Turning now to those of the lower organisms that are somewhat more definitely animal in nature, we may describe the common Amœba. Microscopic in size, this creature consists of a speck of semi-liquid protoplasm, which is irregular and ever-changing in shape. It is continually pushing out finger-like projections from various parts of its surface, feeling, in a dim, vague way, for its food. It moves, if but slowly, by withdrawing its substance in one direction and pouring it forth in another. It indulges in such fare as bacteria or particles of dead organic matter and feeds by the simple method of surrounding the food particle with its protoplasm, and gradually digesting and absorbing whatever it contains of nutriment. Undigested portions are simply left behind as the creature moves on. The waste products are drained into a simple cavity in the protoplasm called the contractile vacuole, which empties itself periodically to the outside. The Amœba reproduces by the ordinary process of simple fission, illustrated, with the creature in its ordinary condition, in Figs. 21 and 22.
Fig. 22—Stages in division of Amœba.
K, nucleus.
Fig. 23.—Paramœcium.
EC, Denser outer layer; EN, inner protoplasm; N, nucleus; PV, contractile vacuole; M, mouth; X, cilia.
From Marshall and Hurst's Practical Zoology (Smith, Elder & Co.).
Somewhat higher than the Amœba, and apparently along the main line of progress, stands the group which includes the slipper animalcule, Paramœcium, shown in Fig. 23. This creature, barely visible to the naked eye, is found in pools of water, or, for example, in drops of rain or dew on plants, and it can generally be obtained in great numbers by soaking a little hay in water for a day or two. It has, as may be seen from the illustration, an elongated shape, with a depression, the mouth, about the middle of one side. The progress made good from the stage of the Amœba has been largely in the direction of a more efficient method of locomotion. Instead of crawling, with painful slowness, the Paramœcium swims freely and rapidly by means of the numerous whip-like projections or cilia which cover it, and with which it lashes the water. An advance is also to be recognised in the fact that the organism is surrounded by a dense outer wall; and that its shape is consequently fixed. Hence also the Paramœcium cannot take in food at any part of its surface, as the Amœba can, but only through the special depression already mentioned. Excretion is carried on in the same manner as in the Amœba. The Paramœcium is a water animal, yet it can resist drying, and remain alive in the absence of water, for a long period. This it accomplishes by becoming encysted, that is, by contracting into a ball and surrounding itself with a resistant shell, from which it can emerge when suitable conditions for active life return. It is worth passing notice that there exist a number of forms occupying a position intermediate between the two types which we have described, and indicating that the second has, in all probability, been derived from the first. One of these is shown on Fig. 24.
