Animals can reproduce in two ways: asexually or sexually. Both modes have advantages and their evolutionary distribution and significance is discussed in this chapter. In sexually reproducing animals, sexes can be separate (gonochoristic) or combined in one individual (hermaphroditic). Gametes can be formed in a variety of ways, but not necessarily in particular structures called gonads. Sponges do not have anything like a gonad, and cnidarians and ctenophores concentrate gametes in particular regions called gametogenic areas. Most bilaterian animals have particular gonads, which can originate in different ways. The gametes also originate in different ways, either very early in development by ‘preformation’ or later by ‘epigenesis’. Gametes are transferred in different ways, producing numerous reproductive structures for gamete storage and transfer.
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