In Hydra the gonads are temporary structures and appear during period of sexual reproduction as small swellings on the surface of the body.
In all multicellular animals above the coelenterates the gonads are developed within the body.
Sexual reproduction involves two most fundamental events, meiosis and fertilization. Meiosis is the means by which gametes from the germinal epithelium of the gonads are formed and reassortment of different genes takes place in the formation of gametes.
Fertilization involves the fusion of two dissimilar gametes or nuclei and is– the means for combining or mixing different genes in the production of offspring.
The two dissimilar gametes on fusion or fertilization form a diploid structure called the zygote which gives rise to the new individual.
Thus sexual reproduction permits the combining within a single individual of the best genes of its parents.
Almost all species of animals have some means of the sexual mixing of the genes. Let us survey some of the methods of sexual reproduction found in animals.