Animals are divided into 2 groups based on reproductive characteristics.
(i) Asexual reproducer: Those animals which do not have any specific reproductive organs and reproduce without the help of opposite sex are called asexual reproducers. Asexual reproduction is of three types. These are-
* Binary fission: The process in which an organism divides to produce two offspring is called bisegmentation. Such as Euglena (Euglena viridis), Hydra (Hydra vulgaris) etc.
* Multiple division: The process in which a single-celled organism divides repeatedly to produce numerous offspring is called multiple division. For example – Entamoeba histolytica, Paramecia (Paramecium caudatum) etc.
* Budding: The process by which new organisms are created by budding from the mother body is called budding. Such as Hydra (Hydray viridis).
(ii) Sexual reproducer: The process of reproduction through the union of sperm and egg of two animals of opposite sex of the same species is called sexual reproduction.
* Oviparous: The animals that lay fertilized or unfertilized eggs and the eggs hatch outside the mother’s body to develop into young animals are called oviparous. For example – fish, birds, snakes, poultry, turtles, frogs etc.
* Endo-viviparous: The animals that lay eggs and after fertilization the eggs develop in the mother’s womb and become babies are called endo-viviparous. For example, sharks, some amphibians etc.
* Viviparous: Those animals whose eggs are fertilized inside the mother’s body and the embryo receives nutrients from the mother’s body through immortality and develops into a full-fledged baby animal are called in utero. For example, mammals.