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.

Jan 15 Bio 325 lecture

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.

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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

Earthworm Society of Britain

Lumbricus castaneus

Animals with a coelom are termed coelomate,

without one acoelomate.

Coelom (defn)fluid-filled cavity forming within mesoderm (one of the primary germ layers of the embryo

Coelomate phyla include Chordata (vertebrates) and Annelida (segmented worms); Platyhelminthes are 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 segmentsbilateral 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

Bio 325 Title

Why segments?

• “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 5-20 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.

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). “...

stereotyped crawling movements: but soil is not this homogenous

Stereotyped burrowing by peristaltic body wavesEach 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

Echinodermataradial 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 armlined with tube feet or podia

Skin of asteroids and ampullae

• 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, SelectedInvertebrateTypes

spiral connective tissue limits response planes of structure and is essential

for protraction

‘plumber’s helperwith vaseline’friction with the substratum is importantstack ossicles, muscles of disc pull up central region, mucus sealsrosette of ossicles gives suction cup shape

Santos, R. et al. 2005. Adhesion of echinoderm tube feet to rough surfaces. J. exp. Biol. 208: 2555-2567.

Fig. 6 external morphology of unattached pedal discs of Paracentrotus lividus (left) [sea urchin] and Asterias rubens

[starfish] (right) .

End of extensible cylinder is the disc, larger in diameter than the stem. There is a central depression.

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.

Tube foot functions in predation: pulling with tube feet adhering and starfish arm muscles to open the protective valves of shellfish Mollusca

Virginia Living Museum‘off the beaten path’

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 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 basesequencing was a grouping of coelomate bilateria into twomajor clades: Lophotrochozoaand EcdysozoaI want you to learn these two namesand the major phyla within them.Here is a simplified Fig. 1 from Dunn.

ProtostomesDeuterostomes

Lophotrochozoa: Mollusca, Annelida, Brachiopods, Entoprocta: lophophores and trochophores