1. In animals like insects, reptiles, birds and mammals, the male animals deposit the sperms inside the body of the female animals, where fertilization occurs. This is called Internal Fertilization. This is most common in terrestrial animals.
2. In majority of the animals, sexes are separate and male and female animals are distinct. This is called sexual dimorphism and animals are said to be unisexual.
3. The external features by which the males and females can be distinguished are called Secondary sexual characters.
4. There are some animals in which male and female sex organs are present in the same animal. This is called Hermaphroditism and such animals are called Hermaphrodites or Bisexual.
5. Hermaphroditism is seen in some of the members of protozoa, Coelenterata, Platy- helminthes, Nematoda, Annelida and mollusca. In these animals sperm and ova are formed in the same animal. However self fertilization is prevented by several methods.
