Lecture 5

Labs this week (Lab 3) Fluid skeletons
Read the preamble to lab 3.
Topics: translocation, coelom: fluid filled cavity in mesoderm.
Coelomate animals. The coelomate phyla include Vertebrata , Echinodermata,
Annelida --- contrasts with acoelomates (without a coelom) and haemocoel
animals (insects have a haemocoel).
Functions of fluid-filled body spaces not just locomotion: they transport
hormones, gases, food, gametes. Concerned for the moment with their role in
locomotion and the giving of body shape.
Metamerism: serial repetition of body parts: modules for ‘local locomotion’
“partitioned by septa …allows for independent operation of portions of a
hydrostatic body” (Kier, p. 1255).
Annelids: circular fibres peripheral to longitudinal; segments isolated by septa
fore and aft: changes in metamere shape as force (pressure) translocated by
coelomic fluid of segment (metamere).
Assigned reading Kier, William M. 2012. The diversity of hydrostatic skeletons.
Journal of Experimental Biology 215: 1247-1257
Habronattus Jumping spider from Arizona
Words of the day:
Phidippus putnami
Salticidae {Jumping Spiders}
predatory spiders with good
vision, used to leap accurately
onto prey – no webs but use silk
in safety line and refugia
Spiders have flexor muscles across the dicondylic joints of their
legs but no antagonistic extensor leg muscles; so when a jumping
spider jumps it does so without contracting leg muscles; it uses a
hydrostatic blood pressure, created by muscles elsewhere in the
body, to extend the legs very rapidly. (It would not be correct to
say that muscle is not involved in their leg extension. ) When
spiders die their legs flex.)
Prairie Ridge
Lecture 5
Fluid as skeleton: hydrostatic, hydraulic skeletons
and muscular hydrostats
In which we turn from leverage of solid skeleton to leverage made upon
fluid and muscle itself (muscular hydrostats [later]). Forces can now
become pressures, forces by a different name. Animals with fluidincompressible hydrostatic skeletons are many: cnidarian polyps, annelid
worms, echinoderms, molluscs, nematodes (5 different major phyla:
Cnidaria, Annelida, Echinodermata, Mollusca, Nematoda).
Kier W.M. 2012. The diversity of hydrostatic skeletons. Journal of
experimental Biology 215: 1247-1257. [I am assigning Kier to be read in good
detail – it is a little bit more important than other assigned readings.]
Notice his useful Glossary p. 1255 for terms you may not know: e.g., bulk modulus,
mesoglea, siphonoglyph....
The Introduction of a paper is a good place to find needed background and
more general information: here the writer explains the question and what has
been done in the past. Kier’s first paragraph below is an example of much
useful information.
[gkm comments in square brackets]
“Animal skeletons serve a variety of functions in support and movement. For
example, the skeleton transmits [translocates] 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)
Main functions of skeleton
1. Translocation [of force] and is shaped and made of materials that lend themselves
to this translocation (chitin, bone, calcium carbonate, collagen, resilin...): it can
also consist of FLUID.
2. Antagonism: muscles can’t push, they can only pull. So they typically function in
pairs as antagonists of each other; one pair member contracts and as it does
stretches the antagonist back to its precontracted dimension; or a muscle can
provide for its own subsequent antagonism by stretching or squeezing an elastic
3. Amplification [mechanical]: skeletal leverage via optimal force moments can
increase the force effect.
Principles of support and movement (Kier 2012) hydrostatic
The fluid of hydrostatic skeletons is essentially water; water 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 or shell of a mollusc, the direction of the force
matters: pull, push, or slide stresses give rise to forces acting in different
directions upon and within 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).
*The bulk modulus of a substance is an index measuring its resistance to
uniform compression (Wikki).
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
Leech Looping
Phylum Annelida,
segmented worms,
includes leeches; they
have no septa; a leech
is like a single
metamere moving by
changing its shape via
the incompressibility
of coelomic fluid.
Earthworm Lumbricus
changing the shape of
metameres in waves.
Upper drawings show a
leech looping anterior
sucker to posterior sucker
to anterior again etc.; fixed
to the substratum by the
suckers its body extension
via contracting circulars
and coelomic fluid, gains
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 (cavityless
skeleton)’some create
torsion, i.e., twisting about
the structure’s long axis.
torsion: twisting due to applied torque
Re levers: remember distance/speed advantage [Kier calls speed
advantage velocity advantage] and some levers are distancespeed increasing, i.e., some levers are velocity increasing.
Put a little distance in and get a lot of more distance out
Put a little velocity in and get a much greater velocity out.
You can design a fluid skeleton to give you velocity advantage.
“Although hydrostatic skeletons lack the fulcrums and lever arms present in rigid
skeletons that allow amplification of force, displacement and velocity, their
geometry …provides a mechanism for mechanical amplification of muscle
“For an initially elongate cylinder, it is apparent that a given percentage
decrease in diameter (caused by shortening of the circular, radial or
transverse muscle fibers) will cause a larger percentage increase in length
compared with a short cylinder…”
For a fixed volume (see Fig. 2 above) the percentage increase in length,
brought about by shortening of [fibres] becomes greater as diameters
A small decrease in diameter of a cylindrical fluid-filled constant-volume
appendage – e.g., from B to D -- causes a much much larger increase in length
in the same time interval. An animal can use this velocity-increasing (= speedincreasing) geometric relationship to improve the rapidity of its strike at a prey
Speed advantage is behind the rapid deployment of the specialized
prey-capture tentacles of squid; these “...require only a 25% decrease
in diameter to generate nearly an 80% increase in length”
A squid striking
at a lure.
Supercontracting muscle: specialized muscle in prey-catching squid tentacles, the
protrusible tongues of amphibians, etc. Longer muscle fibres have evolved – with a
“greater range of shortening and elongation than is typical for most vertebrate and
arthropod muscle fibres”.
Collagen an important material in hydrostatic skeletal systems
as ‘crossed fibre helical connective tissue array’ [CFHCTA]
Collagen is a important body
material, a protein and an
important consitutent of fibrous
connective tissue; it takes the
form of long fibrils that are the
basis of tendons, ligaments and
skin; it is made by fibroblasts
during development.
Useful to memorize
crossed fibre helical
connective tissue array.
Susan Barker
Extension of the inextensible
Connective tissue fibres can be arranged to both effect and affect the
shape changes of a fluid-filled cavity. (These fibres are not contractile;
they are laid down as extracellular material: only muscle fibres can
contract. )
The walls of 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”. As shape varies so these fibre arrays vary.
Note his language: the fibres are “stiff in tension” meaning that when you
pull on opposite ends there is negligible 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 (later 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
fibre’s long axis, decreases) and shortening (the fibre angle increases)...”
A helical array can also store energy for later release.
Ortho- Orthoptera is the
order of insects to which
locusts belong. Orthoptera
means ‘straight-winged’.
For fibres to meet
orthogonally means that
they cross each other
‘straightly’: i.e., at rightangles (A). Pressurized
cylinders of fluids whose
walls are reinforced by
fibres show different
behaviours when stressed.
Orthogonality of
reinforcing fibres allows
cylinder torsion (D) but
helical arrangements (H)
resist this shape change.
Phylum Cnidaria: polyps and medusas
Gastrovascular (coelenteron) cavity as a fluid-cavity skeleton.
Hydras, jellyfish, sea anemones, corals,
colonial hydrozoans.
Radial body symmetry; morphs: polyp
(sessile) medusa (free-swimming).
Gastrovascular (GV) cavity as internal body
space; siphonoglyph pumps seawater into this
space) GV opening only via a mouth, no anus;
so no one-way assembly-line (think motor
cars) food processing, Whorl of tentacles, GV
cavity extends these; crustaceans captured
via batteries of stinging organelles
(nematocysts) on tentacles.
Diploblastic: epidermis and gastrodermis (2
primary germ layers not 3).
In swimming morph, medusa is mesoglea.
{Rheology: Wikki, study of flow of matter in a
liquid state or soft-solid state where material
responds with plastic flow}: mesoglea is a
viscoelastic composite behaving differently at
different scales: macro micro: more sol, more
gel (recall mucus).
Hydrostatic Skeletons
Tubastria polyp upper right, shows oral disc
and slit mouth very clearly, as well as batteries
of nematocysts on the tentacles. The
siphonoglyphs open at either end of the slit
mouth to draw seawater down into the GV
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 – functions
as a hydrostat: its 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 [septa: but certainly not
homologous with worm 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
Fig. 5
Transverse section
level of pharynx; see
retractor muscles on
the radially projecting
mesenteries (= septa).
These vertical septa
supporting the
pharynx at this level,
may function to
increase the surface
area available for
assimilating food.
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: serial repetition of body parts. It is a feature of vertebrates, think
vertebral column. It is a feature of many arthropods (e.g, Scutigera) and of
segmented worms (Phylum Annelida). Why repeat limbs, ganglia, nephric tubules
in a linear progression? The answer is largely for reasons of locomotion. One
vertebra moves upon another and the result is a snake backbone that can send the
body into waves for moving across the substrate. Metamerism in earthworms is a
way of producing local changes in girth and elongation in a wave sequence that
serves for burrowing locomotion.
Metamerism and hydrostatic skeletons
Phylum Annelida
segmented worms.
Most species are marine,
Annelida have a
Leeches are
also annelids,
freshwater, not
specialized for
blood feeding.
ABC News
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.
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: coelom formation in
annelids by splitting
Development of primary germ layers:
ectoderm, endoderm, mesoderm
(embryonic relatively undifferentiated
tissue). Free-living trochophore larva is
one stage in annelid development.
Budding occurs of embryonic tissue at
the posterior end of the trochophore
larva, forming a series of segments;
bilateral spaces appear in segment
mesoderm and enlarge until, right and
left joining, the mesoderm becomes a
layer applied against the gut
(endoderm) and the skin (ectoderm).
Mesoderm forms mesenteries, dorsal
visceral and ventral supporting the gut.
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.
A quick ‘course’ in
embryology. At
first three
embryonic tissue
layers, endoderm
(blue), mesoderm
(red), ectoderm
developing into
the trochophore.
pauses while the
trochophore lives,
as a decidedly not
creature, moving
in the sea by its
rings of cilia.
Then growth is
renewed by the
budding of a
trochophore larva
series of posterior dispersal stage
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.
Lumbricus castaneus
Clitellum is
a tagma
Earthworm Society of Britain
Annelida diagnosis: [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 of high pressure blood in
vessels, excretion by metameric nephidrida.
Each compartment has its own pair of nephric tubules. Why?
Two and one-half segments of an earthworm, drawn in cut-away to show
the arrangement of the circular (outermost) and longitudinal (innermost)
antagonistic muscles. The coelom is interrupted by a succession of septa
that localize the effects of the muscle contractions along the length of the
body. The gut tube (digestive tract) runs through the septa.

similar documents