Copulation is considered to occur when both of two conditions are met: (a) spermatozoa are deposited by a specialized male organ in the female genital tract or in female structures leading to the site of fertilization, and (b) fertilization is internal. As such, copulation is known to occur in eight of the twenty-one animal phyla, including both the simplest and most complex organisms; it can be inferred to have developel inlependently several times during the course of evolution; it is notably associated with a terrestrial habitat or a parasitic status. Many morphological details of copulatory organs that differ between species have no obvious functional significance and probably reflect variation under low selection pressure.