1. 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

2. Habronattus Jumping spider from Arizona

3. Words of the day: saltatorial, vermiform, hydrostat. 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.) Phidippus putnami Prairie Ridge Ecostation

4. 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 fluid- incompressible 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....

5. 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)

6. 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 material. 3.Amplification [mechanical]: skeletal leverage via optimal force moments can increase the force effect.

7. Principles of support and movement (Kier 2012) hydrostatic skeletons 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).

8. 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. 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 ground. Leech Looping

9. 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. torsion: twisting due to applied torque 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.

10. Re levers: remember distance/speed advantage [Kier calls speed advantage velocity advantage] and some levers are distance- speed 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 contraction.” “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…”

11. For a fixed volume (see Fig. 2 above) the percentage increase in length, brought about by shortening of [fibres] becomes greater as diameters decrease. 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 (= speed- increasing) geometric relationship to improve the rapidity of its strike at a prey item.

12. 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” 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”. A squid striking at a lure.

13. 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. Susan BarkerUseful to memorize crossed fibre helical connective tissue array.

14. 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. 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. )

15. 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 right- angles (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.

16. 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).

17. Jennifer Goble Hydrostatic Skeletons Tubastrea DiveGallery Ryan Photographic 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 cavity.

18. Fig. 5 Kier 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 wall.” Transverse section at 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.

19. 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.”

20. 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.

21. ABC News Leeches are also annelids, freshwater, not marine, specialized for blood feeding. Transverse grooving along leech body does not represent its ancestral segmentation and is not homologous with grooving of Nereis. Phylum Annelida segmented worms. Most species are marine, polychaetes. Annelida have a coelom. Metamerism and hydrostatic skeletons Coelomate animals Metameres are segments grouped sometimes into tagmata: a tagma is a series of metameres specialized for a shared function. Nereis

22. Animals with a coelom are termed coelomate, animals without one are acoelomate. What is a coelom? It is defined as a fluid- filled cavity forming within mesoderm (mesoderm being one of the primary germ layers of the embryo). 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’. chaetae

23. 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. cronodon.com Free-swimming trochophore larva dispersal stage A quick ‘course’ in annelid embryology. At first three embryonic tissue layers, endoderm (blue), mesoderm (red), ectoderm (green) developing into the trochophore. Development pauses while the trochophore lives, as a decidedly not worm-shaped creature, moving in the sea by its rings of cilia. Then growth is renewed by the budding of a series of posterior segments.

24. 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 septa- partitioned hydrostatic skeletal units bounded by muscle: its body is a flexible digging machine for making its way through soil. o 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. Earthworm Society of Britain Lumbricus castaneus clitellum Each compartment has its own pair of nephric tubules. Why? Clitellum is a tagma

25. 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.