Jan 15 Bio 325 lecture
Announcements
Papers mentioned in sources are not all to be read with equal intensity.
Something like Prud’homme is pretty much all worth assimilating; but
Vincent is really for reference – to pick up bits and pieces of information
about what comprises cuticle. I will asterisk sources that require more
intensive attention.
Lumbricus castaneus
Earthworm Society of Britain
o
What is the function of the annelid
coelom?
This is an animal adapted for
burrowing, a cylindrical anteriorly
pointed probing snout with serial
partitioned fluid units bound by
muscle: it is a digging machine far
more flexible than any shovel that
makes its way through soil.
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 metamere has its own pair of kidneys
Coelomate phyla include Chordata (vertebrates) and
Annelida (segmented worms); Platyhelminthes are acoelomate.
Coelom (defn)
fluid-filled cavity forming
within mesoderm (one of
the primary germ layers
of the embryo
Animals with a coelom
are termed coelomate,
without one acoelomate.
To burrow effectively through soil,
searching out softer regions or
crevices, going around or under
rocks etc. it needs to twist and turn
its body into all sorts of planes. It
needs to push; and for push you
need purchase. For this you need
chaetae; pro and retractable
‘hobnails’.
notice larger longitudinals
Schizocoel: splitting coelom
formation in annelids
Development, primary germ layers:
ectoderm, endoderm, mesoderm;
budding behind the trochophore larva in
a series of segments
bilateral spaces appear in mesoderm
and expand 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 a septum
Why segments?
Bio 325 Title
Why septa? Muscle fibres on septa allow
them to be stiffened to resist pressure
forces. Septa are... crucial in allowing
large lateral forces to be exerted during
burrowing” (Kier 2012, p1250). “...
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“at a given pressure, the stresses in the circumferential direction {hoop stress of
McCurley} are twice those in the longitudinal direction.” [sounds like this ought to be
Kier’s Law] “...the volume of the longitudinal muscles is greater [than the circular]”
“...the resulting pressure produced by the longitudinal muscles may be as much as 520 times greater than those produced by the circular muscles” This is adaptive in
obtaining purchase, i.e., less force is needed to contract circulars and reach a part of
the body forward or backward.
stereotyped crawling
movements: but soil is not
this homogenous
Stereotyped burrowing by peristaltic body waves
Each segment goes through a sequence of contraction and
relaxation the longitudinals and circulars apropriately out of phase
as antagonists.
Body waves of muscle contraction which travel in the opposite
direction to that of the resulting body displacement are termed
retrograde.
Body waves of muscle contraction which travel in the same direction
as that of the resulting body progress are termed direct.
For fluid skeletons in cylinders “at a given
pressure, the stresses in the
circumferential direction are twice those in
the longitudinal direction” Kier
Echinodermata
radial body not serial
oral and aboral surfaces
•
Water vascular system of
these animals is unique to
them; it is a vessel system,
‘pipes’ arising from a
tubular ring around the
mouth called the ring
canal; in an asteroid one
radial canal travels into
each of the arms
ambulacral grooves below each arm
lined with tube feet or podia
Skin of asteroids and ampullae
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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 protein collageous fibres (connective tissue)
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
From Brown,
Selected
Invertebrate
Types
‘plumber’s helper
with vaseline’
friction with the
substratum is
important
stack ossicles,
muscles of disc pull
up central region,
mucus seals
rosette of ossicles
gives suction cup
shape
spiral connective tissue
limits response planes of
structure and is essential
for protraction
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.
Virginia Living Museum
‘off the beaten path’
Tube foot functions in
predation: pulling with
tube feet adhering and
starfish arm muscles to
open the protective
valves of shellfish
Mollusca
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.
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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.
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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 foot)
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
Storing information by phylum
• Useful to have a very simple phylogenetic cladogram in your head of
the major phyla.
• Dunn, C.W. et al. [18 authors!] 2008. Broad phylogenomic sampling
improves resolution of the animal tree of life. Nature 452: 745-749.
• See Figure 1 of Dunn. Which I have simplified in the following way.
The result of molecular base
sequencing was a grouping of
coelomate bilateria into two
major clades: Lophotrochozoa
and Ecdysozoa
I want you to learn these two names
and the major phyla within them.
Here is a simplified Fig. 1 from Dunn.
Protostomes
Deuterostomes
Lophotrochozoa:
Mollusca, Annelida,
Brachiopods,
Entoprocta:
lophophores and
trochophores