ZoologicalJoumal ofthe Linnem Society (198l), 72:93-106.With 9 figures
Functional aspects of the integument in
polypterid fishes
D. M. PEARSON
Department of Biologtcal Sciences, University ofMaiduguri,
PMB 1069 Maiduguri, Nigeria'
Acceptedjar publication June 1980
The integument and the axial musculoskeletal system of fishes belonging to the family Polypteridae
(Osteichthyes, Actinopterygii) are re-examined with the purpose of clarifying the mechanical
functions of the squamation. The major integumentary axes of bone and collagen are laid down in
such a pattern as to resist that deformation of the skin resulting from axial body torsion. This
resistance will have the effect of reducing or preventing such torsion and maintaining the strictly
transverse plane of locomotory undulations.
KEY WORDS:-
polypterids - squamation - collagen fibres - function - torsion.
CONTENTS
Introduction . . . . . .
Material and methods . . . .
The scale-rows . . . . . .
Thescales . . . . . . .
The fibrous dermis . . . . .
The myotome muscles . . . .
Discussion . . . . . . .
Conclusions and summary. . .
Acknowledgements. . . . .
References . . . . . . .
Abbfeviations used in the figures .
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93
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106
I NTRO DUCT10 N
The fishes of the aberrant actinopterygian family Polypteridae (Polypterus and
Erpetoichthys) have long been known as the possessors of many features primitive
for either actinopterygians or osteichthyans in general. These features include
lobed bases to the paired fins, a spiracular opening, a bilobed ventral lung and a
spiral valve in the intestine (Abdel Magid, Vokac 8c Ahmed, 1970; Gardiner,
1973; Schaeffer, 1973). These and other adaptations have been deemed sufficient
by many authors to warrant the erection of a separate osteichthyan subclass, the
Brachiopterygii, for these forms (but see e.g. Gardiner, 1973:130).
Perhaps the most impressive archaic character of these fishes, however, is the
type of squamation. This takes the form of a heavy jacket of rhomboidal bony
* Present address: 15 DeVilliers Avenue, Liverpool L23 ZTH, U.K
0024-4082/8 11050093+ 14$02.00/0
93
0 1981 The Linnean Society of.London
94
D. M . PEARSON
scales articulating in such a way as to give regular, slightly-overlapping scalerows. The ganoid structure of these scales has been cited (Goodrich, 1928; Kerr,
1952) as a strong piece of evidence supporting the actinopterygian affinities of
the family, while the individual shape of the scales, their relationships to one
another, and the form and disposition of the scale-rows conform to a pattern
which is found not only in primitive actinopterygians (including the living
lepisosteids) but also in early members of the Dipnoi and Rhipidistia (Jarvik,
1948; Denison, 1968; Thomson 8c Campbell, 197 1).
N o major mechanical function has been convincingly ascribed to this type of
squamation, beyond the obviously necessary compromise role of an armour-plating which at the same time allows body mobility (Aleev, 196927; Waterman,
1977 : 146). I t has, however, been suggested that by restricting the amplitude of
the locomotory wave, the scale-jacket increases the power of the thrust delivered
to the water (Lund, 1967). Alternatively, the scales may prevent any body-wall
wrinkling that occurs as a result ofmyotome contraction CGuunann, 1975; 1977).
To the first of these it may be objected that the body of Polypterus is extremely
flexible in the transverse plane, being capable of ‘serpentine’ undulations
(Budgett, 1899; Harrington, 1899). Secondly, any wrinkling of the dermis could
be prevented by an appropriate arrangement of collagen rather than bone, with a
considerable weight saving. Bony scales have a specific gravity of about two, and
the squamation of a small Polypterus weighs about lOOg k g ’ of body-weight
giving the living fish a small positive submerged weight TAIexander, 1975: 168).
MATERIAL AND METHODS
With an end to determine the function of the polypterid squamation,
specimens of Polypterus senegalus Cuv., P . bichir lapradei Steindachner (both from
Lake Chad), Erpetoichthys calabaricus (Smith)from Calabar, and Polypterus endlicheri
congms Boulenger (from Lake Tanganyika) were purchased from local fishermen
in Nigeria and Tanzania.
Whenever possible, live specimens were observed in aquaria ( P . senegalus,
P. endlicheri congas, Erpetoichthys). In addition, examination was made of dead
material in the fresh state, or after having been sun-dried and cleaned by insects,
or sun-dried following ten days’ immersion in 10% formalin. The last method
proved very useful in studying myotome-fibre orientation.
The bulk of the material consisted of about 400 specimens of P. senegalus and
the descriptions and conclusions which follow have been based principally upon
this species.
The drawings of the scales were made with the aid of a Wild M5 binocular
microscope with drawing attachment.
T H E SCALE-ROWS
The abdominal squamation of polypterids is organized into a series of helical
scale-rows (Fig.1). These take the form of bilaterally symmetrical nesting
geodesics (Breder, 19471 of bone, each half-member consisting of a single row of
articulating scales on each side, meeting its op osite number at the mid-dorsal
and mid-ventral lines at approximately rig t-angles (Fig. 2). The anterior
(preaxial) edge of each scale-row is overlapped slightly (Fig. 3) by the projecting
R
PO LY PTERID INTEGUMENT
95
U
10mm
Figure 1 . Polypterus sentgalus, dorsal fin partially raised. Arrow indicates posterior limit of abdominal
cavity.
A
1
B
m vs
Figure 2. Polypteru senegal&, scale row. A, Left half-member with dorsal rib attached, medial view; B,
bilateral scale-row, dorsal ribs and a single vertebra, dorsal view.
border of the one lying in front. This overlap is greatest in the anterior parts of
the body, where it reaches 40% of scale-row horizontal width in the immediately
post-opercular region.
There are no bony links between adjacent scale-rows, so each is rather
independent in its general skeletal relationships (Kerr, 1952; Norman 8c
Greenwood, 1963:57). It is this absence of articulations between the scale-rows
that gives the body its great lateral flexibility.
The scale-rows cross the horizontal at c . 4 5 O in the posterior region of the
body; this angle increases to c. ~ 5 0 - 5 5 ~anteriorly.
The varying bone thickness of a component scale is continuous with that of its
D. M . PEARSON
96
epidermis
myoseptum
scale
muscle
fibres
fibrous region
of dermis
Figure 3. Po[yperuJ renegalus, integumentar). tissues. Diagrammatic longitudinal section of two
ad,jacent scale-rows in the dorsolateral region of the trunk. Anterior to the left.
neighbours over the interscale sutures, and this continuity gives each scale-row a
distinctly unital appearance when viewed medially. Thus, an axial thickening,
present on each scale, runs the entire length of the scale-row and forms the area
of attachment of the row to the underlying fibrous region of the dermis (Figs 2,
3). This axial ridge on the medial surface reaches its maximum thickness in the
dorsal regions of the scale-row: the mid-dorsal scales anterior to the dorsal fin are
the thickest in the body. There is a general diminution of axial thickness in the
more lateral and ventral regions and the thinnest scales occur close to the ventral
midline.
Either side of the scale-row axis, the preaxial regions are thicker than the
freely-projecting postaxial regions (Fig. 3). Each scale-row is widest in its lateral
region, where the degree of overlap between adjacent rows i? greatest. The most
anterior few rows of the trunk, furthermore, are greatly extended rostro-caudally
when compared to more posterior rows.
Attached to the inner face of one scale on each side of the bilateral scale-row is
a dorsal rib. The scale concerned is the lateral-line scale for most of the length of
the abdomen, but not in the first few rows (the exact number differs with the
species) where the canal climbs the sides of the trunk to run towards the skullroof. The contact between rib and scale involves the swollen distal end of the rib
anchored by ligaments in a conspicuous depression in the preaxial region of the
scale (Fig. 4B). The rib lies in the horizontal plane and slopes anteriorly to where
its tapering proximal end meets the tapering distal end of the transverse process
of a vertebra in a ligamentous junction (Fig. 2).
THE SCALES
The main body of descriptive information concerning the scales is embodied
in the figures (Figs 4, 5 ) . For the purposes of scale description, a scale-row will be
held to commence anteriorly, along the dorsal midline of the body and terminate
midventrally. The application of the terms proximal and distal, within the
context of the scales, will stem from this usage.
Scale shape and size varies along the scale-row. This variation is regularly
repeated down all the scale-rows of the abdominal region. On the basis of this
POLYPTERID INTEGUMENT
n
91
A
fib at1
Imm
Figure 4. Polypterus senegalus, variation of the morphology of the scales from a single scale-row,
medial view. All to the same enlargement, anterior is to the left. A, Dorsal scales; B, lateral-line scale
and distal neighbour; C, ventral scales.
Figure 5. Polypterns senegalus, external view of peg processes. Anterior to the right; scale-line 1 mm.
A, Dorsal scales; B, lateral scales; C, ventrolateral scales; D,ventral scales.
98
D. M. PEARSON
variation, in Polypterus senegalus scales 1-4 (Fig. 2) of each scale-row are here
called dorsal scales, scales 5-9 are called lateral scales and the remaining scales in
the row (1O-c. 1 7 ) are called ventral scales. Despite this categorization, the
variation is fundamentally continuous and every scale is slightly different in
shape from its two neighbours in the row.
Each scale is essentially rhomboidal, with approximately parallel edges.
Because of a superficial overhang, the interscale sutures seem to cross the scalerow axis at right angles, when viewed externally, but the line of the functional
suture is rather closer to the horizontal than it appears from this viewpoint. In
the living fish, the superficial suture-lines are overlain by epidermis and are
almost invisible. The functional suture takes the form of an externally-bevelled
proximal edge of the scale overhung by the counter-bevelled distal edge of the
proximally-neighbouring scale. The adjacent bone faces are planar and aligned
precisely opposite (i.e. two-dimensionally parallel) to each other.
Across the scale-row axis, the simplicity of the suture is disturbed by the
presence of the well-known peg and socket articulation between the adjacent
scales. A bifaced peg process borne on the proximal scale edge is inserted into a
bifaced socket on the distal edge of the neighbouring scale. One facet of the peg is
directed anterolaterally and the other posterolaterally (Fig. 5 ) . The anterolateral
face is generally the more prominent of the two and this is also reflected in the bone
disposition of the socket, whose anterior wall is thicker and more robust than the
posterior wall. The posterolateral face of the peg attains its greatest surface area
and prominence in dorsal scales, where the two articulatory faces of the peg are
almost symmetricallydeveloped (Fig. 5A).
Apart, then, from in the dorsal scales, the more anteriorly-lying surfaces of the
peg and socket articulation seem to be functionally more important than the
posterior ones. In this preaxial predominance the peg and socket apparatus
resembles the structure of which it is a part, the interscale suture, and this
circumstance, of course, is a reflection of the generally thicker bone of the
preaxial region of the entire scale-row.
The whole interscale suture and peg and socket articulation is extensively
reinforced by ligaments running parallel to the scale-row axis and inserting into
the bony tissue either side of the line (Ken-, 1952).
The anterior proximal corner of each scale is drawn out into a thick and strong
‘anterior process’ whose degree of development forms part of the positional
variation of the scales in the scale-row. It is most prominent in lateral scales (Figs
2 , 4).While remarkably thick, the anterior process remains slightly thinner than
the axial region of the scale.
Occupying the ventral midline of each scale-row is a median scale articulating
with each of its lateral neighbours by a peg process borne on each anterolateral
edge (Fig. 2 ) . Mid-dorsally, the scale-rows in front of the dorsal fin possess a
median scale (with a socket on each posterolateral edge) while scale-rows along
the field of the dorsal fin have no median single element: the first scale on each
side meets the other one down the midline in a sinuous suture.
Scales in the reduced caudal region of polypterids (Fig. 1) lack the peg and
socket articulations, as well as the clear organization into rows, and the
characteristic positional morphology of the abdominal scales. No dorsal ribs
occur postanally. Any scale-row organization seems to be as V-shaped rows with
the apex of the ‘V’ facing anteriorly and formed by the lateral-line scales on each
side.
POLYPTERID INTEGUMENT
m
99
100
D. M. PEARSON
The regularity of the abdominal squamation is such that any scale is a member
not only of a true scale-row, but also of a second geodetic line of scales (Breder,
1947) running in an opposite direction around the trunk of the fish and
intersecting with the true scale-row at an angle which in the living animal varies
from c. 90° in posterior regions to c. 7 5 O more anteriorly (Fig. 6A). These
‘intersecting scale-rows’ do not possess any bony articulations linking the
constituent scales, but there is a parallel and opposite bevelling of their
contiguous faces. This forms part of the architecture of the overlap between the
adjacent scale-rows.
T H E FIBROUS DERMIS
The integument beneath the squamation consists of a fairly thick collagenfibre layer. The fibres run in two principal directions (Kerr, 1952:60),
terminologically distinguished here as ‘paraxial fibres’ running along the lines of
the scale-rows, and ‘intersecting fibres’ crossing the scale-row axis in such a way
as to lie along the lines of the intersecting scale-rows. There does not seem t o b e
a clear segregation of the two fibre-types to form separate layers, although the
paraxial fibres are concentrated more superficially (and along the axial region of
the scale-row), while the intersecting fibres run more deeply. The latter are
attached to, and therefore mechanically link, the component scales of the
intersecting scale-rows. They run from the immediately preaxial region of one
scale to the immediately postaxial region of the scale distal to it in the true scalerow in front (Fig. 6). The area of the postaxial attachment of these fibres usually
leaves a visible scar on the inner face of the scale.
Across the dorsal and ventral midlines, the paraxial fibres of one side of the
body are continued as the intersecting fibres of the other side, and vice-versa,
since along these lines each scale-row makes a right-angle turn while the fibres
follow helical paths (Fig. 9A).
T H E M Y O T O M E MUSCLES
The myotomes of polypterids have the 3 shape common in gnathostome
fish. The anterior cone is bisected by the horizontal septum and is actually
double (Alexander, 1969). The relationships of the superficial regions of the
myotome to the overlying integument is shown in Figs 3 & 7.
Immediately dorsal and ventral to the horizontal septum occurs the greatest
concentration of red muscle-fibres. They are few in number when compared to
the white fibres and they run longitudinally between successive myosepta.
The superficial white myotome-fibres do not run in a truly anteroposterior
direction (Alexander, 1969). There is a vertical component to their alignment
which is most marked in the hypaxial region (Fig. 7). Here, the fibres run
posteroventrally at an angle of c. Z O O from the horizontal. More deeply, the
hypaxial white fibres run posterodorsally c. 2 5 O from the horizontal.
DISCUSSION
Manipulation of fresh material confirms the impressions gained by the
examination of the scale and scale-row structure : when under axial compression
POLYPTERID INTEGUMENT
101
Figure 7. Polypterus senegalus, relations of the integument to the underlying structures.
the abdominal-region scale-row is a structure which possesses most of the
characteristics of a strut (i.e. a compression-resisting device). The peg and socket
articulations, the precise opposition and the flat bone surfaces across the
interscale sutures give an overall tightness and firmness to the contact of the
adjacent scales when these are pushed together. The entire scale-row, as a result,
displays an impressive rigidity when under axial compression. In effect, it
appears to function as a unit under these conditions (Kerr, 1952).
The hypothesis that a major mechanical function of the polypterid scale-row is
the resistance of a compressive stress applied along the axis of the row is apriori
reasonable, since the principal mechanical function of bone in vertebrates
generally is to resist compression (Young, 1957 :132).Furthermore, the scale-row
axes take the form of geodesics. Geodesics are the shortest lines which can be
drawn on a surface so as to connect two points, assuming these to be less than
half a revolution apart. Any strut or tie laid down in the form of a geodesic curve
around the body will therefore simultaneously possess optimal configuration to
maintain the distance between adjacent points on the curve and optimal
economy of deployment of tissue while so doing.
The major mechanical function of collagen-fibres is to resist tension developed
along their axes (Young, 1957 ).
The regularity and distinctive overall pattern of the integumentary mechanical
tissue of polypterids suggest that a major supportive role of the skin involves the
resistance of compression habitually developed along the scale-row axis and of
tension occurring along the axes of the intersecting and paraxial fibres.
The intersecting-diagonal configuration of these apparent struts and ties in the
integument is strongly reminiscent of the pattern of compression and tension
generated at the surface of a cylinder under axial torsion (Fig. 8A; Olsen, 1966)
where orthogonal compressive and tensile forces of equal magnitude occur at
4 5 O to the long axis and whose principal paths take the form of intersecting
helices wrapped around the cylinder (Case 8c Chilver, 1959277; Crandall &
Dahl, 1959).Torsion would tend to deform a square of integument into a rhomb
(Fig. 8B), with one diagonal lengthened (i.e. extended by tension) and the other
shortened (i.e. linearly compressed). Appropriate reinforcement of these in-
D. M. PEARSON
102
Fixed
C
Figure 8. Engineering principles of the polypterid integument. A, Surface pattern of compression
tension ( u t ) forces generated by the axial torsion of a cylinder (Olsen, 1966: fig. 5.20b,
&drawn). B, Deformation of a square ‘panel’ of integument (solid lines) into a rhomb (broken lines)
by axial torsion of the body. Note that diagonal a’c is shorter than ac, and that d‘b is longer than
db. C, Essential architecture of the polypterid integument, simplified to emphasize ‘panelled’
arrangement. Each ‘panel’ possesses reinforced diagonals. Thick solid lines indicate compressionresisting axes (scale-rows),interrupted lines indicate tension-resisting axes (collagenfibres).
(uc) and
tegumentary diagonals would prevent this deformation and thereby the torsion.
Any integumentary-based torsion-preventer operates at considerable
mechanical advantage, since the torsional rigidity ( z ) of any cylinder of length L
and radius r is given b z =nxr4/2L (Newman 8c Searle, 1957 :1 lo), where n is the
modulus of rigidity or the particular material and can, for our purpose, be
ignored. We see from this that it is advantageous to place the torsion-preventer
as far along the radius away from the axis as possible, to exploit the r4 term to the
full (Wainwright,Vosburgh 8c Hebrank, 1978).
If a tendency to axial torsion does occur in polypterids, and presumably it
would occur as a result of myotome contraction, then the intervertebral joint
cannot prevent it. Polypterids lack the zygapophyses spanning this joint in
teleosts (Schaeffer, 19671, and which restrict angular displacement between
adjacent vertebrae.
The suppression of axial torsion is desirable on the grounds of maintaining
the strictly transverse plane of locomotory undulations and the resultant
efficiency of the action of the posterior body and the caudal fin.
In order to transmit a compressive load applied along the scale-row axis, not
only is it necessary for the scales to come together and ‘lock up’, but some
resistance must be provided to prevent the scale-row from moving as a whole
under the action of the force produced by the torsion. Each myotome is attached
to nine scale-rows (and each scale-row to an equal number of myotomes). A
contracting myotome is therefore linked superficially to nine scale-rows to each
of which are also connected a number of non-contracting myotomes. The
i
POLYPTERID INTEGUMENT
A
B
103
C
w
Figure 9. Engineering principles of the polypterid integument (continued). A, Block-diagram of
the polypterid integument, showing a dorsal view of the major axes of collagen in the fibrous dermis
(upper)and of bone in the squamation (lower). B, Integumentary resolution and resistance of the
principal surface stresses generated by the axial torsion of the polypterid body. The interrupted line
represents the mid-dorsal line of the body. (i) Scale-row axes; (ii) intersecting scale-row axes; (iii)
paraxial fibres, sutural ligaments, scales; (iv)intersecting fibres, scales, C, as B, but resisting torsion in
the opposite direction.
strength of the attachment of the scales to the fibrous dermis, and the inertia of
the immobile regions of the body-wall, will ultimately provide the resistance to
the torsional force. This resistance will cause the scale-row to develop rigidity,
and this rigidity will prevent the integumentary distortion and with it the torsion.
Judging from the superficial muscle-fibre orientation (Fig. 7), one result of
myotome contraction is a slight ‘closing-up’movement of the adjacent scales in
any scale-row to which the myotome is attached. The consequent scale-row
rigidity would clearly be advantageous in resisting any torsion subsequently
tending to increase the compressive load on the row.
In the efficient transfer of compression from one scale to its (apparently
proximal) neighbour in the row, the scarf-jointing of the suture seems to be
adaptive, since such a joint leads to an equal distribution of transmitted load
(Hildebrand, 1974:450). The axial and preaxial regions of the suture, where the
area of opposed bone is large, must constitute the major surfaces of load
transfer. The peg and socket articulation possesses an important function in the
prevention of adjacent-scale slippage during compression of the scale-row; in
this way it maintains the architectural integrity of the scale-row and all the
104
D. M. PEARSON
advantages deriving from the precision of its construction. The occurrence of a
considerable stress down the axis of each scale, between its proximal peg and its
distal socket, is indicated by the thickening invariably present in this region.
Bone is an economically-deployed tissue (Pritchard, 19741, hence where there is a
locally-heavy occurrence, this is probably the response to a locally-heavy
compressiveload.
As can clearly be appreciated by reference to Fig. 9A, B, a tendency towards
torsion in one direction will be resisted by the following components of the
integument:
(a) the scale-rows of one side of the body, strongly resisting the prevailing axes
of compression (Fig. 9B ‘i’),
(b) the intersecting fibres of that side of the body, and the scales of the
intersecting scale-rows which they connect, strongly resisting the tension
developed orthogonally to the direction of the compression (Fig. 9B ‘iv’),
(c) on the other side of the body, the paraxial fibres, sutural ligaments and the
bone of the individual scales of the scale-rows all resisting tension along their
axes (Fig. 9B 5 ’ 1 ,
(d) on the other side of the body, the compressed scales of the intersecting scalerows may play a minor role in resisting the compression developed along
their axes (Fig. 9B ‘ii’).
Torsion in the opposite direction will be resisted by the appropriately-situated
tissues on the other side of the body to those above (Fig. 9C).
Wainwright et af. ( 1978)have demonstrated the torsion-resisting properties of
the shark integument which result from the multi-layered collagenous dermis.
Here, the fibres are arranged in helices whose axes intersect at c. 90°. The
arrangement in Polypterus seems very similar, but it is provided with
compression-members, and involves a less-obviously organized fibrous dermis,
perhaps as a result.
I am indebted to Prof. R. McN. Alexander for pointing out that the probable
original rationale for scale-rows at c. 45O to the longitudinal axis of the body is to
allow locomotor-generated shear to occur between them. Swimming
undulations involve the production of compression and extension forces parallel
not only to the longitudinal axis, but also to the circumference of the bodysection. Thus shear is generated at 45O to the longitudinal axis. Since the scales in
any fish must be arranged so as not to restrict the swimming movements, the
scale-rows are laid down at 45O to the long axis. This configuration is also ideal
for the prevention of torsion.
A further dynamic function of the squamation is indicated by the attachment
of the long series of abdominal dorsal ribs to the scale-rows. The occurence of
dorsal ribs, where successive myosepta cross the horizontal septum (Fig. 71, seems
to be a response to the need for a reinforcement of the collagen at this
intersection point. This is because either side of a myoseptum the muscle tension
is developed in opposite directions, while dorsal and ventral to the horizontal
septum there is an additional antagonistic component to the local strains, that
resulting from the epaxial fibres of the superficial region running
posterodorsally while the superficial hypaxial fibres run posteroventrally. This
occurs not only in PolyPterus but in many if not most bony fishes (Alexander,
1969:270). Dorsal ribs, in sum, contribute to the mechanical efficiency of
myotome muscle action (Carter, 1967:157; Romer, 1970:164) and a firm
POLYPTERID INTEGUMENT
105
attachment of their lateral ends to the squamation will have its effect in the
process of generating locomotory thrust. The nature of this effect is not clear, but
one possibility is the production of a high force of contraction at the movable
end of a muscle trajectory by means of immobilizing the trajectory’s origin on
the myoseptum, close to the dorsal rib.
Among other actinopterygians, a rib-squamation contact occurs in some
recent ‘armoured’ catfish (Alexander, 1966) but it is unknown in other fishes,
including those which (like Polypterus) possess a squamation of rhomboid scales
articulating by means of a peg and socket apparatus. Because of this
circumstance, the rib-scale contact in polypterids is probably best regarded as
secondary, but the generally poor preservation of the axial skeleton in the earliest
bony fishes does not allow certainty on this point.
Examination of the integument of Lepisosteus reveals many of the features
encountered in the polypterids : true and intersecting scale-rows, paraxial and
intersecting collagen-fibres. It is clear that in its general mode of action and
effect the skin performs a similar anti-torsion function to that which it does in
PolyPterus and Erpetoichthys. While palaeoniscoid integumentary data are
unavoidably incomplete, the same general pattern and organization of the scales
occurs as in the living forms above, and so the functions, again, appear to be
similar.
CONCLUSIONS AND SUMMARY
The squamation of polypterid fishes is organized into a series of scale-rows
whose structure indicates the ability to develop considerable rigidity when a
compressive load is applied along their axes.
(2) The fibrous region of the polypterid integument consists of collagen-fibres,
some running parallel to the scale-row axis while others run along a line
intersecting this axis. Fibres of the latter type link up scales in adjacent scalerows to give a series of rows of scales which intersect with the true scale-rows.
(3) The body of polypterids may be considered to be a cylinder with a sheet of
strongly-reinforced integument wrapped around it. This sheet is cross-tied
and cross-braced in such a way as to produce an overall pattern of
intersecting geodesics of bone and collagen whose major axes form a
diagonal pattern strongly reminiscent of the surface pattern of compressionand tension-lines generated by the axial torsion of a cylinder.
(4)I t is suggested that the suppression of any tendency to axial torsion of the
body will inevitably result from an integument of this nature and pattern.
Axial torsion, if unchecked, would result in the loss of the strictly transverse
plane of locomotory undulations and is therefore undesirable.
( 5 ) I t seems certain that the abdominal myotome muscles derive mechanical
benefit from the strength of the squamation, as a result of the strong
articulation between each scale-row half-member and a dorsal rib.
( 1)
ACKNOWLEDGEMENTS
I t is a pleasure to record my appreciation and thanks to the following, without
whom this work could not have been carried out: Mischa and Ithaki Abramowici
for their hospitality at Kigoma; Jane Mathison for large quantities of reprints
and for the opportunity to examine Lepisosteus; Mike Talbot for sectioning and
staining; Brian Gardiner for instructive conversation and the chance to watch
D. M. PEARSON
106
live Erpetoichthys; Colin Patterson and Peter Forey and the authorities of the
British Museum (Natural History) for the opportunities of looking at
palaeoniscoid scales and of using the library; Roger Thompson for discussions
about solid mechanics; Elaine Robson for suggesting productive lines of
enquiry.
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ABBREVIATIONS USED IN FIGURES
aP
amrib
fib. att.
mds
anterior process
depression for the articulation of the
dorsal rib
scar left by attachment ofcollagen fibres
of the dermis
mid-dorsal scale
mvs
oa
P
s
sr
mid-ventral scale
overlap area
peg of peg and socket articulation
socket of peg and socket articulation
axial ridge of scale