Lecture 6
Fluid as skeleton: hydrostatic, hydraulic skeletons
and muscular hydrostats
In which we leave solid skeleton and leverage for skeletons made upon
fluid and muscle itself. Animals with fluid-incompressible skeletons are
many: cnidarian polyps, annelid worms, echinoderms, molluscs (4
differnent major phyla: Cnidaria, Annelida, Echinodermata, Mollusca).
Assigned reading:
Kier W.M. 2012. The diversity of hydrostatic skeletons. Journal of
experimental Biology 215: 1247-1257. ,
Notice Glossary p. 1255 for terms you may not know: e.g., bulk modulus,
mesoglea,siphonoglyph, etc.
The Introduction of a paper is often the best place to find useful general information as
the writer explains the problem and what has been done in the past.
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“Animal skeletons serve a variety of functions in support and movement. For
example, the skeleton transmits the force generated by muscle contraction,
providing support for maintenance of posture and for movement and locomotion.
Also, because muscle as a tissue cannot actively elongate [muscles can’t push],
skeletons provide for muscular antagonism, transmitting the force of contraction of
a muscle or group of muscles to re-elongate their antagonists. In addition, the
skeleton often serves to amplify the displacement, the velocity or the force of
muscle contraction [mechanical amplification]. A wide range of animals and
animal structures lack the rigid skeletal elements that characterize the skeletons of
familiar animals such as the vertebrates and the arthropods. Instead these
animals rely on a [fluid skeleton]... in which the force of muscle contraction is
transmitted by internal pressure” (Kier 2012)
3 main functions of skeleton
1. transmits/translocates force and is shaped and made of materials that lend
themselves to this translocation (chitin, bone, collagen, resilin...)
2. muscular antagonism: muscles can’t push, they can only pull. So they typically
function in pairs that are antagonists of each other; one pair member contracts
and as it does stretches the antagonist back to its precontracted dimension.
(mandible adductor and abductor are a good example: often there is a difference
in the power/size of the two antagonists: opening mandibles doesn’t require the
same amount of force as crushing food.
3. mechanical amplification: skeletal leverage via optimal moments of force can
increase the force effect
The exoskeleton of a locust includes cranium, mandible, and the inflections of mandible
cuticle, the two apodemes*; the mandible is an appendage jointed to the head.
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Pinnately arranged muscle fibres
originate on the inner cranium and
angle downward, converging and
inserting on the apodemes. Muscle
contraction pulls on the apodeme
which translocates forces and so
moves the mandible.
The contraction of the adductor
muscles is antagonized by the
abductor muscles.
The moment of force (red
perpendicular distance from
apodeme insertion to axis ) is
greater for the adductor than the
abductor (blue), because the
adductor inserts farther from the axis
of mandible rotation, this ‘lever arm’
in effect amplifying its muscle power.
*do not forget that the muscles have been omitted; apodemes don’t contract
An adaptation aside
This insect, Romalea, conspicuous to
colour-vision capable predators , is
warningly coloured : it sequesters
noxious chemicals.
Principles of support and movement (Kier 2012) hydrostatic
skeletons
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The fluid of hydrostatic skeletons is essentially water ; it has a high bulk modulus,
i.e., resists significant volume change. Fluids are effectively incompressible. If you
stress fluid (apply a force per unit area to it) its pressure increases without
appreciably changing its volume.
When stresses (force per unit area) are applied to solid skeletons, non-fluids like
the exoskeleton of an insect, the direction of the force matters: pull, push, or slide
stresses give rise to different forces acting in different dirctions inside the
skeleton: tensile, compressive, shear). But stress applied to a fluid is
omnidirectional in effect : “press air into a tire and the tire inflates in any direction
it can get away with” (Vogel).
With fluid skeletons ,“contraction of circular, radial or transverse muscle fibres will
decrease [chamber] diameter, thus increasing the pressure, and because no
significant change in [chamber] volume can occur, this decrease in diameter must
also result in an increase in length.” The reverse occurs to re-expand the diameter
and re-elongate the muscle fibres.
Muscle fibre orientations:
Circular, radial and transverse
affect cross-sectional area of a
fluid-filled chamber, a hydrocoel:
their contraction causes the fluid
skeleton to lengthen and
supports bending.
Muscle fibre orientations:
longitudinal muscle fibres
shorten the hydrocoel
Some muscle fibres run
helically in muscular
hydrostats (cavity less fluid
skeleton) and create
torsion, twisting about the
long axis of the structure.
torsion: twisting
Amplification
by hydrostatic
skeletons.
Consider
relationship
between
diameter and
lengthening.
Muscle contraction forces can still be amplified by hydrostatic skeletons even
in the absence of “fulcrums and lever arms”. For a constant volume (see Fig. 2
above) the percentage increase in length, brought about by shortening of
circulars [or radial or transverse fibres], becomes y greater as diameters
decrease.
Collagen an important material in hydrostatic skeletal systems
as ‘crossed fibre helical connective tissue array’
Collagen is a important body
material, a protein, important
consitutent of fibrous connective
tissue; it takes the form of long
fibrils becoming the basis of
tendons, ligaments and skin; it
is made by fibroblasts during
embryogeny
Susan Barker
Connective tissue – collagen fibres
Don’t confuse connective tissue fibres with muscle fibres: only muscle
fibres can contract.
• The walls of the hydrostatic chambers are often reinforced with
connective tissue fibres that “control and limit shape change”.
• These fibres are typically arranged in a “crossed fibre helical
connective tissue array”.
• Note his language: the fibres are “stiff in tension” meaning that when
you pull on opposite ends there is negligable extension. But
because they are structured as a helix (a ‘spring’ if you like) these
relatively inextensible fibres in the wall of a structure can allow the
structure to extend (see echinoderm tube foot).
• “Elongation and shortening is possible because the pitch of the helix
changes during elongation (the fiber angle, which is the angle
relative to the long axis, decreases) and shortening (the fibre angle
increases)...”
Phylum Coelenterata/Cnidaria
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Hydras, jellyfish, sea anemones, corals.
Radial symmetry.
Gastrovascular (GV) cavity (internal space,
filled with seawater) opening via a mouth; no
anus, no assembly-line digestion.
Whorl of tentaclesare extensions of body
wall and GV cavity aid in food capture using
stinging organelles (nematocysts).
Diploblastic: epidermis and gastrodermis.
Mesoglea layer may give some elasticity
“The mesoglea ranges from a thin,
noncellular membrane to a thick, fibrous,
jelly-like, mucoid material with or without
wandering cells” (Barnes).
Two ‘structural types’ or morphs occur:
polyp is sessile; medusa is free swimming.
Very limited development of organs: e.g.
siphonoglyph might be called an organ.
DiveGallery
Tubastrea
Hydrostatic Skeletons
Jennifer
Goble
Ryan Photographic
The body of a sea anemone is “a
hollow column ...closed at
the base ...at the top with an oral
disc that includes a ring of
tentacles surrounding the
mouth and pharynx”. “By closing
the mouth, the water in the internal
cavity –the coelenteron/GV –
cannot escape, and thus the
internal volume remains essentially
constant. The walls of an
anemone include a layer of circular
muscle fibres. Longitudinal muscle
fibres are found on the vertical
partitions called septa that project
radially inward into the
coelenteron, including robust
longitudinal retractor muscles
along with sheets of parietal
longitudinal muscle fibres adjacent
to the body wall.”
Fig. 5
Kier
Phylum Cnidaria
sea anemones,
corals, jellyfish etc.
“With the mouth closed,
contraction of the circular muscle
layer decreases the diameter and
thereby increases the height of
the anemone. Contraction of the
longitudinal (or R =retractor)
muscles shortens the
anemone and re-extends the
circular muscle fibres.”
“...with this simple muscular
arrangement a diverse array of
bending movements and height
change can be produced.”
Metamerism and hydrostatic skeletons
Phylum Annelida
segmented worms.
Most species are marine,
polychaetes.
Annelida have a
coelom.
Coelomate
animals
Leeches are
also annelids,
freshwater, not
marine,
specialized for
blood feeding.
ABC News
Nereis
Metameres are segments
grouped sometimes into
tagmata: a tagma is a series of
metameres specialized for a
shared function.
Transverse grooving
along leech body does not
represent its ancestral
segmentation and is not homologous
with grooving of Nereis.
What is a coelom?
It is defined as a fluidfilled cavity forming
within mesoderm
(mesoderm being one
of the primary germ
layers of the embryo).
Animals with a coelom
are termed coelomate,
animals without one
are acoelomate.
chaetae
To burrow effectively through soil, searching out softer regions and crevices, going around or
under rocks etc. the worm needs to twist and turn and push its body. For push you need
purchase. For this it has chaetae; producible and retractable ‘hobnails’.
Schizocoel: splitting coelom formation
in annelids
Development, primary germ layers:
ectoderm, endoderm, mesoderm
(embryonic);
Budding occurs of embryonic tissue
behind the trochophore larva, in a series
of segments; bilateral spaces appear in
mesoderm and enlarge until the
mesoderm becomes a layer applied
against the gut (endoderm) and the skin
(ectoderm).
Mesoderm forms mesenteries, dorsal
visceral and ventral. The mesoderm
against the forming body wall
differentiates into the circular and
longitudinal muscles.
Each somite space expands also to
form fore and aft the worm’s septum.
free-swimming
trochophore larva
dispersal stage
cronodon.com
What was the primitive function of the
segmentation of annelids? Why
partition the coelom?
Earthworm is adapted for burrowing,
being able to change body shape
locally: a cylindrical anteriorly pointed
probing snout, backed with serial septapartitioned hydrostatic skeletal units
bounded by muscle: its body is a
flexible digging machine for making its
way through soil.
o
Lumbricus castaneus
clitellum
Earthworm Society of Britain
Annelida: 8800 spp. Triploblastic coelomate
bilateria,
body cavity a schizocoel, metamerically
segmented, longitudinal and circular muscles
around a hydrostatic skeleton, extracellular
digestion in a straight digestive tract running
from anterior mouth to posterior anus; gut
supported by longitudinal mesenteries and
septa, ventral nerve cord with segmental
ganglia and anterior brain, circulatory system
high pressure blood in vessels, excretion by
metameric nephidrida.
Each compartment has its own pair of nephric tubules. Why?
For cylindrical fluid skeletons “at
a given pressure, the stresses in
the circumferential direction are
twice those in the longitudinal
direction” Kier 2012
Septa function in allowing large lateral forces useful in burrowing