The Amoeba is among the simplest of the Protozoa. The body of an Amoeba consists of a minute portion of naked protoplasm, which is not constant in its form, but variable from moment to moment. Amoebae are very minute ; with the aid of a microscope they may be found in various situations among vegetation in fresh -water ponds, in water which contains rotting organic material, and also as parasites in the intestines of many of the higher animals. In company with other kinds of Protozoa they may he obtained from what are called infusions. To make an infusion a quantity of hay or leaves must be placed in water. After about twenty-four hours or more, according to the temperature, the water looks yellowish, and the surface becomes covered with an opaque film. If some of this film be removed and examined under the microscope, it will be seen to contain numerous minute rod-like bodies, many of which move actively ; these are bacteria which have emerged from minute round bodies called spores, just as a chicken emerges from its egg. The spores of the bacteria are present everywhere, attached to the hay and floating in the air. They are not killed by ordinary heat and drought; they are reproduced in the bodies of parent bacteria. The appearance of the bacteria in the water marks the first stage of putrefaction. Soon after the appearance of the bacteria, various Protozoa may be found in the infusion ; these have emerged from bodies called cysts, which also are distributed everywhere. The Protozoa feed actively upon the bacilli and upon one another.
As an example of an Amoeba, a large form which is sometimes found in infusions will be first described.
To obtain it, place a piece of straw from a two or three days' infusion upon a glass slide, and scrape it with a Knife. Place a cover-slip over the dirty greyish-looking fluid which has exuded, and examine it under the low power of the microscope. Among the various organisms in the fluid may sometimes be seen an Amoeba large enough to be recognized without much difficulty. When found, examine it under the high power.
The Amoeba is a piece of living material or protoplasm ; it is translucent, but not equally so throughout ; it is surrounded by an outer layer, or ectoplasm, which is almost as clear and transparent as glass. The more central portion, or endoplasm, is opaque, and contains numerous granules. The shape of the Amoeba is not constant for any length of time, but changing, owing to the protrusion of blunt lobes which may be thrust out from any part of the surface. When such a lobe or pseudopodium is thrust out, a flowing movement of the granules of the endoplasm may be observed. The granules stream down the central axis of the pseudo podium until they reach the blunt end, where they are reflected outwards, so that in returning to the main body they take a superficial course. The movement within a pseudopodium is therefore somewhat like the movement within a fountain of water which is forced vertically into the air. When the animal wishes to move in a certain direction, a large pseudopodium is thrust out in that direction and the bulk of the endo plasm is poured into it, so that the whole animal shifts its position. Pseudopodia are also thrust outwards towards particles of food material, which become engulfed within the ectoplasm to be passed into the endoplasm, where they are digested.
Within the endoplasm is a spherical body which is somewhat opaque, this is the nucleus; sometimes two nuclei are present. (Every cell of a multicellular animal contains a nucleus; it is because of this fact that the body of an Amoeba or other Protozoon appears to be a single cell.) There are many particles in the endo plasm which have the same appearance as the various particles in the surrounding water; they are ingested food material. Sometimes they can be seen to lie within minute spherical spaces in the endoplasm, which are called food vacuoles. Another kind of vacuole can always be seen in the endoplasm; this is much larger, and is called the contractile vacuole. It should be watched while it expands slowly until it bursts like a bubble. When large it is near the surface ; and if then it bo viewed from the side, the ectoplasm which covers it may be seen to bulge and become thinner and thinner, until it ruptures at a point and the contents of the vacuole are discharged. If it is viewed from above and not from the side, the ectoplasm around the point of discharge assumes a radiating appearance, which is perhaps due to minute folds of the surface, which must naturally form as the bulged ectoplasm sinks back into its normal position. As soon as the contractile vacuole has discharged its contents from the body, it again commences to fill and expand as before. The contractile vacuole is doubtless an excretory organ, the fluids that are discharged must contain the waste products of metabolism. Other Kinds of Amoebae. A smaller kind of Amoeba is common among the dark green slime which accumulates in wet gutters. With the help of a microscope we can see that the slime from a gutter consists of a tangled mass of green filaments, which are minute plants called Algae. A small piece of such material should be placed upon a slide in a drop of water, and broken up very finely with needles. By this means the Amoebae become dislodged from among the green filaments. The slide should be covered and examined. Amoebae found in this way, as well as those found among Algae in fresh-water ponds, are smaller than the kind just described, and have long, thin pseud opodia which are composed of clear ectoplasm throughout ; that is to say, a granular axis can not be seen in them. Otherwise, these Amoebae are like the kind just described, but they can be obtained with much greater certainty. Another kind may generally be obtained from the intestine of the common cockroach.
Amoebae which live in the intestines of multicellular animals are called Eridamoebae ; one kind of Endamoeba causes dysentery and liver abscess in man. To obtain Amoebae from the cockroach, kill the insect with chloroform and pull off the end of the abdomen. The intestine will be seen as a black chord stretching between the torn -off portion and the rest of the body. Cut out a portion of this and place it upon a slide in a drop of salt solution. The intestine is a membranous tube with black contents ; it must be cut open with fine scissors, the contents scraped from the inner wall and mixed in the salt solution. The mixture must be covered and examined ; various parasites can usually be seen thread worms (Oxyurus), large ciliated Protozoa (Nyctotherus), and numerous Endamoebae.
The Endamoebae are small, and must be sought for under the high power of the microscope. They are peculiar in being very trans parent, as though entirely composed of ectoplasm, in which are embedded a few large food particles. They arc sometimes very active, and thrust out rounded pseudopodia with great rapidity; at other times they assume a spherical form, as though resting. Although large Amoebae are not easily obtained in infusions, minute Amoebae may almost always be found in them.
Such Amoebae are often very active ; but vacuoles and even nuclei cannot easily be seen in them: they are perhaps stages in the life-history of other Protozoa.
When the water in which Amoebae live is becoming dried up, they undergo encystment, assuming a spherical shape and surrounding themselves with an impervious coat or cyst. In this form they can resist drought and moderate heat ; they may be scattered by the wind until they find themselves in water again.
Amoebae have been seen issuing from such cysts.
The reproduction of Amoeba is by simple division of the body, and is preceded by simple division of the nucleus. The round nucleus assumes the shape of a dumb-bell or hour-glass, and the two halves become separated. After division of the nucleus, the entire body undergoes a similar change ; on separation, each half of the body "contains one-half of the old nucleus. This method of reproduction is essentially different from that of multicellular animals. In reproduction of higher animals, portions of the body are thrown off from time to time to form offspring, Jmt the parent body must finally undergo death. In Amoeba, the parent body divides and forms two offspring, so that it does not die, but becomes continuous in the offspring. Much remains to be discovered about the life-history of Amoebae.
Protozoa, such as Amoeba, which change their shape and thrust out pseudopodia are placed together in a group and named Lobosa. Some Lobosa form hard protecting shells for themselves. Of these the form known as Diiftugia is sometimes found in Indian ponds. This Protozoon forms a balloon-shaped shell by cementing together particles of sand. The nucleus and most of the protoplasm remains within the shell, active pseudopodia are thrust out through an opening in the shell which represents the mouth of the balloon.
There are certain groups of Protozoa which must be passed over hurriedly, although each group is large enough to form the study of a lifetime.
