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Locomotion
blackworms move by means of circular
and longitudinal muscle contractions acting on the worm's hydrostatic
internal skeleton, that is, the fluid-filled body cavity (coelom) in each
segment. However, blackworms are quite acrobatic. Several very different
forms of locomotion are possible, depending on the sensory cues present in
the worm's immediate environment. For example, if you place the worm on
wet filter paper and stroke the tail lightly with a hair, it crawls
forward by reflexive peristalsis. In contrast, if you stroke the head
lightly under these same conditions, reverse peristaltic crawling moves
the worm backwards.
To see very different movements, place the worm underwater on a flat, bare
glass or plastic surface, such as the bottom of a dish containing at least
a centimeter of clear water. Now, prodding the worm's tail initiates
several cycles of forward swimming. To accomplish this, the worm suddenly
and repeatedly twists its body into a helical or corkscrew-like shape.
Each helical twist then passes rearward, as a coordinated wave, thus
propelling the worm forward and away from the stimulus. Each wave cycle
alternates between clockwise and counter-clockwise orientations (Fig. 3).
Prodding the worm's head while it is underwater initiates an unusual
reversal behavior in which the body suddenly coils and then unfurls so
that the head and tail ends rapidly reverse their original positions.
Since the worm can't swim backwards and has no means of traction, this
seems to be a novel but practical way for the worm to make a 180° turn
and get its head removed from a menacing stimulus.
 
Figure 3 Freeze-frame
video images of clockwise (a) and counter-clockwise (b) twist of body
during corkscrew swimming in a small specimen (arrow indicates direction
of worm's forward movement). Elapsed time = 0.1 sec. 10x actual size.
 Figure
4 The worm's tail at water surface
(side view).
When the blackworm occupies natural sediments, it prefers to protrude
its tail vertically out of the bottom debris and toward the water surface.
This allows the head to continue feeding or probing in the bottom debris
while the tail extends up into the more oxygen-rich water column.
If the water is shallow enough, the worm stretches its tail all the way
up to the surface where it actually breaks the surface tension of the
water (Fig. 4). While doing this, the worm bends its tail at a right angle
with the dorsal surface facing skyward and exposed to air. Although this
is an optimal position for gas exchange, it also makes the worm's tail
especially vulnerable to predation.
To offset this problem, the worm uses its rapid escape reflexes, in
which the tail end rapidly shortens in response to the sudden onset of
threatening stimuli. Some stimuli that readily elicit this response
include direct touch, substrate vibration, or the abrupt onset of a
shadow. In fact, photoreceptors used to detect a shadow are present in the
worm's tail!
To produce the escape reflex, these sensory inputs initiate electrical
impulses in rapidly conducting, giant nerve fibers found within the worm's
ventral nerve cord. These impulses, in turn, trigger the synchronized
motor outputs and muscle activity needed for rapid tail withdrawal.
part 5
©
Carolina Biological Supply Compagny, Article used by permission
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