•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
Univ of Wisconsin
• 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
Coelomic cavity
as a hydrostatic
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
or may be ball and socket and move in
multiple planes like monocondylic of
articular capsule contains synovial fluid
ligaments bind the bone ends near each
Endoskeletal muscle
antagonists involved in
frog jump:
gastrocnemius vs
tibialis anticus longus
Current reference
• Azizi E., Roberts T.J. 2010. Muscle performance during frog jumping:
influence of elasticity on muscle operating lengths. Proceedings Royal
Society (series) B 277: 1523-1530
• 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
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.

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