Fig.16 A creeping amœba. Fig. 16—A creeping amœba (highly magnified). The whole organism is a simple naked cell, and moves about by means of the changing arms which it thrusts out of and withdraws into its protoplasmic body. Inside it is the roundish nucleus with its nucleolus.
For a long time now we have comprised under the general name of amœbæ a number of microscopic unicellular organisms, which are very widely distributed, especially in fresh-water, but also in the ocean; in fact, they have lately been discovered in damp soil. There are also parasitic amœbæ which live inside other animals. When we place one of these amœbæ in a drop of water under the microscope and examine it with a high power, it generally appears as a roundish particle of a very irregular and varying shape (Figs. 16 and 17). In its soft, slimy, semi-fluid substance, which consists of protoplasm, we see only the solid globular particle it contains, the nucleus. This unicellular body moves about continually, creeping in every direction on the glass on which we are examining it. The movement is effected by the shapeless body thrusting out finger-like processes at various parts of its surface; and these are slowly but continually changing, and drawing the rest of the body after them. After a time, perhaps, the action changes. The amœba suddenly stands still, withdraws its projections, and assumes a globular shape. In a little while, however, the round body begins to expand again, thrusts out arms in another direction, and moves on once more. These changeable processes are called “false feet,” or pseudopodia, because they act physiologically as feet, yet are not special organs in the anatomic sense. They disappear as quickly as they come, and are nothing more than temporary projections of the semi-fluid and structureless body.
Fig.17 Division of a unicellular amœba. Fig. 17—Division of a unicellular amœba (_Amœba polypodia_) in six stages. (From _F. E. Schultze._) the dark spot is the nucleus, the lighter spot a contractile vacuole in the protoplasm. The latter reforms in one of the daughter-cells.)
If you touch one of these creeping amœbæ with a needle, or put a drop of acid in the water, the whole body at once contracts in consequence of this mechanical or physical stimulus. As a rule, the body then resumes its globular shape. In certain circumstances—for instance, if the impurity of the water lasts some time—the amœba begins to develop a covering. It exudes a membrane or capsule, which immediately hardens, and assumes the appearance of a round cell with a protective membrane. The amœba either takes its food directly by imbibition of matter floating in the water, or by pressing into its protoplasmic body solid particles with which it comes in contact. The latter process may be observed at any moment by forcing it to eat. If finely ground colouring matter, such as carmine or indigo, is put into the water, you can see the body of the amœba pressing these coloured particles into itself, the substance of the cell closing round them. The amœba can take in food in this way at any point on its surface, without having any special organs for intussusception and digestion, or a real mouth or gut.