Metamerism (i.e.,division of a body into serially repeated sections along the anterior/posterior axis) appears to have evolved in the annelids as an adaptation to burrowing. This occurs both externally and internally.

Primary locomotory structures in annelids are Setae,Parapodia and Suckers

Body muscles present below the epidermis assists in locomotion.

Each segment is controlled independently.

Metamerism permits modification of body regions & creates hydrostatic compartments that help in supportive and locomotive functions.

Coelomic fluid functions as a hydraulic skeleton against which the muscles act to change the body shape.

Organs of locomotion are Parapodia -(biramous appendages) with many setae.-used both in creeping and in swimming.

Locomotion in polychaetes is by the combined action of parapodia, body musculature and to some extent the coelomic fluid.

CLASS POLYCHAETA(Gr., poly = many + chaete =

bristles)

Each parapodium( biramous structure) -2 parts an upper or dorsal blade,the notopodium a lower or ventral blade,the neuropodium Each of these is further divided into 2 lobes -an

upper and a lower. Dorsal margin of notopodium-short, cylindrical,

tactile appendage- dorsal cirrus. Ventral margin of the neuropodium - ventral cirrus. Both the notopodium & neuropodium have a

bundle of bristle;like setae or chaetae lodged in a setigerous or chaetigerous sac.

In the middle of each bundle of setae & deeply embedded in the parapodium is found stout, straight, thick and dark coloured chitinous rod the aciculum which projects only a short distance bt does not project beyond the outer edge or the parapodium.

At its inner end the aciculum has attachad muscles by which protrusion and retraction of the parapodium occurs.

The two acicula constitute the

endoskeleton of parapodium and serve to support

and for attachment of the

setal muscles.

SURFACE DWELLERSLocomotion in free-moving polychaetes is by the action of parapodia, body musculature and coelomic fluid.

For ex, when a worm such as Neanthes moves slowly the contractual force comes from the sweeping movements of the parapodia.

During locomotion each parapodium performs two strokes:an effective or back stroke and recovery or forward stroke.

In the effective stroke,the aciculum is extended so that the parapodium is lowered to come in contact with the substratum and moves backwards against the substratum.

In the recovery stroke, the aciculum is retracted so that the parapodium is lifted above and moves forward.

The combined effective and recovery strokes of numerous parapodia propel the worm forward.

The parapodia of the two sides work alternatively causing successive waves along each side of the worm.

LOCOMOTORY PATTERN IN NEREIS

FAST CRAWLINGBody undulations, which help the worm to crawl or swim rapidly, are produced by the contraction of longitudinal muscles stimulated by the backward stroke of parapodium of a particular segment.These contractions coincide with the alternating waves of parapodia on the two sides.The longitudinal muscles of one side contract when the parapodia of that side are moved, the muscles relax when parapodia sweep backwards.

Polynoidae use their muscular parapodia as efficient walking device.In the scale worm Aphrodita (sea mouse), the entire dorsal surface is covered by hairlike “felt”, composed of setae that arise from the notopodia & trail back over the dorsal surface of the animal.

BURROWERSMany polychaetes have become adapted for burrowing.Lumbrinerids and Capitellids, move through the substratum by peristaltic contractions. The circular muscle layer is well developed, and the septa effectively compartmentalize the coelomic fluid and localize its skeletal function.Nephtys enters its head first into the substratum, anchors the body by extending the chaetae laterally from the buried segments & then extends the proboscis deeper into the sand by a swimming motion.

Highly efficient burrowers have secondarily lost most of the intersegmental septa, or have septa that are perforated (e.g.,Arenicola, Polyphysia).The loss of complete septa means a loss of coelomic fluid from one body region causes a corresponding gain in another.They have reduced parapodia;the chaetae ,or simply the surface of the expanded portions of the body, serve as anchor points.In Polyphysia, peristaltic waves move constricted body regions forward while the anchored parts provide leverage; the constricted areas are reduced both in diameter & in length by simultaneous contraction of both the circular & longitudinal muscles.

In Arenicola, movement through the burrow is usually by peristaltic contractions; the parapodia are greatly reduced & are in part represented by transverse ridges provided with setae modified into hooks, called uncini for gripping the tube wall.

Pectinariid worm bears rows of large, conspicuous golden setae that are used in digging in soft sand/mud.

Glycera, a long, sleek worm, burrows rapidly using its large, muscular proboscis bearing hooks.The proboscis is thrust into the substratum and swelled;then the body is drawn in by contraction of the proboscis muscles.

PELAGIC POLYCHAETESTomopteridae, have lost the

setae and possess membranous parapodial pinnules;Their swimming movements are similar to the crawling species.

In Nephtys, the large, fleshy parapodia serve as paddles.

CLASS OLIGOCHAETA (Gr.,oligos= few +chaete=bristles)

No specialised locomotory organs are found.

The process of locomotion is a cumulative effect of contraction & relaxation of both the muscle layers of the body wall coordinatedby the nervous system) aided by setae and

the hydrostatic pressure created by the coelomic fluid.

EARTHWORM

During forward movement, circular muscles at the anterior end contract due to an increase in the hydrostatic pressure of the anterior segments; this wave-like contraction then passes backwards.This results the anterior region to extend forward and at the same time making it thinner in diameter. The anterior end now grips the substratum & the setae acts as hooks by their posteriorly directed points.

When the wave of contraction nears the mid-region of the body, circular muscles relax & longitudinal muscles of the anterior end contract, this shortens and thickens the anterior end causing the posterior body of the worm to be dragged forward.The setae are extended to prevent backward movement of the segment.The wave of contraction of the longitudinal muscles passes backwards. Again a wave of contraction of circular muscle starts from the anterior end before the first one has reached to the posterior end.

Thus locomotion is brought about by alternate contractions of circular & longitudinal muscles causing wave of thinning and thickening to pass backward.This involves partly a pushing of the anterior end and partly a pulling of the posterior end, the setae playing an accessory role.

The direction of contraction waves and setae can be reversed, thus enabling the worm to crawl backward.

When moving on smooth surfaces, earthworms employ mouth as a sucker as the setae cannot anchor the substratum.

CLASS HIRUDINEA(L., hirudo=leech)

Move in looping/ inchworm-like motion or swim with undulations.Without setae and parapodia, the anterior and posterior suckers serve as points of contact with the substratum against which the muscle action can operate.

INCHWORM-LIKE MOTION OF INCHWORM-LIKE MOTION OF LEECHLEECH

When the posterior sucker attaches to a surface, the circular muscles contract, beginning at the posterior end. The leech thus elongates and the anterior sucker fastens to the surface.Then the posterior sucker is released, a wave of contraction of the longitudinal muscles moves in a forward direction; this completes one cycle.

During swimming, the dorsoventral muscles maintain a contracted state, and undulatory waves pass in a vertical plane over the body from the anterior to the posterior end due to contraction of the longitudinal muscles.

BIBLIOGRAPHY[Brusca & Brusca, 1990] R.C. Brusca, & G.J.

Brusca, Invertebrates, Sinauer Associates, 1990.