Reproductive System of Animals
. In many invertebrate species individual animals bear both testes and ovaries (see Hermaphroditism ). In some invertebrates, and in most vertebrates, individuals bear either testes or ovaries, but not both sets of organs. In invertebrates, a single animal may have as many as 26 pairs of gonads; in vertebrates, the usual number is 2. Cyclostomes and most birds are unusual among vertebrates in possessing only a single gonad; owls, pigeons, hawks, and parrots are unusual among birds in having two gonads. The size of gonads increases at sexual maturity because of the great number of germ cells produced at that time; many germ cells are also produced during breeding seasons so that many animals have a seasonal increase in size of the gonads. During the breeding season of fish, the ovaries increase in size until they constitute about one-quarter to one-third of the total body weight. The testes and ovaries of mature animals differ greatly in structure. The testes are composed of delicate co