Skeletons Continuing with three contrasting skeletal forms: hydrostatic, endoskeleton, exoskeleton: annelid coelom, insect mandible, frog leg Why do earthworms have a metameric body? Metamerism extends to nervous, circulatory and excretory systems: leech looping, segment shape: look with the adaptive eye and imagine it otherwise: flatworm in a burrow? Grasshopper mandibles: dicondylic joints and axis of rotation, flexible and lubricating (?) articular membrane part of continuous exoskeleton; (planes of joint axes change along arthropod limbs), role of tentorium truss, muscle fibres dominate interior of head; adductor and abductor apodemes: unshortening muscles: muscle antagonists, pinnate muscle arrangement: high force since total length of fibres greater, but shorter distance to move apodeme Frog: skull and axial skeleton, appendicular skeleton, girdles, hinge joint, synovial fluid lubricates Appendage movments: retractor vs protractor, adduction vs abduction; promotor vs remotor, depressor vs elevator and extensor vs flexor Frog hind limb: femur, tibiofibula, astralagus (ankle); gastrocnemius [plantaris], Achilles’ tendon: pinnate fibre arrangement: Unshortening muscles: another muscle: another material Anatagonists : tibialis anticus longus vs plantaris; adductor mandibular muscle vs abductor mandibular muscle; circulars vs longitudinals Elastic energy storage; scallop hinge; abductin, resilin Muscles of frog jump: elasticity influences muscle operating length

Phylum Annelida mostly marine Lumbricus earthworm The adaptiveness of a segmented body: outer circular and inner longitudinal muscles, septa (septum sing.) fore and aft compartmentalize the coelom; muscles made antagonists by the fluid skeleton: the coelomic fluid, which translocates forces; moving in a burrow Univ of Wisconsin

Coelomic cavity as a hydrostatic skeleton

Metamerism: adaptation for locomotion

From Wikkimedia Commons pictures by Hans Hillewaert Nereis succinia epitoke of polychaete worm

Exoskeleton: dicondylic joint, mandibular apodemes Role of tentorium in resisting stresses and strains, some produced by the chewing actions of the mandibles in crushing food

Exoskeletal muscle antagonists: apodemes of the insect mandible

Crayfish cheliped: segments articulate by dicondylic joints Successive axes change angle By about 45 degrees

Synovial hinge joint: may also move mainly in one plane like dicondylic of arthropods or may be ball and socket and move in multiple planes like monocondylic of arthropods articular capsule contains synovial fluid ligaments bind the bone ends near each other

Endoskeletal muscle antagonists involved in frog jump: gastrocnemius vs tibialis anticus longus

Current reference Azizi E., Roberts T.J Muscle performance during frog jumping: influence of elasticity on muscle operating lengths. Proceedings Royal Society (series) B 277: See also ‘Outside JEB’: small articles that summarize papers: Gary B. Gillis volume 213 of J. exp. Biol. ‘Frog muscles start stretched’ The frog’s ankle-extending gastrocnemius (= plantaris) muscle contracts over a relatively long distance: it shortens by 30% of its resting length: it starts to contract at 1.33 its resting length. But most muscles “ have a relatively narrow range of lengths over which they generate their highest forces. It seemed unlikely that muscles that shorten over great distances… will spend much time at lengths where forces can be maximized. Bullfrogs; force-length curve with ascending ‘limb’, plateau, descending ‘limb’

An example of an antagonist to a muscle that is not another muscle; the Pecten adductor stores the energy of distortion that will later restore it to its precontracted state.