J. Exp. Biol. (1969), 51, 47-58
With 9 text-figures
Printed in Great Britain
47
LOCOMOTION AND COELOMIC PRESSURE IN
LUMBRICUS TERRESTRIS L.
BY M. K. SEYMOUR
Department of Zoology, The University, Hull
(Received 4 November 1968)
INTRODUCTION
The movement of earthworms has been considered from practical and theoretical
standpoints by many authors, for example, Gray & Lissman (1938), Yapp (1956),
whose contributions, recently brought together by Gray (1968), have facilitated
interpretation of the results presented here. On the basis of this previous work, normal
locomotion in the earthworm can be summarized briefly as follows: in forward
locomotion a wave of circular muscle contraction, causing segmental elongation,
passes back from the anterior end. When this retrograde wave has passed about halfway down the body, a wave of longitudinal muscle contraction, resulting in fattening
of the worm, begins to follow it from the anterior end. Longitudinally contracted
segments form a foot or point d'appui in contact with the substratum, and as the cycle
is repeated each segment, as it elongates and then shortens again, is lifted and moves
forward a step of 2-3 cm. (Gray & Lissman (1938) and Yapp (1956)). If the retrograde
locomotory wave disappears before reaching the posterior end, the remaining segments are pulled forward passively. Backward locomotion usually follows a noxious
stimulus to the anterior end; there is dorso-ventral flattening of the posterior end and
the passage of a locomotory wave similar in form to that in forward locomotion but in a
posterior to anterior direction (Yapp, 1956).
The purpose of this study has been to correlate coelomic pressure and locomotory
activity in the earthworm using sensitive continuous recording techniques. Coelomic
pressure has been recorded by Newell (1950) from 'actively wriggling worms', but
not apparently during normal locomotory movement. Although pressure fluctuations
occurring over less than \ min. could not be shown with the techniques then used,
nevertheless, Newell observed large fluctuations of pressure, noting that the highest
pressures occurred when the circular muscles were fully contracted.
MATERIAL AND METHODS
Internal pressure fluctuations were recorded from single segments of Lumbricus
terrestris L. (4-6 g. wt.) by the insertion of one or two recurved stainless-steel cannulae
of 07 mm. bore through the body wall. These were linked via 1 mm. bore flexible
polyethylene tubing to Statham P23BB physiological pressure transducers. In some
cases the cannula was fixed, immobilizing the segment from which recordings were
being obtained relative to the substratum; in others the cannula was free to move with
the worm. Transducer output was displayed using an E & M Instrument Co. Inc.
48
M. K. SEYMOUR
multichannel pen recorder, the Physiograph. Pressure was recorded during locomotory
movement over wax, sand, earth or damp filter paper and during burrowing into loose
earth. Directly observed events were marked synchronously on the trace by manual
key and correlation of variations in segmental dimensions and pressures was achieved
by the analysis of 16 mm. film taken simultaneously with internal pressure records.
EXPERIMENTAL OBSERVATIONS
(a) Introduction
The segments are functionally separate during active movement (Newell, 1950) and
' The internal hydrostatic pressure induced by a shortening of the circular muscles
provides the force which elongates the segment against all the forces, both internal
1 sec
20
1 sec
(b)
10
1 sec
1sec
1 sec
1 sec
Fig. i. Pressure gauge records from the body cavity of Lumbricus during locomotory movement
on a smooth wax surface to which the cannula was fixed. Timing and duration of observed
thinning of the recording segment by circular muscle contraction are marked by elevation of
the time-scale baseline in the left-hand sequences. Thickening, by longitudinal contraction is
marked in the right-hand sequences, a is from segment 10, showing backward locomotory
waves. Time between sequences is 40 sec. a and c are from segment 82 of another individual
and show backward and forward waves respectively. Times between sequences are 113 and 51
sec. In c, circular muscle (CM.) and longitudinal muscle (L.M.) peaks are indicated.
and external, which oppose elongation; similarly it (i.e. internal pressure) provides the
force which stretches the circular muscles when the longitudinal muscles shorten'
(Gray, 1968). It could be inferred from these considerations that coelomic pressure in a
segment would vary regularly with each locomotory wave and would increase upon
contraction of the longitudinal and circular muscles, and that similar variations would
occur successively in successive segments. Pressure gauge recordings such as those
reproduced in Fig. 1 have verified these inferences.
(b) Pressurefluctuationsin relation to activity
A pressure pulse one to several seconds long corresponds to the observed elongation
of the cannulated segment caused by contraction of the circular muscles. The wave of
shortening and fattening which next affects the segment as a locomotory wave passes
Locomotion and coelomic pressure in Lumbricus terrestris L.
49
may or may not be manifested as a second pressure peak on the record (compare
Fig. 1 c, left-and right-hand records, and Fig. za, b). In normal crawling (see Fig. 3a),
the longitudinal muscle ('L.M. ') peak more rarely exceeds the circular muscle ( ' C M . ')
peak in amplitude, as in Fig. 36.
Fig. 2. Generalized diagrams summarizing coelomic pressure fluctuations (vertical scale) and
observed changes in single segments of Lumbricus during the passage of either forward or backward locomotory waves. Horizontal hatching represents thinning and elongation of the
recording segment by contraction of the circular muscles; vertical hatching, thickening and
shortening by contraction of the longitudinal muscles, a illustrates duration of setal protrusion (horizontal bar), large and small pressure pulses associated with elongation and
shortening respectively and the timing of segmental shape changes most commonly observed.
b illustrates backward slip of the anterior of the two possible co-existing feet when immediately
in front of the recording segment (horizontal bar), the frequent lack of any pressure pulse associated with segmental shortening, and a common variant of the timing of segmental shape
changes.
O 10
X 8
S 6
u
8
4
0
10
8
6
4
2
0
CO
(a)
•
8 S
8 r
6
4
2 •
0 •
(6)
Fig. 3. Lumbricus coelomic pressure readings at 0-5 sec. intervals from a pressure gauge record
during the passage of backward locomotory waves are plotted above successive diagrams of the
recording segment. Segmental length and diameter are measured from simultaneously exposed
cine film and both drawn to the scales shown, a and b are from segments 15 and 77 respectively.
4
Exp. Biol. 51, 1
So
M . K. SEYMOUR
Measurements from cine film of segmental diameter and length during crawling
movement are plotted below the simultaneous pressure record in Fig. 4. Figures
4a and b illustrate the correspondence of higher and lower pressure pulses with segmental elongation and thickening respectively, as already noted.
Simultaneous pressure records from two segments, twenty-six segments apart,
demonstrate the passage of a retrograde wave in forward crawling (Fig. 5). Successive
+5
Segmental
0
Length
(mm.)
-5
Segmental
Diameter
(mm.)
Pressure
(cm.HaO)
5 sec.
5 sec.
(.b)
Fig. 4. Simultaneous graphs of coelomic pressure (upper traces) and dimensions of the
recording segments (middle and lower traces) in Lumbricus during the passage of backward
locomotory waves. Recording conditions are as for Fig. i. The pressure record is continuous.
Segmental dimensions were measured from cine film at 0-5 sec. intervals, a and b are from
segments 70 and 15 respectively. Further information is in the test.
diagrams of the anterior half of a worm (after Gray & Lissman, 1938) indicate the
state of contraction of the cannulated segments. Segment 38 shows little or no discrete
L.M. pulse, whereas segment 12 does show such a pulse (compare Fig. zb and a).
Pulse amplitude is markedly different in the two recording segments, demonstrating
that the musculature of different parts of the worm may show different levels of
Locomotion and coelomic pressure in Lumbricus terrestris L.
51
activity, although both contributing to the same locomotory sequence. This, and the
difference in phase between the two records, show clearly how the anatomical isolation
due to the presence of septa allows the functional isolation evident in and necessary
for the earthworm's mode of progression. In contrast to these features in Lumbricus,
Arenicola has a virtually undivided trunk coelom yielding simultaneous anterior and
posterior pressure records showing peaks identical in form, timing and amplitude
(personal observation).
I
II
III
IV V V I
Fig. 5. Simultaneous records of coelomic pressure of Lumbricus in segments 12 (above) and 38
(centre) during the passage of a backward locomotory wave resulting in forward locomotion.
The cannulae were free to move with the worm, which was crawling over damp earth. Diagrams
of the anterior half of a worm showing the shapes and relative positions of the recording segments
at various stages of the locomotory cycle are shown below (partly after Gray & Lissman (1938)).
Stages I to III show the thinning phase passing back from the anterior end, and IV to VI the
thickening phase following it; stage I is the same as stage VI except that by the latter stage the
worm has moved forward by one step of 2-3 cm. Further information is in the text.
A coelomic pressure record from an anterior segment (10) during burrowing into
soil (Fig. 6) shows very brief thinning plus elongation phases associated with a
reduction in pressure, separated by longer intervals of thickening plus shortening
developing relatively high pressure. The latter phase under these conditions serves
two functions; to pull the more posterior segments into the soil and to dilate the
burrow being formed. In fact to be physically able to shorten the buried segments
the worm must be able to expand them against the considerable resistance of the soil,
and it is for this dilation, which also produces the necessary anchorage in the soil, that
high pressure is primarily required (see Trueman (19666) on the dilation anchor in the
burrowing of Arenicola). In the sequence described here the soil contained narrow
4-2
52
M. K. SEYMOUR
fissures easily penetrated by the anterior segments (which, as noted below, are
thinned and extended only by their circular muscles). It is of interest to note that
the pressure record before the cannulated segment entered the soil is of the typical
crawling type, with high CM. peaks. Thus, although the muscular sequence was
qualitatively identical, the reversal of phase of the pressure record clearly shows the
emphasis on the circular and longitudinal muscles in crawling and burrowing respectively. These results tend to support Chapman (1958) in his view of the importance
25
r
20
15
Fig. 6. Record of coelomic pressure from segment 10 oiLumbricus burrowing into soil of open
texture in a glass dish. The worm was about one-quarter buried, the cannula being free to move
with the worm. Elongation of the recording segment is marked from direct observation by elevation of the time-scale baseline.
Fig. 7. Pressure records from the body cavity of Lumbricus during the passage of backward
locomotory waves. Recording conditions are as for Fig. i. a, is from segment 62 and duration
of protrusion of setae is marked from direct observation by elevation of the time scale baseline.
b, is from segment 82 of another individual and backward dragging of the foot formed immediately anterior to the recording segment is marked.
in burrowing of the longitudinal muscles which he calculates can exert ten times the
force of the circulars. In this connexion Manton (1965) points out the need for
maintenance of the burrow by actively pressing the walls outwards, showing that
large Sicilian earthworms do this when shortened and thickened by the longitudinal
muscles. It must be stressed, however, that contraction of the longitudinal muscles
cannot extend a worm into its burrow and the force used for further penetration is
limited to that developed by the circular muscles.
Frequency of locomotory waves varies from 5 to 20 per min.; comparable to the
figure given by Gray (1968), and as frequency varies so also does the shape of the
recorded pressure peak (see Fig. 1). Protrusion of the setae has been observed and
marked upon the pressure record (Fig. ja). It occurs as the longitudinal muscles
contract to form a foot, increasing the resistance of the foot to backward slip, and the
setae are retracted as elongation begins. Yapp (1956) has described backward slip as
occurring when the anterior segments shorten, as shown in Fig. 1 of Gray & Lissman
(1938). Such slip has been observed and marked (Fig. 76), but its precise timing
depends on which segment is providing the pressure record; the timing shown in
Figs. 76 and zb refers most accurately to the region of segment 10.
Locomotion and coelomic pressure in Lumbricus terrestris L.
53
(c) Pressure values and activity
Table 1 illustrates mean CM. and L.M. peak pressure values over ten cycles when a
clear locomotory wave was passing, CM. pulse pressure values show a marked excess
over L.M. Negative pressures appear during the latter and are discussed below. Overall
mean values are lower than those recorded by Newell (1950) using a manometer and
Table 1. Circular muscle ( C M . ) and longitudinal muscle (L.M.) peak pressures during
locomotory movement in Lumbricus. Units are centimetres of water. The overall mean for
each worm is the mean of all L.M. and C M . peak values for that worm. Further information
is in the text.
Segment
S
S
s
10
12
17
99
120
Direction
of movt.
CM. range
Forward
Forward
Backward
Forward
Forward
Forward
Forward
Forward
4-0
7-0
7-0
8-5
6-5
9-5
16-0
7-5 to io-o
to
to
to
to
to
to
to
25-0
15-0
11-5
12-0
14-5
17-0
250
CM. mean
890
13-05
H-4S
9'6S
8-17
9-00
13'iS
20-90
L.M. range
o-o
-2-0
i-o
2-5
6-5
to
to
to
to
to
i-o
i-o
4-0
4-5
10-5
— I-O tO 2 - 0
i-o to 4-0
4-0 to 1 i-o
L.M. mean
O-2O
-o-55
2-15
3 9°
775
-0-30
2-05
6-6o
Overall
mean
4-55
6-25
6-8o
678
7-96
4-35
7-60
13-25
Worm
no.
24
25
25
15
8
17
26
28
O 80
ffi 70
60
e
•^
50
|40
Z 30
TrnTTTTTH 111111111 111 ITT 1 11 11 111 1111 11111111111111111
Fig. 8. Record of coelomic pressure from segment 31 of Lumbricus during violent squirming
movements. The worm was tapped firmly with the fingers just before the start of the sequence
and as shown by the horizontal bars. Recording conditions as for Fig. 1.
spoon gauge (12 and 11 cm. H2O respectively) but his worms were wriggling and this
activity produces higher pressures than crawling. Indeed the highest pressure
recorded (75 cm. H 2 O; Fig. 8) accompanied violent squirming, with evident contraction of all the body-wall muscles following firm manual stimulation.
No definite anterior-to-posterior gradient of mean internal pressure as implied by
Newell (1950) emerges from Table 1, but the data therein were deliberately taken
from segmental pressure records during clear locomotory movement. I suspect that a
gradient usually exists, its slope depending on the direction of locomotion; for instance,
a retrograde wave, vigorous at the anterior end, may die out completely before reaching
the posterior end of the worm (Yapp, 1956). Pulse pressure would then undoubtedly
be higher in the region in which the waves begin than in the tail. In Fig. 9 posteriorto-anterior locomotory wave affects the cannulated segment (5) (to the left of the
arrow). A retrograde wave initiated at the head end begins (at the arrow) and the CM.
pulses increase dramatically in amplitude, suggesting that locomotory waves may
produce maximal pressure in the first segments they affect and lower pressures in
the later ones.
M. K. SEYMOUR
54
Resting pressure in Lumbricus is here defined as a low steady pressure recorded in the
absence of organized movement such as crawling or wriggling. Range and mean of
not less than fifteen readings over at least half a minute are given for each cannulated
segment in Table 2. Resting pressures are very low, and as in crawling, may show
negative values. Newell's (1950) lowest value in a non-anaesthetized worm is 2 cm. H2O
and only in narcotized worms did he record zero pressure. The baseline of locomotoiy
pressure records usually rises during a phase of activity and may fluctuate (Fig. ya).
When locomotory movements cease the pressure falls steadily over several minutes to
a resting level.
^
25
§ 20
*
15
-Z 10
I 5
£ °
~ S ~ rr-rTT
Fig. 9. Record of coelomic pressure from segment 5 of Lumbricus. To the left of the arrow a
direct locomotory wave is passing from the posterior region. At the arrow this is replaced
by a backward wave and at once the amplitude of pressure pulses associated with segmental
elongation increases. Elongation is shown by elevation of the time-scale baseline. Recording
conditions are as for Fig. 1.
Table 2. Resting pressure in Lumbricus. Units are centimetres of water. Further
information is in the text
Segment
4
10
17
17
20
42
73
90
no
Range
13
-1-9
o-i
o-o
to
to
to
to
22
i-6
i-o
0-2
O-2 to O'7
o-o to o-o
o-6 to 1-5
- 3 - 0 t o — i-o
0-2 to 0-7
Mean
1 99
Worm no
036
24
IS
0-48
16
0-03
0-41
i7
14
14
IS
17
16
o-oo
1 03
— I-go
037
In Lumbricus resting pressures are remarkably low and free from fluctuations when
compared to those of Arenicola marina (L.), the lugworm (see Trueman, 1966a). In
the trunk of Arenicola, which is functionally non-septate, resting pressure never
ceases to fluctuate, usually around 5 cm. H2O, in healthy specimens and this is
attributable to continual muscle stretch-receptor feedback in virtually the whole
trunk body wall surrounding the large coelom. The fully segmented structure of the
earthworm is more dimensionally stable since the septa do now allow such extreme
dilation and divide the body into a series of small compartments so preventing distortion of the worm under its own weight. The small but appreciable pressure recorded in Arenicola seems necessary to prevent such distortion. Additionally, the
inherent stiffness of the body wall in Lumbricus means that high pressure is necessary
only for activity (such as locomotion) involving rapid and pronounced changes of
Locomotion and coelomic pressure in Lumbricus terrestris L.
55
shape; the resting body shape can be maintained even when, with complete muscular
relaxation, the stiff, elastic nature of the body wall permits zero or even slightly negative
pressures to be developed and maintained.
Negative pressures, of the order of those observed in resting, i.e. — 1 or — 2 cm.
H2O, have been recorded during both gentle (Fig. 4a) and vigorous crawling (Fig. 9,
right-hand half). In both cases the outward rebound of the body wall on relaxation
of the circular muscles is almost certainly responsible and the recording of a negative
pressure shows that longitudinal muscle tension at that instant is slight. It is suggested
that the body wall, which is stiffened by the cuticle and subepidermal connective
tissue, may act as a spring, storing potential energy upon circular muscle contraction
which can be released to aid the longitudinal muscles in shortening on the relaxation
of the circular muscles.
DISCUSSION
The results presented here indicate that, as suggested by Gray (1968), the force
exerted on the fluid contents of each segment by one set of muscles builds up a pressure
which is applied to and stretches the antagonistic set of muscles, resulting in overall
locomotory movements. Thus contraction of circular muscles extends, and contraction of longitudinal muscles thickens, the worm, the contents of each segment acting
as a hydraulic fluid. Figure 3 a summarizes this, showing changes in length and diameter of a segment (15) at 0-5 sec. intervals and the associated changes in internal
pressure. Indeed the results shown in Figs. 3 and 4 demonstrate the point, implicit in
Gray's statement, that segmental length and diameter are always found to vary
reciprocally, and this observation particularly supports the validity of Chapman's
(1950) consideration of a worm as a closed cylinder of fixed volume.
In forward progression, that anterior part of the worm undergoing thinning before
the formation of a foot at the extreme anterior end can only be elongated through the
agency of the circular muscles; this is a restriction of the fully septate condition which
is not applicable to a largely non-septate worm such as Arenicola, in which part of the
body can be extended using pressure built up by either circular or longitudinal muscle
contraction elsewhere in the body wall. Figure 3 a illustrates the relatively high pressure necessary to extend an anterior segment using the circular muscles only. Any
segment more than about half-way back, however, is during its elongation also pulled
longitudinally by the shortening of segments in front of it. In Fig. 3 b, from the posterior extending segments is emphasised, L.M. peaks being higher than CM. peaks. It
should be noted that in forward locomotion segments which are in between one foot
and the next are moving forward relative to the substratum and, as noted by Yapp
(1956), are lifted off it. This lifting, occasioning some increase in turgidity during the
forward-moving phase, may thus partly account for the amplitude of the CM. pressure
pulse and if so must necessitate some antagonistic contraction in the form of controlled
relaxation of the longitudinal muscles during the contraction of the circular muscles.
For greatest economy of energy expenditure, however, such antagonism must be kept
to a minimum and the resistance to contraction, shown to be present by the appearance
of a pressure increase on the recording, probably consists of unavoidable external and
internal friction and of inertia.
From the foregoing discussion of pressure and activity data it is clear that the
demands made on the musculature of any segment vary according to the level of
56
M. K. SEYMOUR
activity, the type of activity and the stage reached in the cycle of movement. At times
one set of muscles is predominantly active and this predominance is usually reflected
by the pressure record; thus the circular muscles alone are responsible for elongating
the head end before an anterior point d'appui is formed; the longitudinal muscles
alone can dilate the body forming an anchor in the burrow and are then able to draw the
posterior segments into the burrow. The presence or absence of a discrete L.M. pulse
seems to depend on the segment position and the extent to which dilation or shortening are required by the worm. For example, burrowing requires active shortening and
dilation, as noted above, and in crawling the extent to which the longitudinal muscles
actively pull forward more posterior segments, or shorten more passively through the
extension of those segments probably determines the presence and relative amplitude
of the L.M. pulse.
Gray & Lissman's (1938) and Yapp's (1956) description of locomotion, summarized
in the introduction to this paper, may now be supplemented in the light of these
investigations. Changes in one segment are described with particular reference to the
information contained in Figs. 2 and 5. (a) Circular muscle contraction builds up a
pressure of about 12 cm. H2O which extends the segment, elongating the longitudinal
muscles. Rate of contraction may decrease abruptly as in Fig. 2 a, or more gradually as
in Fig. zb.(b) As contraction of the circular muscles ceases and that of the longitudinal
muscles begins, the segment is at its longest. Continuation of longitudinal contraction
shortens the segment and may develop a pressure of about 7 cm. H2O peak amplitude,
dilating the segment and elongating the relaxing circular muscles. During the period
of shortening the setae emerge (Fig. 2a). The group of fully dilated and shortened
segments with protruded setae forming a foot may slip backwards as segments immediately behind it shorten (Fig. zb).
From equations given by Chapman (1950) the following can be derived:
(a) Tc = 2 g./cm.2;
T, = ^
g./cm.2,
where Tc and Tx are the tension per cm.2 of cross-sectional area in the circular and
longitudinal muscles; r is the radius of the worm; t the radial thickness of the circular
muscle layer; Ax the cross-sectional area of the longitudinal muscle layer and p the
internal pressure. The highest pressure recorded, of 75 cm. H2O. (Fig. 8), was in an
unsupported worm (of 6 g. weight) relying on its circular muscles to withstand the
antagonistic contraction of the longitudinals. For this worm, substitution of measured
dimensions and pressure in the above expressions gives tension values of 1323 g./cm.2
for the circular muscles and 265 g./cm.2 for the longitudinals. Thus the longitudinals
are 'understressed* by a factor of 5 relative to the circulars. Although this factor is less
than that given by Chapman (1950), it clearly shows the greater potential strength of
the longitudinals at a given pressure, in accord with their function of dilating the body
and thrusting aside soil particles as emphasized by Chapman. If the mean locomotory
L.M. and CM. peak-pressure values (7 cm. H2O and 12 cm. H2O respectively) are
substituted in the above expressions, the mean circular muscle tension in crawling is
shown to be 212 g./cm.2 and the longitudinal tension only 25 g./cm.2; in comparison
with the calculated maximum figures these values are clearly well within the worm's
capacity to develop.
Locomotion and coelomic pressure in Lumbricus terrestris L.
57
Roots & Phillips (i960) describe soil entry by Lumbricus terrestris and figure
dilation of that anterior part of the worm below the soil. The maximum tension found
here for Lumbricus is, as noted above, 1323 g./cm.2 for the circular muscles of an unconstrained worm developing 75 cm. H2O pressure. Assuming that this tension
is attainable also by the longitudinal muscles (when the body wall is supported by the
soil over the region of longitudinal contraction) the internal pressure available for
dilation of that part of the body wall against the soil would be as great as 376 cm. H 2 O.
This calculated maximum indicates the magnitude of forces which are available for
burrow formation and maintenance. Manton (1965) gives a value of 80-110 g./cm.2,
exerted against a glass slide, for a specimen of Allolobophora, comparable to the 75 cm.
H2O noted in Lumbricus and suggesting similar burrowing and 'burrow-packing'
forces for both species.
SUMMARY
1. Crawling movement and burrowing of Lumbricus terrestris (L.) have been studied
by continuous recording of internal pressure, direct observation and analysis of cine
film. Frequency of locomotory waves is from 5 to 20 per min. Timing of protrusion of
setae and of backward slip of points d'appui in locomotion have been observed and
recorded.
2. In normal locomotion elongation of segments by contraction of the circular
muscles gives rise to a discrete pressure pulse; shortening, by contraction of the
longitudinal muscles, may or may not do so, depending on the position of the segment
in the worm and the relative extent of contraction of the longitudinal and circular
muscles.
3. Consideration of crawling and burrowing pressure records emphasizes the
importance of (a) the circular muscles in extension of the head end in crawling and in
initial penetration of the soil, and (b) the longitudinal muscles during burrowing, in
dilating the burrow and drawing in more posterior segments
4. Mean pressures at circular and longitudinal muscle contraction are 12 and 7 cm.
H2O respectively. The highest pressure recorded was 75 cm. H2O and accompanied
violent squirming with evident contraction of all the body wall muscles. Resting pressures, shown in the absence of organized movement, are low (mean 0-26 cm. H2O).
In both resting and crawling negative pressures sometimes occur and these are considered in relation to the inherent stiffness of the body wall and to the septate condition.
5. Tension in the longitudinal and circular muscle layers of a worm developing
75 cm. H2O internal pressure are calculated to be 265 and 1323 g./cm.2 respectively,
demonstrating in this example that, relative to the circulars, the longitudinal muscles
are understressed by a factor of 5. Mean locomotory L.M. and CM. peak values yield
tension values of only 25 and 212 g./cm.2 respectively, and these are clearly well
within the worm's capacity.
I thank Dr E. R. Trueman for advice and encouragement, and for reading the
manuscript; and Professor J. G. Phillips for research facilities. This work was carried
out under the tenure of a Science Research Council studentship.
58
M. K. SEYMOUR
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