Rhizopoda

“ These ever-changing little pseudo-feet serve both for locomotion and for taking food.
The class of the Rhizopoda is divided into three different legions, viz. the chamber-shells, or Acyttaria, the sun-animalcules, or Heliozoa, and the basket-shells, or Radiolaria. The Chamber-shells (Acyttaria) constitute the first and lowest of these three legions; for the whole of their soft body consists merely of simple mucous or slimy cell-matter, or protoplasm, which has not differentiated into cells. ”

“ At the same time, Siebold united the Infusoria with the Rhizopoda, under the name of Protozoa (lowest animals) , into a special main division of the animal kingdom. By this the number of animal types was increased to six. It was finally increased to seven by the fact that modern zoologists separated the main division of the articulated animals into two groups: (a) those possessing articulated feet (Arthropoda) , corresponding to Linnæus’ Insects, namely, the Flies (with six legs) , Myriopods, Spiders, and Crustacea ”

“ Finally, the Proteomyxa forms a sort of lumber-room for beings which are intermediate between the Heliozoa, Rhizopoda, and Flagellata, usually passing through an amoeboid stage, and for the most part reproducing by brood-formation. Zoospores that possess flagella are certainly known to occur in some forms of Foraminifera, Rhizopoda, Heliozoa, and Radiolaria, though not by any means in all of each group. ”

“ Strongly to be contrasted with the above groups belonging to the Rhizopoda are the Infusoria proper, which are characterized by the usual possession of cilia. Cilia (literally "eyelashes") are fine hair-like processes of the protoplasm of the cell, which fringe its exterior; by their constant movement they enable the animal to swim, and at the same time they create a current in the water, which washes up to the region of the mouth particles which may serve for food ”

“ If we expected to find organic remains at all in these most ancient deposits of the primordial period, we should certainly look for such of the most simple Protista as are covered with a solid shell, and in the organization of which the difference between animal and plant is as yet not indicated.
We know of but few species of the Sun-animalcules (Heliozoa) , the second class of the Rhizopoda. One species is very frequently found in our fresh waters. ”

“ Most of the Rhizopoda live among filamentous Algae in pools, ponds, and in shallow seas, etc.; some are "sapropelic" or mud-dwellers (many species of Amoeba, Pelomyxa, Difflugia, etc.) , others frequent the roots of mosses. Amoeba coli is often found as a harmless denizen of the large intestine of man. Amoeba histolytica, lately distinguished therefrom by Schaudinn, is the cause of tropical dysentery. It multiplies enormously in the gut, and is found extending into the tissues, and making its way into the abscesses that so frequently supervene in the liver and other organs. ”

“ And in like manner, judging from the characters of the pseudopodia in Schultze's genera Lagynis and Squamulina, it may be doubted whether the true place of those genera is in association with the Foraminifera, or whether their relation is not really more intimate with the Actino- phryna. But the existence of such osculant forms by no means invalidates the principle of our classification, since their presence only serves to supply, between the Orders into which I propose to divide the Rhizopoda, the link which is necessary to their completeness as natural groups.
It is an. ”

“ In most cases during such a resting period the rhizopod surrounds itself with a firm, impervious coat, called a ‘cyst.’ When the unfavorable conditions cease the sarcode divides up into several minute spores, each of which on the dissolution of the cyst becomes a new individual. The growth of these individuals is rapid when food is abundant. ”

“ Generally rhizopods are freely moving animals, but some are attached in adult life by stalks. The individuals are usually distinct, but colonial rhizopods are known, and such colonies are sometimes half an inch across. ”

“ Reproduction takes place by simple budding or fission, the two processes differing only in the relative size of the resulting individuals. The formation of spores, however, occurs in many cases, especially after a resting period. ”

“ The animal part of the rhizopods is a gelatinous body called sarcode, a bit of scarcely organized protoplasm. Food is taken in through the outer wall, and is made into small pellets, which are surrounded by a digestive fluid in extemporized stomachs. Minute granules move about the interior, perhaps the substitute for a circulating fluid; and the outer wall can be moulded into the long processes called pseudopodia, used for locomotion and prehension. When these rhizopods secrete stony matter for a covering, the interior is the same structureless mass ”

“ Rhizophora, rī-zof′ō-ra, n. a small genus of trees, the mangroves.
Rhizophore, rī′zō-fōr, n. the structure bearing the true roots in certain species of Selaginella.—adj. Rhizoph′orous. [Gr. rhiza, root, pherein, to bear.]
Rhizophydium, rī-zō-fid′i-um, n. a genus of unicellular fungi.—adj. Rhizophyd′ial. [Gr. rhiza, root, pheidos, sparing.]
Rhizopod, rī′zō-pod, n. one of the Rhizop′oda, a division of the Protozoa, esp. a class with pseudopodia for locomotion and the ingestion of food.—adjs. Rhizop′odal, Rhizop′odous. [Gr. rhiza, a root, pous, podos, a foot.] ”

“ Not every sample of soil, however, contains all the species capable of living in soil, but the local conditions determining the presence or absence of any species are at present unknown. In general the numbers, both of species and of individuals present, follow the number of bacteria. They are consequently most numerous in rich moist soils. The statement sometimes made that protozoa are most numerous in peaty soils is based solely on the number of Rhizopod shells found in such localities ”

“ When a Rhizopod discriminates between the contact of a large body and that of a small one, and draws in its pseudopodia and shrinks into as compact a shape as possible in order to escape the danger which the past experience of the race has shown to be related to the former sensation, or when it expands its pseudopodia in order to ingulf and digest the body which has caused the second sensation, it furnishes proof that its scientific education has begun. ”

“ The unfortunate shelled animal is like a fish caught on a hook. It struggles vainly to get away. It rushes, but the fungus simply lets out [Pg 47] the line until the rhizopod is brought to an abrupt stop and can be hauled in. The line is a filament connecting the body of the fungus with the bulb in the animal’s mouth.
Once its prey is secure, the fungus proceeds to send out growths from the bulb through the creature’s flesh, literally eating it alive. Very rarely, like a hooked fish, a rhizopod is able to break away. ”

“ Indeed the Rhizopods themselves, on passing into a state of quiescence in which the relations of outer and inner parts are fixed, become encysted: there is formed a hardened outer coat different from the matter which it contains. And what is here a temporary character answering to a temporary definiteness of conditions, is in the Infusoria a constant character, answering to definite conditions that are constant. Each of these minute creatures, though not coated by a distinct membrane, has an outer layer of excreted substance forming a delicate cuticle. ”

“ The choice was necessarily limited to those the chief forms of which were easily to be obtained alive in some abundance. The Crabs and Macrurous Crustacea, the Stomapoda, the Diastylidæ, the Amphipoda and Isopoda, the Ostracoda and Daphnidæ, the Copepoda and Parasita, the Cirripedes and Rhizocephala of our coast, representing the class of Crustacea with the deficiency only of the Phyllopoda and Xiphosura, furnished a long and varied, and at the same time intimately connected series, such as was at my command in no other class. ”

“ We might, of course, trust to empirical hunting for organisms, or to chance discoveries such as led Goodey to find the mysterious Proteomyxan Rhizopods, which cannot yet be cultured with certainty, so that they are rarely found by soil workers. It is possible that there are many such organisms, and it is even conceivable that these unknown forms far outnumber the known. The defect of the present method is that it always leaves us in doubt as to the completeness of the list, and so we may have to devise another. ”

“ The parasitic habit is most common among the Copepoda and Isopoda, where it leads to complex modifications of structure and life-history. Perhaps the most complete degeneration is found in the Rhizocephala, which are parasitic on other Crustacea. In these the adult consists of a simple saccular body containing the reproductive organs and attached by root-like filaments which ramify throughout the body of the host and serve for the absorption of nourishment (fig. 1) . ”

“ They live in minute shells, the largest of which may be almost the size of a grain of wheat, but by far the greater number are invisible as shells without a microscope, and merely show as fine dust. The [pg 33] rhizopods are of different shapes, sometimes round, sometimes spiral, sometimes having only one cell, sometimes having several cells. In the latter case a separate animal lives in each cell. The animal is of the very simplest as well as the smallest kind. He has not even a mouth or a stomach but can take in food at any part of his body. ”

“ At the end of vegetative life the needle-bearing form fragments into numerous mononucleate parts; these develop into adults similar to the parent, but without the spines. At the end of its vegetative life this new individual fragments into biflagellated swarm-spores which may conjugate, reproducing the form with needles. Size up to 2 mm.
Trichosphærium sieboldi Schneider. Fig. 3.
With the characters of the genus. A form which I have taken to be a young stage of this interesting rhizopod is described as follows:
A minute, almost quiescent, form which changes its contour very slowly. ”

“ Some dozen years ago it was discovered that in many cases vegetal cells are connected with one another by threads of protoplasm—threads which unite the internal protoplasm of one cell with the internal protoplasms of cells around. It is as though the pseudopodia of imprisoned rhizopods were fused with the pseudopodia of adjacent imprisoned rhizopods. We can not reasonably suppose that the continuous network of protoplasm thus constituted has been produced after the cells have become adult. ”

“ The most noticeable feature about the Myxosporidian trophozoite is its amoeboid and Rhizopod-like character. Pseudopodia of various kinds, from long slender ones (fig. 3, B) to short blunt lobose ones, are of general occurrence, being most easily observed, of Morphology. course, in the free-living forms. The pseudopodia serve chiefly for movement and attachment, and never, it should be noted, for the injection of solid food-particles, as in the case of Amoebae. The general protoplasm is divisible into ectoplasm and endoplasm. ”

“ The general trait which here concerns us, is that while its plastic or semi-fluid sarcode goes on protruding, in irregular ways, now this and now that part of its periphery, and again withdrawing into its interior first one and then another of these temporary processes, perhaps with some small portion of food attached, there is but an indistinct differentiation of outer from inner (a fact shown by the frequent coalescence of the pseudopodia in Rhizopods) ”

“ There are large amœbæ, which thrust out strongly mobile feet from their [130] unicellular body, broad, flaplike processes of the naked cell body which constantly change their form, size, and place. If they are killed and examined with the aid of the best methods of coloring, it is quite impossible to detect any structure in them; and this is also true of the pseudopodia of the mycetozoa and many other rhizopods. Moreover, the slow flowing movement of the fluid protoplasm shows clearly that there cannot be any composition out of fine fixed elements in the body. ”

“ Its form is not constant during activity, for fingerlike processes called pseudopodia are pushed out tentatively in many directions to be followed as circumstances direct by the materials of the whole cell body. Other protozoa differ in possessing constant forms, or in having constant vibratile processes, or shells of some kind, while in still other cases like individuals combine to make colonies which are more or less definite and permanent. Here at the very foot of the organic scale are found animals which seem to be entirely different from those above. ”

“ America, as we have seen, possesses only one of the three species of Rhizophora, and this is the only representative that it owns of the four Asiatic genera (Rhizophora, Kandelia, Ceriops, Bruguiera) that constitute the tribe Rhizophoreæ. The rule prevailing with current-dispersed plants that America is a distributor and not a recipient evidently does not apply to the Rhizophoreæ; and to explain their distribution we must go back to some epoch very remote from the present. ”

“ In the course of its life, a single one of these thread-like fungi will capture many of the shelled animals, lining them up securely mouth-to-mouth on both sides of itself. It absorbs their substance at its leisure. Other predaceous fungi have definite external organs for capturing their prey. This particular species, however, has no external appendages and appears completely inert and innocent until it is stimulated to action by the sucking of the rhizopod. ”

“ In Amoeba binucleata two nuclei are always present; and some genera are permanently plurinucleate (Pelomyxa, Arcella, fig. 1, 7) . It often gives forth fragments into the cytoplasm, the “chromidia” of R. Hertwig, which, as in Foraminifera (q.v.) , may play an important part in reproductive processes. The contractile vacuole (there are two in Arcella, fig. 1, 7) in actively progressing Rhizopods always discharges at the hinder end. Absent or sluggish in marine forms, it is of constant occurrence lin all fresh-water Rhizopods except Pelomyxa.
The pseudopods vary greatly in type. ”