The animal, on the other hand, lives by appropriating, either directly or indirectly, what the plant has produced. All flesh is indeed grass, in a different sense from that originally intended by the statement. It is this essential difference which lies at the root of all the plain and obvious distinctions between animals and plants. The plant has neither the necessity to go forth in search of its food materials, which nature brings to it, nor has it to spare of its painfully collected energy for the labour of locomotion. Hence it remains stationary. The animal must of necessity go to seek its more elaborate fare, therefore it moves. Moreover, to be successful in its search, the animal obviously requires a nervous system to direct and control its movements, which system, except in the simplest and crudest forms, is absent from the plant. In the main, then, the plant builds up and saves, the animal breaks down and spends. The plant is the producer, the animal the consumer. [Illustration: FIG. 21.--Amœba. _K_, Nucleus; _V_, contractile vacuole.] 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. [Illustration: Fig. 22--Stages in division of Amœba. K, nucleus.] [Illustration: 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.).]