Lower animals like the sponge and hydra can reproduce through asexual methods. They can, however, also reproduce sexually. In all animals, the female produces eggs and the male produces sperms. An egg and a sperm fuse to form the zygote which then develops into the embryo and the embryo into a full-fledged individual.

The development may occur partially or completely, inside the egg. Such animals that lay eggs include fish, frog, reptiles and birds and are thus called oviparous. In mammals such as cats, dogs, cows and humans, the baby develops inside the mother’s womb. They are termed viviparous.

In tapeworms and earthworms both female and male sex organs are in the same individual. Such individuals are termed hermaphrodite. Sexes are separate in all other animals, and the male individual has male organs like testes, etc. while the female has ovaries etc. Humans also belong to the animal kingdom. They are mammals and hence viviparous.