Lecture 7

Bio 325 Lecture 7
Nematoda: roundworms have no circular muscles to antagonize their
longitudinals; they locomote using helical crossed-fibre arrays of inextensible
Echinodermata: starfish tube feet as an example of hydraulic skeletons; and they
also use helical crossed-fibre arrays of collagen in the walls of their tube-feet.
Mollusca: foot of burrowing bivalves as another example of hydraulic-based
Muscular hydrostats: when no cavity is needed: tongues, trunks, mantles.
left over from
last lecture
Phylum Nematoda, roundworms
Collagen cuticle, cylindrical
to flat, “body cavity...is
derived from the embryonic
blastocoel and is called a
Longitudinal muscles send
extensions to the nerve cord
rather than having motor
nerves extending to the
muscles. There are no
circular muscles acting as
antagonists in roundworms.
from Barnes;
Classic paper on ‘crossed-fibre helical connective tissue arrays’
Shadwick assigned reading not Clark & Cowey
Shadwick, R.E. 2008. Foundations of animal
hyraulics: geodesic fibres control the shape of
soft bodied animals. J. exp. Biol. 211: 289-291.
“In 1958 R.B. Clark and J.B. Cowey published a
paper in which they presented a simple
geometric model, based on the idea of a fibrereinforced cylinder, to explain the mechanism
underlying shape changes in ribbon worms and
“the essential idea of this paper, [is] that a
structure composed of inextensible fibres could
accomodate large extensibility”, a paradox*
Clark R.B., Cowey J.B. 1958. Factors controlling
the change of shape of certain nemertean and
turbellarian worms. J. exp. Biol. 35: 731-748.
Fig. 4 of Kier is from Clark &
Cowey (Fig. 3) 1958.
CURVE shows the way a
helical fibre array angle
will relate to the volume
of a cylinder (a worm is
modelled as a cylinder).
If fibre angle increases above 75,
volume must decrease: but the
fluid in the a nematode’s
pseudocoelom, is
“incompressible and cannot
escape”. So what changes, given
the fixed volume, is the internal
pressure within the fluid cavity.
The increased pressure tends to
return the longitudinals of one
side to their precontracted state.
Clark & Cowey
model used to
calculate this
volume-fibre angle curve.
Clark & Cowey model: “...a right circular cylinder wrapped
with a single turn of an inextensible helical fibre ... is used
to solve for the enclosed volume of the cylinder as the
fibre angle varies from 90 deg to zero deg, with the
maxiumum volume occurring at a fibre angle of 54 deg 44
Nematodes - roundworms
Roundness relates to adaptation for locomotion: so does high internal
What is the antagonist of nematode
longitudinal muscle?
Answer is the crossed-fibre helical array
of collagen in the cuticle .
In Shadwick’s article see the paragraph p. 290,
beginning “An interesting outcome...”. The
idea here is that the antagonist to the
longitudinals is elasticity stored and supplied
by the fibre-reinforced cuticle with its helical
array of collagen fibres.
refer to Fig. 4: longitudinal muscle contracts
increasing fibre angle; increase from 75 deg requires vol drop
but this not possible so pressure increases
Phylum Echinodermata
radially rather than bilaterally
symmetrical, with oral and
aboral surfaces.
Water vascular system is unique to
these animals among phyla; it is a
vessel system filled with coelomic
fluid. ‘Arm vessels’ arise from a
tubular ring canal. In an asteroid
(starfish) five radial canals branch,
one into each of the arms.
Ambulacral groove on underside of
each arm lined with tube feet also
called podia.
Asteroid echinoderms have an exoskeleton. In their dermis are embedded calcareous plates called
ossicles (inorganic salt Calcium Carbonate is the material). The skin thus consists of stiff relatively
shape-stable elements, in a matrix of flexible collageous fibres (i.e., connective tissue): tough, solid,
thick yet flexible protective armour.
Above each tube foot, within the arm, is a vesicle called an ampulla, encircled by ampullar muscles;
contraction of ampullar muscles will displace fluid out of the ampulla into the tube foot because fluid
is incompressible. There is a valve in the side branch to the radial canal – a one-way valve -- that
closes to prevent backflow of the fluid into the water vascular system. So for each ampulla +tube foot
as it operates the volume of fluid in the ampulla and tube foot lumen is fixed because the one-way
valve closed. But this fixed volume of fluid moves back and forth from ampulla to tube-foot lumen;
the system in the starfish is HYDRAULIC rather than hydrostatic as in earthworm segments.
From Brown,
Circumferential stress in a
pressurized cylindrical vessel (e.g.,
worm, tube foot) is exactly double
the longitudinal stress ‘Kier’s Law’.
Stress distribution in a fluid-filled
cylinder is not uniform (as per
annelid metameres): hoop stress
[force acting to increase diameter] is
twice as large as longitudinal stress.
Imagine it as it isn’t: no helical
array in the tube foot wall.
When the ampulla pushes fluid
into the podium lumen there
will be an increase in diameter
rather than a lengthening
Rosette of ossicles with
intrinsic musculature that
pulls up the disc middle
creating suction to
End of extensible cylinder is the disc, larger in diameter than the
stem. There is a central depression.
Santos, R. et al. 2005. Adhesion of
echinoderm tube feet to rough
surfaces. J. exp. Biol. 208: 25552567.
Fig. 6 External morphology of
unattached pedal discs of
Paracentrotus lividus (left) [sea
urchin] and Asterias rubens
[starfish] (right) .
Temporary adhesion: the epidermis of the disc contains glands which produce two secretions:
glue/bonder and de-bonder, i.e., adhesive secretions and de-adhesive secretions. The glue is
delivered through the disc cuticle to the substratum where it forms a thin film bonding the foot. The
debonding secretions act as enzymes, detaching the upper coat of the glue and leaving the rest of
the adhesive material behind attached to the substratum as a footprint.
Importance of tube foot in predation
Virginia Living Museum
‘off the beaten path’
Pulling with tube feet
adhering and starfish arm
muscles to open the
protective valves of
shellfish Mollusca
An interesting picture of razor clams packaged for sale in a
chinatown market in Philadelphia
Phylum Mollusca
Razor clam burrowing
Winter A.G. et al. 2012. Localized fluidization burrowing mechanics of Ensis
directus. Journal of experimental Biology 215: 2072-2080.
(See also Inside JEB, Kathryn Knight. 2012. Razor clams turn soil into quicksand
to burrow.)
Blood and foot sinuses serve as hydraulic burrowing mechanism.
Cycle of burrowing movements Fig. from Winter: In stage A, adductor relaxed so shells
braced on surrounding sand by ligaments; protractors start to contract (B) pushing blood
into foot and the foot probes down, gaining ground into the mud; pushing force of foot
makes body move up a little (C) (relative to dashed line).
Stage D, adductors contract, pulling valves together, (red indicates the space they DID
occupy), pushing blood into foot to make an anchor, simultaneously squirting seawater out
around valves from mantle; this water* ‘puddles’ sand; foot swells with blood into this
quicksand region– the localized fluidation; displaced blood is swelling the foot maximally
into the bottom anchor of the TWO ANCHOR SYSTEM. Cycle renews (F).
*Its not clear whether
this involves ocean
water drawn in by
siphons; perhaps it does
if the clam is burrowing
near the surface and
perhaps if lower down it
oscillates (?) its valves
to draw in pore water
Muscular hydrostats
Kier p.1252 Tongues, tentacles, trunks: “lack
the fluid-filled cavities and fibre-reinforced
containers that characterize ... hydrostatic
skeletal support systems” rather they are:
“a densely packed, three-dimensional array of muscle and connective tissue fibres”
Transverse sections showing the muscular arrangement of three examples of
muscular hydrostats.
A. Squid tentacle: T, transverse muscle fibres; L , longitudinal; transverse in the
tentacle core, “and extend to interdigitate with bundles of longitudinal muscle
fibres, notice the suckers.
Kier W M J Exp Biol 2012;215:1247-1257
©2012 by The Company of Biologists Ltd
Transverse sections showing the muscular arrangement of three examples of muscular
B. Elephant Trunk: R, radials ‘extend from centre of the trunk between bundles of
longitudinal muscle that are more superficial, notice nasal passages.
Kier W M J Exp Biol 2012;215:1247-1257
©2012 by The Company of Biologists Ltd
C. Monitor lizard tongue. Circular muscle fibres surround two large bundles
of longitudinal fibres.
“The muscle fibers are typically arranged so that all three dimensions of the
structure can be actively controlled, but in several cases such as the mantle of the
squid [of which more later] and some frog tongues, one of the dimensions is
constrained by connective tissue fibers.”
“Because muscle tissue, like most animal tissues lacking gas spaces, has a high bulk
modulus, selective muscle contraction that decreases one dimension of the
structure must result in an increase in another dimension. This simple principle
serves as the basis upon which diverse deformations and movement of the
structure can be achieved” (Kier 2012).
Read carefully all the section on muscular hydrostats by Kier: complex bending
achieved by interplay of contracting muscles --more subtle than a passive uniform
fluid in a chamber – i.e., some muscles by contracting can affect the bulk modulus
presented to other muscles that are acting upon its incompressibility.
*bulk modulus of a substance measures its resistance to uniform compression Wikki
Muscular hydrostats (Kier contin.)
Selective contraction: “This simultaneous contractile activity is necessary to
prevent the compressional forces generated by the longitudinal muscle from
simply shortening the structure, rather than bending it, and can actually augment
the bending by elongating the structure along the outside radius of the bend.”
“The longitudinal muscle bundles are frequently located near the surface of the
structure, as this placement away from the neutral plane increases the bending
Helically arranged muscle fibres can be present and generate torsion.
Bivalve: Pecten, scallop: water-jet swimming propulsion
anterior adductor lost, posterior
relocated more centrally
Seawater exits from 2 openings near the hinge as the valves are adducted: this jets the
scallop forward.
An example of an antagonist to a muscle that is not another muscle; the
Pecten adductor stores the energy of distortion that will later restore it to its
precontracted state.

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