It permits calcium to leave the sarcoplasmic reticulum and enter the cytosol. During muscle contraction, the binding of acetylcholine initiates an excitatory impulse, which is transmitted to the deep of the muscle via T tubules. T tubules are the invaginations of cell membranes of muscle cells (sarcolemma). When the action potential travel down the t-tubules, they change shape and allow the calcium ions to enter into the sarcoplasm from the sarcoplasmic reticulum.
A single species that has evolved into several different forms that live in different ways has undergone "b. convergent evolution" since the multiple "divergent" species are seen to "converge" into a single species if you go back far enough in time.
