Animals

Animals, form one of the two kingdoms (animals and plants) into which living nature is divided. The main features distinguishing A. from plants are considered to be: mobility, the presence in them of a nervous system, sensitivity and consciousness, and animal nutrition. While distinguishing the two kingdoms for convenience, modern biology adheres to a monistic view of living nature, considering A. and plants as two branches stemming from a common trunk, and denies the presence of a fundamental difference between these groups. The basic similarity of both groups is particularly evident when comparing lower, single-celled A. and plants. Many single-celled algae are indistinguishable from simple A. in their mobility, animal nutrition, etc. This similarity led to Haeckel's failed attempt to create a special, third kingdom of living nature from single-celled A. and plants: protists. However, there is much data that smooths the difference even between higher A. and plants. Thus, there are A. completely lacking a digestive tract and, like plants, feeding osmotically through their coverings (tapeworms), sometimes with the help of outgrowths resembling roots (the parasitic crustacean Sacculina). On the other hand, only green plants are capable of photosynthesis and feeding solely through CO₂ from the air and inorganic salts absorbed from water or soil, while fungi feed osmotically on decomposing organic substances (like tapeworms), and insectivorous plants even capture solid food. Many animals (sponges, corals) are as immobile as plants, and some even lack a nervous system (sponges), while some plants (mimosa, Desmodium) possess considerable mobility and sensitivity to irritations. Finally, the fundamental similarity of A. with plants is demonstrated by the profound analogies that the Indian physiologist Bose discovered in the laws of sap circulation, movement and sensitivity in plants with the phenomena of blood circulation, nervous activity, etc. in animals.