Animals life cycles

Animal group

Scientists put animals into groups. The animals in each group are the same in many ways, and they often have very similar life cycles. The two biggest animal groups are invertebrates and vertebrates.
Invertebrates are animals that have no backbone. More than 95% of the animal species in the world are invertebrates, and most of them are very small. There are many different groups of invertebrates. The biggest group is called arthropods. Insects, spiders, and animals like crabs that have a hard shell are all arthropods.
Animals that have a backbone are called vertebrates. Only a few percent of the animals in the world are vertebrates, and most of them invertebrates. Because of this we see vertebrates more often than invertebrates. Scientists put vertebrates into five big groups __ they are called fish, amphibians, reptiles, birds, and mammals.
                  
                          Breeding
Most species of animal have males and females. Males produce sperm, and females produce eggs. Baby animals are made when sperm from the male joins together with an egg from the female. This is called fertilization.

Some animals give birth to baby animals. Other animals, like reptiles and birds, lay eggs. Their babies hatch - they break the egg open and come out.

Most reptiles and birds care for their babies. Most insects and fish do not care for their eggs or their babies. All mammals care for their babies and feed them milk. Some big mammals stay with their babies for many years.