Locomotion among Invertebrates

Cilliary and Flagellary Locomotion

Swimming locomotion in protozoans is caused by the flagella and cilia. Flagella bring about the movement of some parasites in the body fluids of the hosts. As the movement in this case is caused by the beating flagella and cilia are also known as undulipodia.

Depending on the structure involved swimming movement can be of two types namely,

* Flagellar movement

* Ciliary movement

Flagellar movement

Flagella are the locomotory organelles of flagellate mastigophoran protozoans.

They are mostly thread like projection on the cell surface.

Types of flagellum:

1. Anematic: Flagella are without mastigonemes, e.g., Noctiluca.

2. Stichonematic: Flagella with a single row of mastigonemes on one side of the  

    flagellum e.g., Euglena.

3. Pantonematic: Flagella with two or more rows of mastigonemes on the sides,

   e.g., Peranema, Monas socialis.

4. Acronematic: Flagellum does not bear any arrangement of mastigonemes but a

   terminal filament is seen, e.g., Polytoma, Chlamydomonas.

5. Pentacronematic: When the flagellum bears two rows of mastigonemes on the

   sides and the flagellum ends in a terminal filament without mastigonemes, e.g.,   Urcoclus.

A flagellum pushes the fluid medium at right angles to the surface of its attachment, by its bending movement. The bending movement of flagellum is made by the sliding of microtubules past each other with the help of dynein arms. The dynein arms show a complex cycle of movement with the energy provided by ATP. These dynein arms attaché to the outer microtubule of an adjacent doublet and pull the neighboring doublet. As the result the doublets slide past each other in opposite direction. The arms release and attach a little farther on the adjacent doublet and again pull the neighboring doublet.

The doublets of the flagellum are physically held in place by the radial spokes and thus the doublets cannot slide past much and their sliding is limited by the radial spokes. Instead the doublets can curve causing a bend in the flagellum and this bending has an important role in the flagellar movement.

Flagellum shows the following movements,

Undulation movement: Undulation from the base to the tip causes pushing force and pushes the organism backwards. Similarly undulation from the tip to the base causes pulling force and causes the organism to pull forward. Also when the flagellum ends to one side and shows wave like movement from base to tip the organism moves in laterally in opposite direction. Finally when the undulation is spiral, it causes rotation of the organism in the opposite direction and this is called as gyration.

Sidewise lash movement: The flagellar movement of many organisms is a paddle-like beat or sidewise lash consisting of strokes namely effective stroke and recovery stroke.

Effective stroke-During effective stroke the flagellum becomes rigid and starts bending against the water. This beating in water at right angles to the longitudinal axis of the body causes the organism to move forward.

Recovery stroke– During recovery stroke, the flagellum becomes comparatively soft and will be less resistant to the water. This helps the flagellum move backwards and then to the original position.

Simple conical gyration movement: In this kind of movement the flagellum turns like a screw. This propelling action pulls the organism forward through the water with a spiral rotation around the axis of movement and gyration on its own.

Ciliary movement

Just like the flagellum, the cilium also shows back and forth movements during the locomotion. These back and forth movements of the cilia are also called as effective and recovery strokes respectively. Cilium moves just like a pendulum or a paddle. The cilium moves the water parallel to the surface of its attachment like that of paddle stroke movement. The movement of water is perpendicular to the longitudinal axis of cilium.

Effective stroke: During effective stroke, the cilium bends and beats against water thus bringing the body forward and sending the water backwards.

Recovery stroke: During recovery stroke, the cilium comes back to original position by its backward movement without any resistance.

Cilia shows two types of coordinated rhythms,

* Synchronous rhythm, where in the cilia beast simultaneously in a transverse row.

* Metachronous rhythm, where in cilia beat one after another in a longitudinal row. The metachronal waves pass from anterior to posterior end.

The beating of the cilia can be reversed to move backwards when a Paramoecium encounters any undesirable object in its path. The ciliary movement is coordinated by infraciliary system though neuromotor center called as motorium present near the cytopharynx in the ciliates like Paramoecium. The infraciliary system together with motorium form neuromotor system which helps in coordination of the beating of the cilia. Ciliary movement is the fastest locomotion in protozoans.

Structure of cilia and flagella

Despite their different pattern of beating, cilia and flagella are indistinguishable structurally.

All cilia and flagella are built on a common fundamental plan:

1. A bundle of microtubules called the axoneme (1 to 2 nm in length and 0.2 μm in diameter) is surrounded by a membrane that is part of the plasma membrane.
2. The axoneme is connected with the basal body which is an intracellular granule lying in the cell cortex and which originates from the centrioles.
3. Each axoneme is filled with ciliary matrix, in which are embedded two central singlet microtubules, each with the 13 protofilaments and nine outer pairs of microtubules, called doublets. This recurring motif is known as the 9 + 2 array.
4. Each doublet contains one complete microtubule, called the A sub fiber, containing all the 13 protofilaments. Attached to each A sub fiber is a B sub fiber with 10 protofilaments.
5. Subfibre A has two dynein arms which are oriented in a clockwise direction. Doublets are linked together by nexin links.
6. Dynein is an ATPase that converts the energy released by ATP hydrolysis into the mechanical work of ciliary and flagellar beating.
7. Each sub fiber A is also connected to the central microtubules by radial spokes terminating in fork-like structures, called spoke knobs or heads.

This regular arrangement of microtubules and associated proteins with the nine-way pattern is also seen in centrioles. But unlike centrioles, cilia and flagella have a central pair of microtubules, so that the overall structure is called the 9 + 2 axoneme. 

LOCOMOTION OF COELENTERATA:

HYDRA

Normally, a Hydra remains attached by the basal disc to some suitable object in the water. There it twists about and makes various movements of the tentacles and body in response to various stimuli and for the capture of food. All such movements are caused by the contraction or expansion of the contractile muscle fibres of the muscle processes of both epidermis and gastro dermis.

Actual locomotion is accomplished in several different ways which are as follows:

(i) Looping:

The most common, a type of walking similar to the looping of an inchworm or caterpillar. While standing erect, the body first extends and then bends and fixes the tentacles to the substratum by means of glutinant nematocysts. It then releases the attachment of the basal disc, reattaches the basal disc near the tentacles and again assuming an upright position by releasing its tentacles.

(ii) Somersaulting:

Somersaulting is like the looping. In this type of movement, Hydra extends its body and is bent to one side to place the tentacles on the substratum, the glutinant nematocysts help to fix the tentacles. The basal disc is freed from its attachment, and the animal stands on its tentacles, the body then contracts strongly till it appears like a small knob.

The body is then extended and bent to place the basal disc on the substratum, the tentacles loosen their hold and the animal regains an upright position. These movements are repeated and the Hydra moves from place to place. This is the normal method of locomotion.

(iii) Gliding:

Hydra can glide slowly along its attachment by alternate contraction and expansion of basal disc.

(iv) Cuttlefish-like movement:

The tentacles are fixed to the substratum and with the pedal disc up, Hydra moves over the substratum by pulling its tentacles along.

(v) Floating:

Sometimes, Hydra can produce a bubble of gas secreted by some ectodermal cells of the basal disc which helps the animal to float on the surface of the water and is passively carried from one place to another by water current or wind below.

(VI) Climbing:

Hydra can climb by attaching its tentacles to some distant objects and then releasing the basal disc and by contracting the tentacles the body is drawn up to a new position.

(vii) Swimming:

By freeing itself from the substratum and with the help of wave-like movements of the tentacles, Hydra swims in water.

HYDROSTATIC LOCOMOTION IN ANNELIDS

Annelids possess three types of locomotory structures, namely parapodia, setae and suckers.

LOCOMOTION IN EARTHWORM

Pheretima posthuma moves by alternate contraction and relaxation of circular and longitudinal muscles of the body wall. The setae and the coelomic fluid also assist in the locomotion. The body of earthworm exhibits extension, contraction and anchoring in the anterior and posterior regions during locomotion. The locomotory structures are setae, cir­cular muscle fibres arranged in the form of a ring, longitudinal muscle fibres in bands and the coelomic fluid.

Setae:

The setae are arranged in the form of a ring in all segments, except the first, last and clitellar segments.

1. A seta is a narrow, elongated, chitinous structure hardened by sclerotised pro­tein.

2. It is light yellow; the shape is in the form of a ‘f’ (Fig. 24.13), with a distal pointed end and a blunt proximal end embedded in setigerous sac in epidermal pit. A swelling, nodule, may be present in the middle.

Mechanism of Locomotion:

• Circular and longitudinal muscles of the body wall are useful in extension and contraction of the body respectively. The setae are useful in anchoring. The coelomic fluid causes turgidity during locomotion of earthworm.
• Metamerism is beneficial for the burrowing worms. The individual segment or a short series of segments can be made rigid by contraction of muscles against coelomic fluid.
• Coelomic fluid cannot be displaced as it is trapped within each segment by septa. Thus efficiency of coelom as hydrostatic skeleton is improved.
• The contraction of the muscles is local but it passes backwards from anterior end, mush like a wave.
• Forward progression of the anterior end begins by the contraction of the circular muscles of a few anterior segments.
• The wave of contraction of circular muscles passes backwards over the body of earthworm. When this wave passes over the anterior of few segments of the body, circular muscles at the anterior end of the body relax and the contractions of the longitudinal muscles sets in. thus series of waves of contraction of circular and longitudinal muscles also passes backwards one after the other continuously and pass backward during the locomotion of the earthworm. This movement is coordinated by the nervous system.
• Segments remain at rest when the longitudinal muscles contract to the maximum extent. The protractor muscles of setal sac also contract making the setae in that segments being protruded and directed backwards. With the relaxation of the longitudinal muscles and the simultaneous contraction of the circular muscles, the segments begin to move forwards on the ground. The retractor muscles of setal sacs contract at this time, making the setae in that segments being withdrawn.
• Such series of local contractions and relaxations of circular and longitudinal muscles of the body wall are repeated in the anterio-posterior axis of the body. Earthworm can move upto 25 cm/min. On smooth surface the earthworm uses its mouth as sucker and pulls the body forward. The mucus secreted by the worm helps for easy gliding on the substratum.