Explian phenomenon of locomotion in Invertebrates



 Locomotion:

            The usage of various methods by animals  to move from one place to another for adaptation and survival is called as locomotion.

Type of tissues, organ or organ system used in locomotion

Muscle Fibers: The contractile cells of muscle tissue: smooth, cardiac, and skeletal, are called muscle fibers.

The driving force and the power behind movement in most invertebrates and vertebrates is  the Muscular tissue.

A. Types of  Muscle tissues

Muscle Tissues have following three types:

1.    Smooth Muscles

2.    Cardiac Muscles

3.    Skeletal Muscles

All these three types are described one by one following:

1.     Smooth Muscles:

·         Smooth muscle tissues have; single nucleus, are spindle shaped, and these are arranged in a parallel pattern to form sheets.

·         These muscles are also known as involuntary muscles because their contractions are not under the control of higher brain centers.

·         It contracts slowly, and it doesn’t fatigue easily because it can sustain prolonged contractions.

·         A predominant muscle type in many invertebrates is smooth muscle.

·         The ability of bivalves to “clam up” against predators with little or no energy expenditure is all because of these smooth muscles.

 

2.     Cardiac Muscles:

·         The striated muscle fibers (cells) with their single nuclei are common in invertebrates.

·        These muscles occur in adult vertebrates only in the heart, where they are called as cardiac muscles.

·        Cardiac muscle fibers are involuntary, having a single nucleus, are striated and branched. This branching allows interlocking of fibers for greater strength during contractions.

3.     Skeletal Muscles:

·        Skeletal muscle is a voluntary muscle because it is under the conscious control of the nervous system.

·         Skeletal muscle fibers are multinucleated and striated.

·         These muscles can only pull and cannot push because they are shortened when they contract. Therefore, skeletal muscles work in antagonistic pairs.

 

B. The Muscular System of Invertebrates

 

·         There are a few functional differences among Invertebrates muscles and vertebrate skeletal muscles.

·         In arthropods,  there are at least two motor nerves.

·         One motor nerve fiber causes a fast contraction and the other a slow contraction.

·         Another variation is in certain insects flight muscles. These muscles are called asynchronous muscles because the upward wing movement activates the muscles that produce the downstroke.

C. Locomotion in Invertebrates

Invertebrates are divided into different phyla. Each phylum differs from the other one in many ways.  While keeping in view Locomotion in Invertebrates, we start from phylum Protista and end on echinoderms. Let’s talk about all phyla one by one considering representative animal of each phylum.

A.   Locomotion in Protists

Protists are generally categorized according to their mode of movement i.e. Locomotion. All protists can move either through cilia, flagella or by pseudopodia. Let’s explore each of these movements.

·         Ciliary locomotion:

Ciliary locomotion is a characteristic of ciliates such as paramecium. These ciliates have number of small/little projections called cilia. The protists use their cilia to rhythmically beat against its fluid environment.

·         Flagellar movements:

Flagellar movements are characteristic of flagellates; which are smallest of protists surrounded by one or more cylindrical projections called as flagella. Common example of flagellates can be heterotrophic Euglena.

·         Amoeboid movements:

In contrast to movements by cilia and flagella, amoeboid movements are produced by pseudopodia. The formation of cytoplasmic projections, pseudopodia, is characteristic of unicellular protozoans like Amoeba.

 

B.   Locomotion in Poriferans

·         Sponges are sessile as an adult and these spend their most of lives attached to a fixed substratum.

·         They do not show movement over large distances as do free-swimming marine invertebrates.

·         However, the sponge cells are also capable of creeping along substrata via organizational plasticity.

C.    Locomotion in Cnidarians

Cnidarians possess two body forms:

Medusa: A free- floating form is called medusa. Medusa locomote by swimming and floating. Medusae swims by jet propulsion. However, most do so weakly and are carried passively by currents over long distances.

Polyp: The other form is polyp. Polyps are generally sedentary.

 

D.   Locomotion in Ctenophores

·         Ctenophores are characterized by eight comb rows.

·          Each comb row possesses cilia.

·         These cilia help in swimming of the animal. These cilia beat and propel ctenophores through water.

·         Many ctenophores possess two tentacles and some lack tentacles completely.

 

E.    Locomotion in Platyhelminthes

·         Small flatworms(Platyhelminthes) move along the bottom by ciliary activity.

·         On their ventral (bottom) surface, a dense coat of cilia extends from head to tail.

·         The direction of the ciliary beat is tailward i.e. towards the tail that causes the animal to glide slowly forward.

F.    Locomotion in Aschelminthes

·         Locomotion in nematodes involves somatic muscles.

·         The somatic muscles are present below the ‘cuticle and hypodermis’.

·         During locomotion these somatic muscles are used to apply pressure laterally to the cuticle.

·         This pressure is opposed by the high hydrostatic pressure of the coelom and it causes dorso-ventral bending.

G.   Locomotion in Molluscs

·         Locomotion is achieved either by ciliary action or by muscular locomotory waves.
·         Mucus plays an important role in locomotion both by cilia and by muscle. Whereas adhesion or tenacity of the foot is related both to the area of attachment and to the properties of pedal mucus. 
·         Pedal wave is basically observed in snails.        

H.   Locomotion in Annelids

·         The basic features of locomotion in annelids are most easily observed in the earthworm because it lacks appendages and parapodia.

·         Movement involves extending the body, anchoring it to a surface with setae, and contracting body muscles.

·         The setae are retracted during the circular contraction period.

 

Looping movement in leeches:

·         Leeches move using their longitudinal and circular muscles, which other annelids such as earthworms use for peristalsis, with the addition of the use of their posterior and anterior suckers (one on each end of the body) to enable them to progress by looping.

·         Leeches permanently show this type of locomotion.

I.       Locomotion in Arthropods

1.      Walking or running in arthropods:
 
               Animal uses three appendages to form a plane, two on one side and the third on the opposite side. As the three appendages are on the substrate, other appendages can be moved forward and form a new triangle on the substrate.  The previous appendages can then then be moved forward.  This means that the appendages don't get tangled up with each other as they move, yet, there is always a stable platform for movement.

2.     Flying in arthropods
 
·         Nature of a foil surface--Wing acts as a foil surface, that is, it is bowed so that the air passing over dorsal surface goes at a highly velocity than the air passing over ventral surface.  Thus, the pressure is lower on the dorsal surface producing lift. Angle of attack of the wing can result in net forward motion.
·         Insect exoskeleton is quite functional with respect to flight.
·         Current thoughts are that wings developed from dorsal outgrowths in aquatic insects which were used for cooling/heating purpose. Evidence shows that wings function as radiators are shown by a variety of insects.  Butterflies flap their wings several times before taking off. Bees beat their wings in a hive to heat up the hive.
 

3.    Jumping

·         Some insects can jump. Most often this is an escape reaction.

·         To jump an insect must exert a force against the ground sufficient to impart a takeoff velocity greater than its weight.

·         Long legs increase the mechanical advantage of the 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 a limit to this line of evolution.

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J.       Locomotion in Echinoderms

·         Echinoderms primarily use their tube feet to move about, but some sea urchins also use their spines. 
·         The tube feet typically have a tip shaped like a suction pad in which a vacuum can be created by contraction of muscles.
·         Water vascular system is the locomotory factor in echinoderms.