23. The Amoeba.-Among the simplest one-celled animals living in the ooze at the bottom of nearly every freshwater stream or pond is the Amaba (Fig. 7, A), whose body is barely visible to the unaided eye. Under the microscope FIG. 7.-A, the Amaba, highly magnified, showing c. v., pulsating vacuole; f, food particle; n, nucleus. The arrows show the direction of movement. B, shape of same individual 30 seconds later. C, an amoeba-like animal (Difflugia) partially enclosed in a shell. D, an Amaba in the process of division. E, Gromia, another shelled protozoan.-After SCHULZE. it is seen to consist of an irregular, jelly-like mass of protoplasm totally destitute of a cell wall. Unlike those animals with which we are familiar, the body constantly changes its shape. A rounded bud-like projection will be seen to appear on one side of the body and the protoplasm of adjacent regions flows into it, thereby increasing its extent. Similar projections at the opposite end of the cell are withdrawn, and their substance may flow into the newly formed lobe, which gradually swells in size and pushes forward. Thus, by constantly advancing the front part of the body and retracting the hinder portion, the cell glides or flows along from place to place. Upon meeting with any of the smaller organisms upon which it lives, projections from the body are put out which gradually flow around the prey and it becomes pressed into the interior of the cell. The process is not unlike pushing a grain of sand into a bit of jelly. There is no mouth. Any point on the surface serves for the reception of food. Oxygen gas also is taken into the body all over the surface, and wastes and indigestible material are cast out at any point. Nothing exists in these simple forms comparable to the complex systems of organs that carry on these processes in the squirrel. The bodily size of animals is limited, and to this general rule the Amaba is no exception, for upon gaining a certain size, the nucleus divides into two exactly similar portions, and very soon afterward the rest of the body separates into two independent masses of equal size (Fig. 7, D), each of which, when entirely free, contains a nucleus. In this way two daughter amœbæ are formed possessing exactly the characters of the parent save that they are of smaller size; but it is usually not long before they reach their limit of growth, when division occurs again, and so on, generation after generation. It not infrequently happens, however, that the pond or stream, in which the Amaba and other Protozoa live, dries up for a portion of the year. In such an event the body assumes a spherical shape, develops a firm, horn-like membrane about itself, and thus encysted it withstands the summer's heat and dryness and may be transported by the wind, or otherwise, over great distances. When the conditions again become favorable the wall ruptures and the Amoba emerges to repeat its life processes. 24. Some relatives of the Amoeba.-All amoeba-like forms, to the number of perhaps a thousand species, possess this same method of locomotion, but many present some inter esting additional characters. For example, the form represented in Fig. 7, C, constructs a sac-like skeleton of tiny pebbles cemented together, into which it may withdraw for protection. Others construct similar envelopes of lime or flint, and still others, as they continue to grow, build on additional chambers, giving rise to a great variety of forms often of wonderful beauty. In the tropics, particularly, some of the shelled Protozoa are so abundant that they may impart a whitish tinge to the water, and in some places their empty shells on falling to the bottom form immense deposits. The chalk cliffs of England are in large measure made up of such shells. 25. The Infusoria.-A little over two hundred years ago it was discovered that wherever water remained stagnant it became favorable for the rapid multiplication of a large number of species of Protozoa which live in such situations. These are known as Infusoria, and, like the preceding species, are usually of microscopic size and of the most varied shapes. The first striking feature of their organization is the presence of a delicate though relatively firm external cell membrane known as the cuticle, which preserves a definite shape to the body. Such a method of locomotion as exists in the preceding group is consequently an impossibility, but other and more highly developed structures perform the office. These latter organs are of two types, and their general characteristics may be readily understood from an examination of a few species living in the same localities as the Amaba. 26. The Euglena.-The first type exists in the common fresh-water organism known as Euglena, represented in Fig. 8, A. Here the spindle-shaped body is surrounded by a delicate cuticle perforated at one point, where a funnelshaped depression, the gullet, leads into the soft protoplasmic interior. From the base of this depression the protoplasm is drawn out in the form of a delicate whip-like process known as the flagellum. This structure, always permanent in form, constantly beats backward and forward with great rapidity in a general direction represented in the diagram (Fig. 8, c). The movement from a to b is much more rapid than the reverse, from b to a, which results, like the action of the human arm in swimming, in driving the organism forward. Not only does the flagellum serve the purpose of locomotion, but it also produces currents in the water which may serve to bear minute organisms down into the gullet, whence they readily pass into the soft pro FIG. 8.- Flagellate Infusoria. A, Euglena viridis; c, pulsating FIG. 9.-Paramecium aurelia, a ciliate infusorian. c, cilia; c.v., pulsating vacuoles; f, food particles; g, gullet; m, buccal groove; n, nucleus. toplasm of the body, there to undergo the processes of digestion and assimilation. In some forms the protoplasm in the region of the flagellum is drawn out in the form of a collar (Fig. 8, B), whose vibratory motion also aids in conveying and guiding food into the body. 27. The Slipper Animalcule.-The second type of locomotor organ may be understood from a study of the Slipper Animalcule (Paramecium, Fig. 9), abundant in stagnant water. In this form the cuticle surrounding the somewhat cylindrical body is perforated by a great number www a -n C.V. g m of minute openings through which the internal protoplasm projects in the form of delicate threads. Each process, termed a cilium, works on the same principle as the flagellum, but it beats. with an almost perfect rhythm and in unison with its fellows, drives the animal hither and thither with considerable rapidity. On one side of the body is a furrow which deepens as it runs backward and finally passes into the gullet (g), which leads into the interior of the body. Throughout the entire extent it is lined. with cilia which create strong currents in the surrounding water and in this way conduct food down the gul et into the body. Embedded in the outer surface of the body, in among the cilia, are also a number of very minute sacks, each containing a coiled thread which may be discharged against the body of any intruder, so that this form is supplied with actual organs of defense. Two pulsating vacuoles (c.v.) or simple kidneys are also present, consisting of a central reservoir into which a number tionary individual (one of radiating canals extend. partly united is shown in specimen b). FIG. 10.--Vorticella, an attached ciliate infusori an, highly magnified. a, fully extended individ ual; c.v., pulsating vacuole; g, gullet; n, nucleus. b, contracted specimen. c, small free swimming individual, which unites with a sta 28. The Bell Animalcule and other species. The Bell Animalcule (Vorticella, Fig. 10) is often found in the same situations as the Slipper Animalcule, which in certain respects it resembles. It is generally attached by a slender stalk, and where many |