HYDROSTATIC MOVEMENT

INTRODUCTION

Different organism needs different things for their survival and reproduction. To obtain food and defense itself from predator they must move. Their are different mechanism for movement one of them is hydrostatic movement. Hydrostatic movement occur when dominantly muscles are involved in their movement many organism show such kind of movement including Hydra, Earthworm, Leeches and Echinoderms.

In some of the animal muscle are well develop for example as in the case of annelids while in the case of hydra due to less development of gastrointestinal and other muscle fibres hydrostatic movement is performed with the help of epidermal muscle fibres. Brown and white hydra can remain fixed at one spot for considerable time but green hydra can move often from place to place by several ingenious devices. They twist about or perform movement to change their location either in response to light or some chemical stimulus or to obtain food.

In echinoderm most peculiar and interesting role of water vascular system is to bringing about locomotion by providing hydraulic pressure mechanism.

HYDROSTATIC MOVEMENT IN COELENTERATES

There are various types of organism in coelenterates some of which are sessile while other are movable among lots of different organism of coelenterates locomotion of hydra is describe here in details

Hydra shows movement of following types :-

[a] Expansion and contraction

• Hydra attach to any substratum frequently expand and contract itself at intervals. This behaviour is refer as contraction burst. It is initiated by pacemaker located near the hypostome. This movement allow to bring food to organism with the help of tentacles that are waved around in water. Capture of the prey can be also done with elongation of one side while contraction of other end of tentacles.

[b] Looping

• In looping motion body first extend and then bend over so that tentacles attach to substratum with the help of adhesive glutinant  nematocyst.  Then pedal disc is release and bought closer to circlet of tentacles and then attach. Now tentacles are loosen body becomes erect again and again appears like a series of looping movement of caterpillar or leech.

[c] Somersaulting

• In this process of movement the tentacular end is bought forward and attach to substratum. The pedal disc is freed and moved upwards, thus by causing hydra to assume an invert posture. Now pedal disc is moved forward and attached to new position. By freeing the tentacular end and animal again comes to its upright position.
• An animal perform series of somersault by repeating the process.

[d] Gliding

• For moving the short distance along the smooth surface, hydra simply glides along its basal disc like a snail, it can slide or glide slowly over the substratum with the help of creeping amoeboid movement of cells of pedal disc. A considerable amount of distance can be covered with the help of this motion.

[e] Walking

• Occasionally hydra becomes inverted stands on its tentacles and moves in an inverted condition using the tentacles as its legs. This type of movement takes place on particular objects such as leaves in its resting time.

[f] Climbing

• While changing location in a limited area, Pelmatohydra can even climb by attaching its long tentacles to some objects releasing the foot, and then  contracting the tentacles, so that the body is lifted up.

[g] Floating

• Occasionally, hydra throws its body free and floats on water surface with waves. While other time it attach to a floating leaves or twig through its pedal disc.

[h] Surfacing

• It is a process in which hydra secretes gas bubble by the cell of its pedal disc to elevate in water and float over water surface. It the gas bubble burst, the mucous thread sustain the body on water surface due to surface tension.

[i] Swimming

• It is said that sometimes hydra frees itself from substratum and swims in water by undulating, wave like motion of tentacles and body.

HYDROSTATIC LOCOMOTION IN ANNELIDS

[a] Pheritema posthuma : Indian earthworm

• Movement of body wall involves the musculature of body wall and setae. According to the studies of Gray and Lissman the worm body undergoes the extension, anchoring and progression during the course of its progression.
•  A wave contraction affecting its circular muscles, begins at the anterior end and travels posterior. This causes the body to become thinner and longer. This will allow the body to move forward.
• There will be another wave of contraction followed by previous wave but  this time it will effect longitudinal muscles causing thickening and shortening of body. Here there will be no movement as they become anchor to the ground by protruded setae.
• This again followed by wave of thinning and the process is repeated alternately.
• It has been calculated that by this method earthworm travels about 25cm in 1 minute.
• When direction of wave reversed the worm crawls back.
• During locomotion coelomic fluid serves as hydraulic skeleton. When contraction of circular muscle takes place body becomes stiff  and aid the relaxation of longitudinal muscles.

[b]  Hirudinaria granulosa : Indian cattle leech

• It performs two types of movement looping or crawling and swimming

Looping or crawling movement

• These is perform with the help of muscles and sucker which serves for its attachment.
• Leech fix its posterior sucker firmly on substratum and secretes the slimy secretion from the sucker gland.
• This allow the contraction of circular muscles and relaxation of longitudinal muscles hence the anterior part of body is extended as far as possible.
• When leech fixes the anterior sucker it leads to relaxation of circular muscles and contraction of longitudinal muscles takes place which allow release of posterior sucker and allow to shorten the body
• When posterior end is moved forward closer to anterior sucker loop formation will take place.
• Again same process will be repeated. This type of motion is refer to as leech like locomotion.

Swimming

• It swims very actively in water. Body becomes dorsoventrally flattened during swimming and performs successive undulating movements.

LOCOMTION IN ECHINODERMS

Locomotion is performed with the help of water-vascular system which set up a hydraulic pressure. By the action of cilia lining the ambulacral canal, sea water  enters through madreporite and fills up all canals of the system including tube feet. Body is moved by the stepping action of tube feet which are alternatively adhere to and released from the substratum. One or two arms in desired direction of the movement are  raised from the substratum.

Simultaneously the ampulae of the tube feet to these arm contract by the action of circular muscles. This increase the hydraulic pressure within the tube feet, which consequently elongate, extent forward and adhere firmly to the substratum by the vacuum action of their suckers. Adherences is further strengthen by mucus secreted by the tip of tube feet.  Then by muscular activity, tube feet assumes a vertical postures, dragging the body forward. Tube feet then shorten by contracting their longitudinal muscles and forcing some water back to their ampullae. Consequently the sucker releases their hold on substratum.

During the locomotion one or two arms serves as leading arms and all the tube feet extend in the same direction in coordinated manner. However the tube feet may not work in unison. As a result the sea star move forward steadily but slowly, at a speed of about 15 cm per minute

Sea star can also climb the wall by the combined action of their tube feet

If sea star is accidently turn upside down it can correct it posture by folding of its arms. In folding action tips of one or two arm twist to bring their tube feet in contact with substratum, thus permitting their whole body to fold over and right itself

CONCLUSION

• Different animal posses different ability to move from one place to another, while some of the times single organism  have ability to move in more than one way. For example Hydra can move in different ways such as walking, climbing, surfacing etc. We had seen that in lower invertebrates body surface ejections such as cilia and flagella helps them to move while in other invertebrates their muscles help them to move. Hydra are sessile animal. They live attach by their pedal to the object in water. They are able to change their location with the help of epidermal muscle fibres. As their gastrodermal muscle fibres are less develop.
• Mainly two types of muscles are involve in locomotion in the case of annelids that are circular muscles and longitudinal muscle when first works other relaxes.
• Where as in the case of echinoderms water vascular system help them to move their arm.