Unisexual and Bisexual Animals

In most animals, the male and female gametes are produced by the male and female animal, respectively. This results in animals being classified as unisexual or dioecious. However, some animals possess both ovaries and testes in the same individual, allowing them to produce both male and female gametes. These animals are called bisexual, monoecious, or hermaphrodite. Examples of such animals include the liver fluke, earthworm, leach, and tapeworm. Hermaphrodites have the advantage of doubling their reproductive capacity, as all individuals are capable of producing eggs. In the case of endoparasites like the tapeworm, hermaphroditism allows for self-fertilization, which is essential for the organism's survival.

Animals that undergo internal fertilization and external development are called oviparous. Reptiles and birds are examples of oviparous animals that lay shelled eggs to protect the developing embryo. Ovoviviparous animals, on the other hand, retain their eggs inside their bodies until they hatch. The fully developed offspring are then released. Examples of ovoviviparous animals include oysters, seahorses, garter snakes, pit vipers, and the duckbill platypus.

Mammals are particularly known for their viviparity, where fertilization and development occur internally within the female's body. The female gives birth to fully formed young, and the nutrients required for development are supplied by the mother. Viviparity represents the ultimate form of care for the zygote and embryo. The placenta, a complex structure derived in part from the chorion, first appeared in shelled eggs. Its evolution allowed developing offspring to exchange materials with the mother, ensuring their survival.