locomotion in invertebrates

In Protozoans like amoeba there is amoeboid movement. The plasma membrane of amoeba has adhesive properties since new pseudopodia attach to the substrate as they form the plasma membrane. A thin fluid between plasma membrane and ectoplasm facilitate this sliding. As amoeba moves, the fluid endoplasm flows forward into the fountain zone of advancing pseudopodium. As it reaches the tip of a pseudopodium endoplasm changes into ectoplasm. At the same time, ectoplasm near the opposite end in the recruitment zone changes into endoplasm and begins flowing forward. In flagellates of phylum protozoa there are present one or more whip like flagella as locomotary organs e.g. Euglena.

In ciliates the are hair like cilia present pellicle of the body e.g. paramecium. They are locomotary organs.

With the exception of arthropods locomotary cilia and flagella occur in every animal phylum structurally cilia and flagella are similar but cilia are shorter and more numerous, where as flagella are longer and generally occur singly or in pairs.

Pedal locomotion:

Many soft bodied invertebrates move by means of waves of activity in the muscular system that is applied to the substrate. In land snail Helix, several waves cross the length of foot simultaneously, each moving in the same direction as the locomotion of snail, but at a greater rate. Many large flatworms and most nemertine exhibit a muscular component to their locomotion. In this type of movement, alternating waves of contraction of circular and longitudinal muscles generate peristaltic movement, which enhance the locomotion that the surface cilia also provide. This system is most highly developed in earthworms.

Looping movement:

Leeches and some insect larvae exhibit looping movement. Leeches have anterior and posterior suckers that provide alternating points of attachment. Lepidopteran caterpillars exhibit similar locomotion in which arching movements are equivalent to the contraction of longitudinal muscles.

Polychaete worms move by alternate movement of parapodia, the tips of which move backward relative to the body; since the tips attach to the ground, the body of worm moves forward. In echinoderms e.g. sea star has fine arms with a water vascular system in each. Along each canal are reservoir ampullae and tube feet. Contraction of muscles comprising the ampullae drives water into the tube feet, whereas contraction of tube feet moves water into the ampullae. Thus the tube feet extend by hydraulic pressure and can perform simple step like motions.

Terrestrial locomotion by walking:

Invertebrates (terrestrial arthropods) living in terrestrial environments are much denser than the air in which they live. As a result those that move quickly make use of rigid skeletal elements. These elements include flexible joints, tendons and muscles that attach to a rigid cuticle and form limb. Walking limbs of crustacea are remarkably uniform in structure. Each joint is articulated to allow movement in only one plane. These limbs joints allow extension and flexion of the limb. The limb plane at the basal joint with the body can also rotate and this rotation is laterally projected limbs and movements do not involve raising or lowering the body. Depending on the arthropod, the trajectory of each limb is different and non overlapping. Most arthropods walk forward, rotating the basal joint of the limb relative to the body, but crabs walk in a sideway fashion.

Flight:

Physical properties of arthropod cuticle are such that true flight evolved for the pterygote insects some zoo million years ago. Since then the basic mechanism of flight has been modified. Consequently present day insects exhibit a wide range of structural adaptations and mechanisms for flight.

Jumping:

Some insects like fleas and grasshoppers can jump, mostly an escape reaction. To jump an insect must exert a force against the ground sufficient to impart a take off velocity greater than its weight. Long legs increase the mechanical advantage of leg extensor muscles. This is why insects that jump have relatively long legs. The mechanical strength of the insect cuticle acting as the lever in this system probably determines the limit of this line of evolution.