Locomotion is a characteristic and fundamental attribute of all forms of animals. Unicellular animals swim by cilia or flagella, crawl about with pseudopodia, make withdrawal movements on a stalk that coils up like a stretched spring, or glide along without apparent deformation in shape. Multicellular animals are characterized by the development of muscles—the specialized contractile tissues unique to the animal world.
In general, these different types of motility may be grouped into three broad categories:
(1) Protoplasmic or cytoplasmic movement—the most universal and probably oldest, phylogenetically;
(2) Ciliary or flagellar movement; and
(3) Muscular contraction. At the molecular level, all motile systems depend on a relatively small group of protein molecules, actin and myosin, that polymerize to form elongated microtubules or delicate microfilaments. These two categories of cell organelle are basic to animal movement at all levels in phylogeny.
The survival and success of multicellular organisms depend on the versatility of the muscular system. During evolution, the contractile machinery of muscle system has been adopted to an almost infinite range of movements the sudden withdrawal of the startled tube dwelling worm, the jet propulsion of the squid, the gentle movement of star fish, the rapid oscillation of wings and so on.
All these diverse mode of movement depend on very similar biochemical mechanisms.
However, it was found that muscular tissue alone is not sufficient in most metazoan animals to bring about effective movement of the body. The reason is that the contraction of muscle fibre is an active process whereas their relaxation is not. So the contracted fibres must be restored to their original length by some force, if the form of the body is to be maintained.
In higher animals (vertebrates), the force is external and applied through the jointed skeleton and attached muscles. In order to achieve this force, soft-bodied animals have developed a special type of skeletal system known as hydrostatic skeleton. Its function depends upon the musculature being so arranged that it surrounds an enclosed volume of fluid.
The contraction of any one part of the musculature system sets up a pressure in the fluid which is then transmitted in all directions to the rest of the body and thus making its movement possible. With the advancement in the organization of body structure, coelomates evolved.
In coelomates the coelom or body cavity with its contained fluid provides more highly organized structural basis for the hydrostatic skeleton than that of the acoelomate animals like coelenterates and platyhelminthes.
Coelomates viz., annelids, echinoderms and many other small groups move upon hydrostatic principles but they show a greatly increased flexibility and speed of response.
Along with the coelom, another innovation in the structural organization is the development of metamerism—a plan of structure in which the body is differentiated along long axis into a series of units, each of which contains elements of some chief organ systems like appendages, nerve ganglia, excretory organs etc.
In some metameric annelids, the locomotion depends upon the segmental partitioning of coelomic fluid as well as the refinement of integration exerted by the metamerically segmented nervous system.
So the precision of various movements depend on the architecture of muscular system and concomitant specialization in associated tissues, hydrostatic skeletal system, metamerism and nervous integration. Here we shall discuss how various type of movements are performed by animals belonging to different phyla in different habitats.