There is a family of fresh-water testaceous rhizopods of which one group secretes its shell and the other builds it. The horny shell secreted by the group of the Arcella presents various degrees of plano-convexity, the convexity in some cases amounting to a hemisphere. They rarely, if ever, have mineral matter on their surface, which is studded with regular but very minute hexagonal reticulations. The aperture or mouth of the shell is small, and invariably occupies the centre of the plane surface, its margins being more or less inverted. The form of the shell is exceedingly varied, sometimes it even has horns indefinite in number, sometimes symmetrical, sometimes not; when its test or covering becomes too small for its increasing size, it quits it, and secretes a new one. The filamental pseudopodia proceed from the mouth of the shell only, and by means of these it creeps about on its mouth in search of food.
Fig. 95. Simple Rhizopods.—A, B, Difflugiæ; C, D, Arcellæ.
The Difflugia build their own shells, which are usually truncated spheres, ovate, or sometimes elongated into the form of a pitcher or flask. The most minute recognisable of these shells is about the 1⁄1000 of an inch in diameter, but they are constructed with the most perfect regularity. The Difflugia pyriformis or symmetrica has the form of an egg with an aperture at the small end. It is entirely made up of rectangular hyaline plates, arranged with the greatest regularity in consecutive transverse and longitudinal rows, the smaller ones being at the extremities, while the larger ones occupy the central and widest portion of the structure. The inhabitant of this abode is an Amœba with a sarcode body covered with a thin film, from whence it sends off pseudopodia through the mouth of its shell. The Difflugia is propagated by conjugation, but before that takes place it becomes densely charged with chlorophyll-cells and starch-grains. The former disappear during the subsequent changes, and are replaced by a mass of colourless cells full of granules which are supposed to be the elements of a new generation. The embryo or earliest form is a minute truncated sphere, but the animal builds up its habitation very much according to local circumstances.
The greater number of the Difflugiæ secrete a substance which forms a smooth layer in the interior, which the animal covers with sarcode from its mouth, and then it drags itself with its pseudopodia to the particles which it selects, and they adhere to it. The particles selected are invariably mineral matter. ‘The selective power is carried to such an extent that colourless particles—sometimes quartzose, sometimes felspathic, sometimes micaceous—are always chosen.’ ‘The particles seem to be impacted into the soft matter, laid on the exterior in the same way that a brick is pressed into the yielding mortar, and that too, in so skilful a manner as to leave the smallest possible amount of vacant area; whilst in the specimens of Difflugia in which tabular or micaceous particles are used, they are sometimes disposed with such nicety that there is no overlapping, but the small fragments are placed so as to occupy the space left between the larger ones. These excellent architects seem to know that in the valves of the Diatoms are combined the properties best suited to their wants, that is, transparency and form, capable of being easily arranged.’
Both the Difflugia and Arcella are Amœbæ in the strictest sense of the word; their bodies consist of sarcode, which sends out finger-like lobes from the mouth of the shell at one end, while the other end has an adhesive property, which fixes it to the bottom. The nucleus and contractile vesicles are identical in character with those of the Amœbæ, and exhibit the same tendency to subdivision at certain periods of the creature’s history that is witnessed on a large scale in the Amœba proper; and the reproductive process is the same.[[5]]
The Difflugiæ are found in rivulets and pools containing aquatic plants; the condition of the water and the nature of the soil have a great influence on the form of their shell.
The Euglyphæ is the third group of fresh-water rhizopods. They are extremely minute, and there are no mineral particles whatever on their shells, the axes of which do not coincide with the aperture. The interior of the animal is like that of the Arcella and Difflugia, but it differs from them in as much as the pseudopodia and ectosarc, or external coat, are finely granular, and the whole mass of the body possesses a decided degree of adhesive viscidity. The pseudopodia are filiform, tapering, radiating, and readily coalesce; and ‘as if to compensate for the restricted power of locomotion, compared with that of the Amœba proper, the pseudopodia of the Euglyphæ are much more active. The rapidity with which they admit of being projected outwards, and withdrawn into the shell, is unequalled in any other form, presenting the most wonderful example of inherent contractility in an amorphous animal substance, that is to be met with in either of the great organic kingdoms.’[[6]]
The order Reticularia, with a very few exceptions, are animals dwelling in calcareous microscopic shells, and differing essentially in constitution from all the preceding Rhizopods. The ectosarc or surface-layer of the sarcode in the Amœba and Actinophrys has so much consistence, that their pseudopodia, which are derived from it, have a decidedly firm outline and never coalesce; whereas in the order Reticularia, the sarcode is merely a semi-fluid protoplasm or colourless viscid fluid, without the smallest surface-layer or film, so that their pseudopodia possess no definiteness either in shape, size or number. Sometimes they are cylindrical, and sometimes form broad flat bands, whilst they are often drawn into threads of such extreme tenuity, as to require a high magnifying power to discern them. They coalesce and fuse into each other so freely and so completely when they meet, that no part of their substance can be regarded as having more than a viscous consistence. Their margins are not defined by continuous lines, but are broken by granules irregularly disposed among them, so that they appear as if torn; and these granules, when the animal is in a state of activity, are in constant motion, passing along the pseudopodia from one end to the other, or passing through the connecting threads of this animated network from one pseudopodium to another, with considerable rapidity, analogous to the movement of the particles in the cells of the hairs of the Tradescantia and other plants.[[7]]
The sarcode body of the Gromiæ is inclosed in a yellowish brown horny envelope or test of an oval shape, with a single round orifice of moderate size, through which the pseudopodia extend into the surrounding water, some forms of the animal being marine, others inhabitants of fresh water. When the animal is at rest all is drawn within the test, and when its activity recommences, single fine threads are put out which move about in a groping manner until they find some surface to which they may attach themselves. When fixed, sarcode flows into them so that they rapidly increase in size, and then they put forth finer ramifications, which diverging come in contact with those from other stems, and by mutual fusion form bridges of connection between the different branching systems; for the protoplasm spreads over the exterior of the test, and from it pseudopodia extend and coalesce, wherever they meet, so that the whole forms a living network, extending to a distance of six or eight times the length of the body. [Fig. 96] represents the Gromia oviformis with its pseudopodia extended.