The document summarizes the locomotion mechanisms of annelids. It describes that annelids evolved metamerism which allows modification of body regions and creates hydrostatic compartments to aid supportive and locomotive functions. The primary locomotory structures in annelids are parapodia, setae, and body muscles. Parapodia contain setae and acicula which aid locomotion through effective and recovery strokes. Locomotion varies between surface dwellers, burrowers, and pelagic species through different coordinated actions of parapodia, body musculature, and coelomic fluid.

1.  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.

2. 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.

3.  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)

4.  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.

5.  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.

6.  The two acicula
constitute the
endoskeleton of
parapodium and
serve to support
and for
attachment of the
setal muscles.

8. SURFACE DWELLERS
Locomotion 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.

9. 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.

10. 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.

11. LOCOMOTORY PATTERN IN
NEREIS

12. FAST CRAWLING
Body 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.

13. 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.

14. BURROWERS
Many 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.

15. 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.

16. 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.

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

18. 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.

19. PELAGIC POLYCHAETES
Tomopteridae, 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.

20. CLASS OLIGOCHAETA
(Gr.,oligos= few +chaete=bristles)
No specialised
locomotory organs are
found.

21. The process of locomotion is a cumulative
effect of contraction & relaxation of both the
muscle layers of the body wall coordinated
by the nervous system) aided by setae and
the hydrostatic pressure created by the
coelomic fluid.

22. EARTHWORM

23. 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.

24. 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.

26. 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.

27. 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.

28. 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.

29. INCHWORM-LIKE MOTION OFINCHWORM-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.

30. 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.

31. BIBLIOGRAPHY
[Brusca & Brusca, 1990] R.C. Brusca, & G.J.
Brusca, Invertebrates, Sinauer Associates,
1990.