Amphibians such as frogs and newts are more dependent on the aquatic habitat than reptiles. Their eggs are protected only by a thin membrane and are laid in water. Many of them go through several developmental stages, till they become adults. Unlike amphibians, reptiles such as tortoises, crocodiles or snakes have a scaly skin. Their eggs are covered by a hard calcite shell and are laid on land.
Female fish lay eggs—they spawn. The male fish eject their semen on the eggs, and thus fertilize them. Many fish like the salmon return to their place of birth for laying eggs. The salmon eggs hatch in streams and from freshwater fish turn into saltwater fish in 1-3 years. Then, they remain in the sea up to 4 years. When they wish to reproduce, they return to their birthplace covering a distance of as much as 6000 km. They have to swim against the current and overcome all the obstacles in their way. Once at the destination they spawn. It is a mystery how the salmons find their birthplace. Scientists assume that they are guided by the magnetic field of the Earth.
Insects have the largest number of species among animals on the Earth. Out of ten different animal species, eight are of insects. They can survive in almost all types of habitats regardless of temperature. Many can survive only in specific environmental conditions, while others are found in all parts of the world. The reason for this difference within the species is not known. Insects have six legs and a body divided into three parts with a solid outer covering called the armour. Many insects are useful (bees which pollinate fruit tree flowers), while others are harmful (plant louse on indoor plants).
‘Mammal’ is derived from the word ‘mammary’. The young ones of mammals suckle milk from the teats of the mother. This milk contains all the essential nutrients for growth. Except for the monotremes, all mammals are born as young ones and are not hatched from eggs. Contrary to almost all other animal species, mammals nurture and train their young ones for a considerable time. Mammals have a constant body temperature of 36-40°C, so they need a hide (like an elephant) or a thick layer of fat (like a whale) as protection against extreme heat or cold.
Biologists have classified the animals according to their common features. Animals that share a lot of common features are known as species. For instance, all members of the dog species have similar body structures and group behaviour. Similar species are grouped together in one family, similar families are grouped in orders, and orders are further combined to form classes. The fox, wolf, and dog species combine to make one ‘family’ of canine predators. Together with the cat and bear families, they belong to the ‘order’ of predators in the ‘classes of mammals, which in turn belong to the vertebrates in the ‘kingdom’ of animals.
There are more than 1.5 million animal species. Ranging from simple animals like sponges and jellyfish, the invertebrates include the arthropod phylum with over 1 million species of insects. The higher- level vertebrates include fish with about 35,000 species. Around 350 million years ago, when aquatic animals moved to land, they adapted to the new environment and amphibians were born. They adapted some more and became reptiles. From reptiles came the birds and around 4000 known species of mammals.
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