This entailed greater complexity in the structure of the nervous system and permitted a variety of movements, broadening the vital possibilities of the organisms. An important stage in the process was the development in vertebrates of the rigid skeleton and striated musculature. Darwin showed that in the course of evolution it was those kinds of movement and locomotor design that were vital and useful for the species that became fixed by natural selection. The movements in air of so-called flying fish, frogs, and mammals (for example, the flying squirrels) are not in fact flying but rather long, gliding jumps accomplished by means of such devices of support as elongated thoracic fins, interdigital membranes on the feet, and skin folds.Īs animals evolved, the kinds of movement changed and became more complex. Active movement in air-flying-is characteristic of most insects, birds, and some mammals (bats). Active movements in water are accomplished by specialized remigial structures (from hairs and flagella to the modified limbs of aquatic turtles, birds, and seals), by flexure of the entire body (most fish, caudate amphibians), and by jet action, ejecting water from body cavities (medusoids and cephalopods). Some aquatic animals have adaptations that maintain their bodies in a suspended state (for example, vacuoles in the external layer of protoplasm in radiolarians and air sacs in colonies of siphonophores). The soaring of birds, using air currents, is also a form of passive movement. For example, certain spiders release their silk and are borne through great distances by the air currents. Movements in water and air can also be passive. Purposeful movements are possible only through the coordinated work of a large number of muscles, which is effected by the nervous system. In all cases, the movements are the result of the interaction of forces external to the organism (gravity, environmental resistance) and internal forces (muscular tension, contraction of myofibrils, movements of protoplasm). Some aquatic animals, such as sponges and corals, which maintain a sedentary mode of life, use cilia and flagella to set their immediate environment in motion and bring them food.Īnimals can move about by (1) moving over a substrate, that is, upon solid or liquid support (walking, running, jumping, creeping, sliding), (2) moving freely in water (swimming), and (3) moving freely in air (flying). The most common construction of the locomotor organs, the limb, is a system of levers activated by muscular contractions. The organs may be pseudopodia (the slow flow from one place to another of protoplasm ameboid movement), cilia and flagella (ciliary and flagellar movement), or special body appendages by means of which the animals cling to a rough area of the substrate (setae, squamellae, scutella) or attach themselves to it (suckers). Movements are effected by specialized organs, the structure of which varies with the type of animal and depends on the type of locomotion and the nature of the habitat-terrestrial, aquatic, or aerial. Movements enable an organism to interact actively with the environment-specifically, to move from place to place and to capture food. (in biology), one of the manifestations of vital activity.Īnimals and man.
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