Although spermatozoa have no organ system, they are the product of organ systems and are of enormous evolutionary importance. Spermatozoa typically possess an acrosome, a nucleus, mitochondria, and a cilium, but these compartments can vary considerably in their shape, arrangement, number, or by being absent. There is a general correlation between external fertilization and the so-called round-headed spermatozoon. In connection with the various modes of modified external and internal fertilization, spermatozoa change their shape to aciliary, filiform, spirally coiled, and others. An evolution from a round-headed to a derived spermatozoon is likely, but some exceptions are possible.
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