The male and female gametes in most animals are usually produced by the male and female animal, respectively. Such animals are called unisexual or dioecious.
Animals with both ovaries and testes in the same individual i.e. male and female gametes are produced by the same organism. Such animals are called bisexual or monoecious or hermaphrodite e.g. Liver fluke, earthworm, leach, and tapeworm. Hermaphrodites double the reproductive capacity since all individuals can produce egg. In case of endoparasites such as tapeworm, hermaphrodites permit self-fertilization which is essential for such organisms.
After internal fertilization external development takes place e.g. reptiles and birds. They lay shelled eggs to protect the developing embryo. Such animals are called oviparous. Some animals are Ovoviviparous that their eggs are retained in the body until they hatch. Then fully developed offspring are released, e.g. Oysters, Sea horse, Garter snake, Pit vipers, Duckbill platypus.
Mammals are particularly viviparous. Here fertilization and development is internal, i.e. inside the female body. The female gives birth to the young ones, and the nutrients needed for development are supplied by the mother.
Viviparity represents ultimate caring for zygote and embryo. The placenta is a complex structure derived in part from the chorion, which first appeared in shelled egg. Its evolution allowed the developing offspring to exchange materials with the mother.