Invertebrate Locomotion notes PDF

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Summary

notes on the locomotion of invertebrates ...

Description

Invertebrate Locomotion •

Many invertebrate animals have hydrostatic skeletons.

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Essentially a fluid filled space surrounded by muscle.

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Contraction of circular muscles narrows the fluid filled space •

But because a property of fluid…is that you can’t alter its volume

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The fluid filled space must become longer (elongate)

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Relaxation of circular muscles and contraction of longitudinal muscles makes the space shorter but fatter

Annelid -oligochaete i.e. earthworm: •

Each cavity has an outer circular and an inner longitudinal layer of muscles.

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Alternating contractions of circular and longitudinal muscles – result in a wave of movement down the body

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Bulging segments (Circular relaxed, longitudinal contracted) anchor the animal to the substrate.

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Setae help anchor these segments to the substrate and these state effectively pull the segments behind the anchor forwards.

Polychaete worms: •

Polychaete worms have many chaetae –

Usually on flesh appendages (parapodia) found on each body segment

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And they move in quite a different way, even though polychaetes and oligochaetes have hydrostatic skeletons

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Polychaete worms move differently according to how fast they need to move and whether they are crawling on the substrate or swimming.

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Crawling slowly: each parapodia is capable of moving –

Thus when walking parapodia lift off the ground and move forwards

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Upon contacting the ground the setae are protruded and the parapoidal/oblique muscle contracts pulling the body forwards (the power stroke)

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The parapodia is now pointing backwards

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At this stage the parapoidal muscle relaxes and setae are retracted causing the parapodia to lift off the ground and swing forwards

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When a parapodium on one side is making a power stroke, the parapodium on the other side is making a recovery stroke

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When coordinated across the entire worm, it moves forwards

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When the polychaete wants to move more quickly it combines parapodia movements with sinusoidal S-shaped body movements.

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Longitudinal muscles in the segments contract (on the inside of the curve), shortening that part of the segment.

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And relax on the other side of the segment, lengthening that part of the segment (outer-side of the curve)

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The parapodia on the outer-side contact the ground and paddle backwards (Power stroke)

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At the same time parapodia on the inside curve lift off the ground.

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Thus the worm ‘pivots’ on those parapodia contacting the ground

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Polychaetes can also swim

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They do this via undulating movements and using their parapodia like paddles – similar to rapid walking

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The key difference is that in swimming there is an increase in the length of the sinusoidal waves AND they in crease in amplitude AND they increase in frequency

Molluscs: •

In gastropods, the foot is ciliated and a large pedal gland secretes copious mucus

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mucus is also produced by numerous single celled glands found in the connective tissue, that pass to the exterior via pores between the epidermal cells.

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In small molluscs movement can be based on ciliary action alone

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In gastropods with shells the columellar muscle connects the foot or operculum to the inner surface of the shell.

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this functions to withdraw the snail into its shell or to clamp down onto a surface (i.e. a limpit)

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The tarsos muscles produce the delicate waves of contraction to produce locomotion

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But in large molluscs contraction of muscle blocks, cause waves to travel along the sole

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In some molluscs the foot muscles are separated by a mid-ventral line- This enables the two sides of the sole to operate independently of each other

Gastropod mucus- The sole is anchored to the substrate by mucus: •

Where the sole moves (when a wave passes) the mucus changes very rapidly from a firm gel to a more liquid form –

This allows the foot to glide over the substrate

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Scallops (bivalve molluscs) have a special form of locomotion, usually employed to evade predators such as starfish: They have two categories of escape locomotion – both based on jet propulsion.

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Swimming: they suck water in through their valves

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And eject it through small holes near the hinge (exhalent apertures)

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A fleshy ‘curtain’ the velum prevents water exiting through the valve apertures

Jumping: they suck in water through the gape between the valves and eject it out the same way it came in. •

A jumping scallop will usually land on the sea floor between each jump

Exoskeletons: •

May be articulating – as in insects, some echinoderms, bivalves.

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Or they may be non-articulating – ie the one-piece exoskeletons of snails.

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Because muscles cannot elongate by themselves, they must be stretched by antagonistic forces – usually other muscles

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Where exoskeletons articulate, antagonistic muscles often appear in pairs •

Flexors and extensors

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Protractors and retractors

Jumping:       

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Certain insects (fleas, grasshoppers, leaf hoppers) are capable of jumping incredible distances. Thanks to the semilunar process on the back leg joint The semilunar process is actually heavily sclerotised cuticle – but functions like a very very stiff rubber band The flexor muscle brings the tibia and femur together The extensor contracts Contraction of the extensor would normally cause the leg to straighten But in this case, the flexor remains contracted…keeping the leg bent But, the act of both muscles contracting ‘bends’ the semilunar process o Essentially the energy generated by the contracting muscles is stored in the now bent semilunar process The flexor muscles then stop contracting All the energy stored in the semilunar process is then suddenly released and the tibia is extended very rapidly The grasshopper then jumps This is a catapult mechanism – Fleas have a similar mechanism (slingshot/catapult) but rely on a resilin pad in the thorax – This pad works on the coxo-trochanteral joint

Insect flight: •

Each wing articulates with the edge of the notum (thoracic tergite)

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Its proximal end rests on a wing hinge (pleural process)

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The hinge contains lots of resilin

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Muscles run from the thorax to the base of the wing

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These direct flight muscles serve to raise and lower the wing – but also change its angle.

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But this is not the main source of power

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Most power come from indirect flight muscles

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These do not directly connect to the wing

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In Dragonflies and mayflies - contraction of the dorsoventral muscle pulls down the notum making the wing raise – the basalar muscle then contracts to pull the wing down, raise the notum and relax the dorsoventral muscle

Dipterans: Dorsoventral muscles contract to bring the notum down (and the wing up) but dorsolongitudinal muscles then contract, arching the notum, and forcing the wings down (& relaxing the dorsoventral muscles)...