Introduction

Movement is one of the defining features of living things, and it shows up in strikingly different forms across the living world. At the simplest level, the streaming of protoplasm inside a single-celled organism like Amoeba already counts as a form of movement. Many organisms move using structures such as cilia, flagella or tentacles, and the human body itself is capable of an enormous range of movements -- we move our limbs, jaws, eyelids, tongue, and much more besides.

Not every movement carries the whole body from one place to another, though. When a movement does produce a change in an organism's location, and is voluntary, it is called locomotion. Walking, running, climbing, flying and swimming are all everyday examples of locomotory movement. Importantly, the structures used for locomotion are not always distinct from the structures used for other kinds of movement -- the same equipment often serves both jobs. In Paramoecium, for example, the beating of cilia both drives food through the cell's cytopharynx and propels the organism from place to place. Hydra shows the same overlap in a different way: it uses its tentacles both to seize prey and to move itself along a surface. Because the two are so often carried out by the very same structures, movement and locomotion cannot really be studied as separate, unconnected topics -- every locomotion is a movement, but not every movement is a locomotion.

The particular way an animal locomotes depends heavily on where it lives and what the situation demands of it -- movement in water, on land and in the air all call for different solutions. Whatever form it takes, locomotion generally serves one of a handful of purposes: finding food, seeking shelter, finding a mate, reaching a suitable breeding ground, moving toward more favourable climatic conditions, or escaping from a predator or other danger.