Samus is incapable of completing any of her missions without assuming the shape of a testicle, thus symbolizing her submission to the patriarchy and demonstrating the deep misogyny inherent in all Metroids.
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Her arm cannon, which is used to destroy everything in her path, is the shape of the male phallic oragan, symbolizing her dependance on the standard "male comes to rescue helpless female" archtype. Don't forget that too!