This order comprises those Rhi-zopoda which are, with few exceptions, naked; have usually short, blunt, lobose pseudopodia, which do not anastomose with one another; and contain a "nucleus" and one or more "contractile vesicles."
The Amoeba, or Proteus-animalcule, may be taken as the type, and a description of it will be sufficient to indicate the leading points of interest in the order. The Amoeba (fig. 8, B) is a microscopic animalcule, which inhabits fresh water,* and is composed of gelatinous sarcode, which admits of a separation into two distinct layers: an outer transparent layer, termed the "ectosarc;" and an inner, more fluid and mobile, molecular layer, called the "endosarc." The "ectosarc"is highly extensile and contractile, and is the layer of which the pseudo-podia are mainly composed; whilst the "endosarc" contains the only organs possessed by the animal - viz., the "nucleus" and "contractile vesicle" or vesicles, along with certain fortuitous cavities termed "food-vacuoles."
Fig. 8. - A, Amoeboe developed in organic infusions (after Beale), greatly enlarged; B, Amoeba princeps (after Carter); v Villous region ; c Contractile vesicle; n Nucleus ; e Ectosarc."
It is believed by some that the ectosarc is surrounded by a colourless and structureless investing membrane or cuticle; but this is denied by others. Be this as it may, there is no oral aperture, so far as has ever been certainly observed, and the food is merely taken into the interior of the body by a process of intussusception - any portion of the surface being chosen for this purpose, and acting as an extemporaneous mouth. When the particle of food has been received into the body, the aperture by which it was admitted again closes up, and the discharge of solid excreta is effected in an exactly similar manner.
In this case, however, the area of the general surface within which an anus may be extemporised, appears to be more restricted, and to comprise a portion only of the body ("villous region").
The "nucleus" (fig. 8) is a solid granular body, or a clear vesicle containing a "nucleolus " in its interior, one or more of which is present within the endosarc of every Amoeba, but its function is not known with any certainty. The "contractile vesicles " are cavities within the endosarc, of which ordinarily one only is present in the same individual, though sometimes there are more. In structure it is a little cavity or vesicle filled with a colourless fluid apparently derived from the digestion, and exhibiting rhythmical movements of contraction (systole) and dilatation (diastole). In some cases radiating tubes are said to have been seen proceeding from the vesicle at the moment of contraction. Regarded functionally, the contractile vesicle may be looked upon as a circulatory organ; in which case, it offers the most rudimentary form of a vascular system with which we are as yet acquainted. By others, however, the contractile vesicle is believed to be filled with water from the exterior, and it is regarded as a rudimentary form of water-vascular system; while others regard it as an excretory organ.
Besides these proper organs, the endosarc usually contains clear spaces, which are called "vacuoles," or, more properly, "food-vacuoles." These spaces (though sometimes rhythmically contractile) are of a merely temporary character, and are simply produced by the presence of particles of food, usually with a little water taken into the body along with the food.
There are no traces of any organs of sense, or of a nervous system, or, indeed, of any other organs in addition to those already described. Locomotion is effected with moderate activity, but in an irregular manner, by means of the blunt, finger-shaped processes of sarcode, or pseudopodia, which can be protruded at will from any part of the body, and can be again retracted within it. The pseudopodia also serve as prehensile organs; but they do not interlace and form a network, nor do they exhibit any circulation of granules derived from the endosarc, as in many others of the Rhizopoda.
As regards the reproductive process in the Amoeba, no differentiated sexual organs have hitherto been discovered, and the true sexual form of the process is therefore unknown. Fresh individuals, however, may be produced in three ways: Firstly, by simple fission, the animal dividing into two parts, each of which becomes an independent organism. Secondly, by the detachment of a single pseudopodium, which becomes developed into a fresh Amoeba. Thirdly, by the production of little spherical masses of sarcode, which may be derived from the nucleus by fission, or may be produced by a segmentation of the endosarc, the animal having previously become torpid, and the nucleus and contractile vesicle having disappeared. These little masses, however produced, develop themselves when liberated into ordinary Amoeboe. This last method of reproduction is obviously very closely analogous to the production of "pseudonavicellae" in an encysted Gregarina.
The remaining members of the Amoebea are constructed more or less closely after the type of the Amoeba itself. In the nearly allied Difflugia, the sarcode forming the body of the animal is invested with a membranous envelope or "carapace," strengthened by grains of sand and other adventitious solid particles, and having a single aperture at one extremity, through which the pseudopodia are protruded (fig. 9). The animal generally creeps about head-downwards, so to speak; that is to say, with the closed end of the carapace elevated above the surface on which it is moving. Difflugioe often exhibit the phenomenon known as "conjugation" or "zygosis." Under these circumstances, two Difflugioe come in contact; the mouths of the two tests are brought together ; the two animals flow backwards and forwards into each other's tests, with an apparently complete incorporation ; and finally they separate again, and each retires to its own test. In Arcella there is a discoid or basin-shaped carapace, secreted by the animal itself, and likewise possessing but a single pseudopodial aperture, placed in this case on the flat surface of the body. One species of Arcella (viz., A. arenaria) is terrestrial in its habits.
In Pamphagus there is no carapace, but the pseudopodia are nevertheless protrusible from one extremity only of the body, the remainder of the surface appearing to be of too resistant a consistence to allow of this. Cochliopodium is like Arcella, but the test is quite flexible. Pseudochlamys, Hyalosphenia, Quadrula, etc, are other fresh-water Rhizo-pods more or less closely allied to Arcella and Difflugia, but often exhibiting interesting and remarkable modifications of structure.
The Amaebea may be divided into two sub-orders : 1. Amae-bina, including those forms which have the body naked; and 2. Arcellina, comprising those in which the body is protected by a carapace. The latter are included by Hertwig and Lesser along with Gromia and the typical Foraminifera in a common group, to which they give the name of Thalamophora. The blunt and lobose character of the pseudopodia in the Arcellina would, however, appear to be a more important character than the possession of a test, and would assign to these forms a position close to Amoeba.
Fig. 9. - Difflugia pyriformis, greatly enlarged. (Altered slightly from Carter.) The test is composed of angular grains of transparent quartz, within which is the transparent ecto-sarc, lined by the finely granular endosarc. n Nucleus ; c c Contractile vesicles.