others are withdrawn,--diminishing and disappearing in one spot, while it spreads and becomes more complex in another spot.
Gromia terricola, with its pseudopodal net fully spread, like its near relatives, reminds one of a spider occupying the centre of a circular web. If we imagine every thread of the latter to be a living extension of the animal under the same control as its limbs, the spider would be a nearer likeness to the Gromia. Over each and every thread of the pseudopodal net Gromia has as complete control as if the threads were permanently differentiated limbs acted on by particular muscles, and directed in their movements by nervous agency. Threads dissolve their connection and are withdrawn; new ones are formed and establish other connections: they bend; they contract into a spiral; they occasionally move like the lashing of a whip, and indeed produce almost every conceivable variety of motion. Not unfrequently spindle-like accumulations of protoplasm occur in the course of the pseudopodal threads. Sometimes, through the conjunction and spreading of several of the latter together, islet-like expansions occur, and become the centres of secondary nets.
The pseudopodal extensions of Gromia consist of pale granular protoplasm with coarser and more defined granules. The latter are observed to be in incessant motion along the course of the threads, flowing in opposite directions in all except those of the greatest delicacy. See fig. 4. In the larger threads, the granules are immersed and near together; in the smallest threads, they are in single rows, more or less widely separated, and thicker than the threads, so that these appear like strings of minute beads.
In the flow of the granules in the pseudopodal threads, they are sometimes seen to slacken their speed, or for a moment become stationary, and then reverse their course. Granules arriving at a dividing branch are sometimes retarded, and then take one or another direction; or, passing from one main branch to another through a by-path, they may take a reverse course from their former one.
The movements of the granules, though apparently independent, are rather due to the currents or flow of the protoplasm constituting the basis of the pseudopodal threads.
Besides the granules, minute vacuoles often make their appearance along the course of the pseudopods. Some of these seem to be of the character of contractile vesicles, -starting as mere points, slowly enlarging, and then collapsing. Other circular spots in the pseudopodal threads, in patches formed by union and anastomosis of the latter, appear as mere circular spaces, due to spreading of the protoplasm in the meshes of the net.
Occasionally, minute diatoms and other objects which come within the territory of the pseudopodal net of Gromia terricola are seen to become immersed in the substance of the threads, and to move along in a manner reminding one of a boat carried along in the current of a river.
Gromia terricola, by means of its pseudopodal net, was observed to be strongly disposed to accumulate around it a quantity of dirt, and especially at the posterior part of the body, as seen in fig. 1. In one individual, after completely surrounding itself with sand and dirt, it entirely withdrew its pseudopodal rays, and nothing that was done could initiate the animal again to protrude them.
Commonly, the specimens under observation remained nearly stationary in position, but occasionally the body appeared to be dragged along with extreme slowness through aid of the anterior pseudopodal extensions.
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