Lecture 7

Lab this week a bell-ringer; starts at 3 pm SHARP!!!
Term test coming up next week Monday October 6 in lecture
(Its not in February as it mistakenly says on the website.)
R.B. Clark, Dynamics in Metazoan Evolution – the coelom as
hydrostatic skeleton
Frank Brown Jr., Selected Invertebrate Types
McCurley R.S. & Kier W.M. 1995. The functional morphology of
starfish tube feet: the role of a crossed-fiber helical array in
movement. Biological Bulletin 188: 197-209.
Metamerism and Tagmata
“Of all the countless shapes and forms that triploblastic metazoan animals
take, the least complicated …the least specialized, is vermiform.
Vermiform: worm-shaped: longer than diameter cylindrical
[A worm consists] …from a mechanical point of view, of a flexible
muscular body wall enclosing an incompressible, but deformable medium
which has a constant volume and in which fluid pressures can be
transmitted.” R.B. Clark p. 31 Dynamics in Metazoan Evolution.
Because the force (pressure) made by body-wall muscle within the fluid
cavity is everywhere the same, to get locomotory changes effective in
different body regions one needs to partition the medium into
metameres/segments. Functionally metameres are locomotory modules
that enable the creation of peristaltic body waves and forces adequate for
Annelids are metameric worms and their coelom is a hydrostatic skeleton.
Caribbean reef advice: don’t touch: fire
coral, Millepora
not a true coral (Anthozoa), but a
Hydrozoa; very variable form per
Hermodice carunculata
Bonaire fringing reef
Photo from Paddy Ryan
Homology of
Filogranella spp.?
Steinbeck: Sea of Cortez, Ed Ricketts, Hansen
Sea Cow (see Wikkipedia)
Protula bispiralis
New Zealand
Good place to see invertebrate diversity. Photos from Paddy
Ryan website ‘worms’
Most species of annelid
are marine, errant*
Annelida have a
*Knight errant
Loss of septa
Leeches: bloodfeeders with gut
tube dominated by
a sacculate pharynx
for blood storage.
annulations are not
homologous with
ABC News
Metameres are grouped
sometimes into
tagmata: a tagma is a series of
metameres adapted for a
Particular function.
How does a leech
move without a
partitioned coelom?
“Arenicola lugworm, both burrows and feeds by forcefully everting its proboscis into
the sand and then retracting it with its load of sand. The burrow is enlarged by firm
muscular peristaltic waves which also pass anteriorly. Organic material in the sand
[falling out of the water column, brought in by the tides] is digested [extracellularly]
and absorbed as the material passes through the gut.” (F. Brown, Selected Invert.
Arenicola, lugworm: ocean flats tidal detritus feeder
Ingests from
shaft of
‘puddled’ sand
and castings
ejected at the
surface at other
end of burrow.
Malcolm Storey
Proboscis eversion in
burrowing see p. 84-85
R.B. Clark, Dynamics…
Coelomic pressure everts
pharynx for burrowing/feeding in
lugworm. Some worms evert
jaws from inside pharynx and
capture prey.
Arenicola, gills in middle third of animal, notopodium protects.
Gills: fine branches increased
surface area, metameric on
annelid body. (What is the
difference between a gill and
a lung?).
Neuropodia are ventrolaterally located and
comprise dorsoventrally elongated muscular
ridges – obtains purchase in burrow. Each
neuropodium has a longitudinal slit-like opening
into a chaetal sac. The sac contains large hooklike setae (chaetae) or crochets (not visible
here). F.A. Brown, Selected Invertebrate Types.
Parapodia comprised of neuropodium (ventrolateral) and notopodium
Parapodia serve as paddles for locomotion and sites of gas exchange.
Neuropodium is the lower half of a parapodium, nearer to the ventrally located
nerve chord. (Annelids and arthropods contrast with vertebrates in this respect:
vertebrate nerve cords are always dorsally situated.) Notum means ‘back’ so the
upper, more dorsal, half of the parapodium is nearer the back, hence notopodium.
Parapodia of an errant polychaete, Nereis.
The parapodia act as paddles, strengthened by aciculae (modified chaetae). There
are muscles that can retract parapodia slightly, reducing impedance forces relative
to power-stroke forces; parapodia protraction by localized increase in coelomic
fluid pressure (?). Parapodium has locomotory function but also a respiratory
function – gas exchange. Blood in a closed vessel system circulates out into the
parapodia where there is a capillary bed for gas exchange. Movement maintains
diffusion gradients steep. Parapodia have at least two functions.
Muscles longitudinals
Setal/chaetal bundle
Lives in U-shaped secreted
parchment tubes
constructed in tidal flats;
notopodia on 12th segment
are wing-like, aliform,
produced; epithelium of
these wings is ciliated and
richly supplied with glands
that secrete mucus.
Notopodia of segments 14,
15 and 16 are modified into
circular fans that just fit
against the tube walls.
Phylum Annelida: concentrates organic food* filtering
it from silt and inorganic particles using mucus and a
seawater current.
paired aliform
fan notopodium
The fans fit piston-like against the cylindrical wall of the tube and are moved in a tofro cycle, making a power then a recovery stroke; there are intrinsic promotor and
remotor fan muscles. Fans create a strong current within the tube (of course only
when the tide is in). Seawater is drawn in the chimney of the U-shaped tube, flows
down across the burrow, up and out the other chimney. Each fan beats about 60
times a minute in active feeding. Ventrally there are suckers gripping the tube wall; if
the fans are to work properly they must be properly anchored.
The paired aliform notopodia are deployed against the walls of the tube and they
secrete a sheet of mucus drawn rearward in the current created by the fans. Initially
the current draws this ‘sock’out (mucus in water retains coherence). Streams of
mucus trailing backward in the current, continually being renewed by fresh secretion
at the wings, tapers ‘downstream’ into a bag. Particulate matter is filtered/trapped in
this mucus bag. (Food of a sediment feeder: micro-organisms such as diatoms,
detritus tissue from prey events etc. Bottoms of water bodies accumulate organic
The 3 fans of this
annelid are a tagma
that functions as a
pump: 3 metameres
specializing to create a
current of water
within the U-shaped
‘parchement’ tube.
Imagine it as it isn’t.
Why 3 fans? Why not
2 or 4? Why not just
Tagmatization supreme
Three fans instead of two or one create a smoother flow by their metachronal
Metachronal: refers to the phase difference between elements of a series. The flow is
steadier with a three-element phased pump and this is important for filtration.
The animal can move the bag about a little within the tube by muscles associated with
the ‘wings’ and so avoid incoming material that is undesirable. The ciliated cup collects
and rolls up the bag and then a midline ciliary tract carries this food ball anteriorly to
the mouth for ingestion.
Some other tagmata
Head is a tagma: derived from fused segments.
Potter wasp pedicel or
Insect has two ‘necks’, one
behind the head, and one
between the thorax and
abdomen: making both the
abdomen and the head
mobile relative to the thorax
(the locomotory tagma).
One might say there are 4
tagmata: thorax, head,
abdomen and the petiole (a
tagma of one segment). TorreBueno: “petiole: a stem or
stalk; the slender segment
between the thorax and
abdomen in certain Diptera
and many Hymenoptera; in
the latter a pedicel formed of
only one segment, or the first
segment of a two-segmented
pedicel in ants*; in plants the
slender stalk of a leaf.”
Petiole provides tagma manoeverability
for stingingand immobilizing prey or for
carrying prey (?).
Orthopteran Cyphoderris
illustrates a segmented
abdomen adapted for
When acoustic insects
stridulate the abdomen
telescopes in and out like a
little bellows, ventilating the
tracheal system (tracheal
sacs). Just as we breathe
more heavily under
exertion, so the katydid
shows more rapid
abdominal movements
when singing. The retention
of telescopic segmentation
in the abdomen of many
insects allows this
ventilating function.
Spiders evolved from serially segmented ancestors; they have two
tagmata: cephalothorax/soma, opisthosoma/abdomen; intervening
pedicel creates mobility of abdomen tip for creating silk structures.
Maydianne Andrade
Spider spinnerets terminal on opisthosoma
‘opistho-’ means behind or to the back. Spider opisthosoma is the ‘to-the-back’
tagma; it houses silk glands and spinnerets and the tagma gets necessary
manoeverability for silk disposition (web-building) from the pedicel, a constricted
‘neck’ connecting the locomotory tagma and the weaving organ. (Some primitive
spiders have segmented opisthosomas; but most have lost this segmentation.
Opisthosomas house book lungs and can be the basis for hydraulic extension of
the legs with fluid.
Phylum Echinodermata
Water vascular system of
these animals is unique to
them; it is a *hydraulic
mechanism (Kier): a
system of vessels and
chambers arising from a
tubular ring around the
mouth called the ring
canal; in an asteroid one
radial canal travels into
each of the arms. They
have a coelom but the
water vascular system is
separate from the coelom
*Where movement or shape
change occurs through
actual displacement of fluid
from one location to another.
Ambulacral grooves below each arm
lined with tube feet or podia.
Asteroid echinoderms have an
exoskeleton. In their dermis
are embedded calcareous
plates (inorganic salt Calcium
Carbonate) called ossicles; the
skin thus consists of ossicles of
various shape separated by
collagen- fibre connective
Above each tube foot inside
the arm is a vesicle called an
ampulla encircled by ampullar
muscles; their contraction will
push incompressible fluid out
of the ampulla, displacing it
into the tube foot.
(in the case of earthworm
segments the fluid is not
displaced from its container
but it is here.
There is a valve within the side
branch to the radial canal – a
one-way valve that closes to
prevent backflow of the fluid
into the water vascular system
Fig. from Frank Brown,
Selected Invertebrate
Ampulla is squeezed
by ampullar or
protractor muscles;
the fluid is displaced
down into the lumen
of the tube foot;
because of the
crossed fibre helical
connective tissue
array in the wall of the
tube foot (CFHCTA)
the accordioned tubefoot wall smooths out
as the foot protracts,
i.e., lengthens.
McCurley R.S. & Kier W.M. 1995. The functional morphology of starfish tube feet: the role of a
crossed-fiber helical array in movement. Biological Bulletin 188: 197-209.
The great importance of helical fibres in the functioning of tube feet is
explained by Kier, but see also this paper by McCurley.
The cylindrical tube foot extends when the ampullar muscles contract and
displace fluid into it.
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 isn’t.] In the absence of connective tissue fibres in the tube
foot walls when fluid pressure increased in the tube foot it would swell more
in diameter than it lengthened; the helical fibres oppose this diameter
increase so that there is an increase in length rather than diameter.
How can a connective tissue fibre which is relatively inextensible (“stiff in tension”)
serve to prevent diameter change while allowing lengthening? The answer is the
pitch of the helix changes.
p. 207 , Fig. 10 McCurley
Tube foot model: treat it as a cylinder and consider this cylinder “wrapped by a single turn of
an inextensible helical fiber.
Fibre angle θ is the angle that the helical fibre makes with the long axis of the cylinder (tube
When tube foot is at its shortest, θ is 90 degrees
When tube foot is at its maximal extension θ is 0 degrees
Confirm this for yourself by drawing a lengthened and shortened version of his modelled
single-turn cylinder, see Fig. 10 of McCurley

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