狠狠撸

external fish skeletal system ligaments elbow shoulder walk walking shark tiger ray sting skate hagfish lampreys sharks gills bony jawless cartilaginous joints knee flexion movement joint abduction extension adduction inversion plantar eversion dorsiflexion hyperextension hip tree frogs amphibians fertilization muscles muscle fiber contraction motor neuron synapse end plate synaptic cleft acetylcholine neurotransmitter lungs breathing gar bichir internal anatomy ventricle atria chambers three circulatory cycle life frog tadpoles toads salamanders