AMŒBA AND PARAMŒCIUM

LABORATORY EXERCISE

Amœba.—Technical Note.—Amœbæ are found in stagnant pools of water on the dead leaves, sticks and slime at the bottom. To obtain them, collect slime and water from various puddles in separate bottles and take them to the laboratory. Place a small drop of slime on a slide under a cover-glass. Examine under the low power first and note any small transparent or opalescent objects in the field. Examine these objects with the higher power and note that some are mere granular jelly-like specks, which slowly (but constantly) change their form. These are Amœbæ.

A teacher of zoology recommends the following method of obtaining a large supply of Amœbæ: "For rearing Amœbæ place two or three inches of sand in a common tub, which is then filled with water and placed some feet from a north window; three or four opened mussels, with merest trace of the mud from the stream in which they are taken, are partially buried in the sand and a handful of Nitella and a couple of crayfish cut in two are added; as decomposition goes on a very gentle stream is allowed to flow into the tub, and after from two to four weeks abundant Amœbæ are to be found on the surface of the sand and in the scum on the sides of the tub; small Amœbæ appear at first, and later the large ones."

Having found an Amœba (fig. [5]) note its irregular shape, and if it moves actively observe its method of moving. How is this accomplished? The viscous, jelly-like substance which composes the whole body of an Amœba is called protoplasm. The little processes which stick out in various directions are the "false feet" (pseudopodia). Note that the outer portion, the ectosarc, of the protoplasmic body is clear, while the inner, the endosarc, is more or less granular in structure. Has Amœba a definite body-wall? Do the pseudopodia protrude only from certain parts of the body? Within the endosarc note a clear globular spot which contracts and expands, or pulsates, more or less regularly. This is the contractile vacuole. Note the small granules which move about within the endosarc. These are food-particles which have been taken in through the body-wall. Note how pseudopodia flow about food-particles in the water and how these are digested by the protoplasm. If an Amœba comes into contact with a particle of sand, note how it at once retreats. Note within the endosarc an oval transparent body which shows no pulsations. This is the nucleus, a very complex little structure of great importance in the make-up of Amœba.

Fig. 5.—Amœba sp.; showing the forms assumed by a single individual in four successive changes. (From life.)

Note that Amœba has no mouth or alimentary canal; no nostrils or lungs, no heart or blood-vessels, no muscles, no glands. It is an animal body not made up of distinct organs and diverse tissues. Its whole body is a simple minute speck of protoplasm, a single animal cell. But it takes in food, it moves, it excretes waste matter from the body, is sensitive to the touch of surrounding objects, and, as we may be able to see, it can reproduce itself, i.e., produce new Amœbæ. Amœba is the simplest living animal.

It is only rarely that we can find an Amœba actually reproducing. The process, in its gross features, is very simple. First the Amœba draws in all of its pseudopodia and remains dormant for a time. Next, certain changes take place in the nucleus, which divides into equal portions, one part withdrawing to one end of the protoplasmic body, the other to the opposite end. Soon the body protoplasm itself begins to divide into two parts, each part collecting about its own half of the nucleus. Finally the two halves pull entirely away from each other and form two new Amœbæ, each like the original, but only half as large. This is the simplest kind of reproduction found among animals.

Amœbæ continue to live and multiply as long as the conditions surrounding them are favorable. But when the pond dries up the Amœbæ in it would be exterminated were it not for a careful provision of nature. When the pond begins to dry up each Amœba contracts its pseudopodia and the protoplasm secretes a horny capsule about itself. It is now protected from dry weather and can be blown by the winds from place to place until the rains begin, when it expands, throws off the capsule and commences active life again in some new pond.

The Slipper Animalcule (Paramœcium sp.)—Technical Note.—Paramœcia can be secured in most pond water where leaves or other vegetation are decaying. However, if specimens are not readily secured place some hay or finely cut dry clover in a glass dish, cover with water and leave in the sun for several days. In this mixture specimens will develop by thousands. Place a drop of water containing Paramœcia on a slide with cover-glass over it. Using a low power, note the many small animals darting hither and thither in the field. Run a thin mixture of cherry gum in water under the cover-glass. In this mixture they can be kept more quiet and be better studied.

How does Paramœcium (fig. [6]) differ from Amœba in form and movement? Has the body an anterior and a posterior end? The delicate, short, thread-like processes, on the surface of the body, which beat about very rapidly in the water are called cilia, and they are simply fine prolongations of the body protoplasm. What is their function? Note a fine cuticle covering the body. Note also many minute oval sacs lying side by side in the ectosarc. These are called trichocysts and from each a fine thread can be thrust out.

Note on one side, beginning at the anterior end, the buccal groove leading into the interior through the gullet. Observe also that by the action of the cilia in the buccal groove food-particles are swept into the gullet. Rejected or waste particles are ejected from the body occasionally. Where? Note about midway of the Paramœcium an ovoid body with a smaller oval one attached to its side, the former being the macronucleus, the latter the micronucleus. Note that there are two contractile vacuoles in the Paramœcium; also that the food-vacuoles have a definite course in their movement inside the endosarc.

Make a drawing of a Paramœcium.

In comparing Paramœcium with Amœba it is apparent that the body of the first is less simple than that of the second. The definite opening for the ingress of food, the two nuclei, the fixed cilia, and the definite cell-wall giving a fixed shape to the body, are all specializations which make Paramœcium more complex than Amœba. But the whole body is still composed of a single cell, and there is, as in Amœba, no differentiation of the body-substance into different tissues, and no arrangement of body-parts as systems of organs.

Fig. 6.—Paramœcium sp.;
buccal groove at right. (From
life.)

Paramœcium may occasionally be found reproducing. This process takes place very much as in Amœba. The animal remains dormant for a while, the micronucleus then divides, the macronucleus elongates and finally divides in two, the protoplasm of the body becomes constricted into two parts, each part massing itself about the withdrawn halves of the macro- and micro-nuclei, and lastly the whole breaks into two smaller organisms which grow to be like the original. After multiplication or reproduction has gone on in this way for numerous generations (about one hundred), a fusion of two Paramœcia seems necessary before further divisions take place. This process of fusion, called conjugation, may be noted at some seasons. Two Paramœcia unite with their buccal grooves together, part of the macronucleus and micronucleus of each passes over to the other, and the mixed elements fuse together to form a new macro- and micronucleus in each half. The conjugating Paramœcia now separate, and each divides to form two new individuals.

[CHAPTER VII]