Even an experienced zoological scientist with full confidence will not be able to say exactly how many species of animals exist now and existed before on our planet. New species are constantly being discovered, and, unfortunately, the old species disappear from the face of the Earth. Perhaps that is why it is so necessary to know more about the reproduction and development of animals in order to try to artificially prevent this extinction, to try to comprehensively study the species that are on the verge of extinction.
A little bit about classification
The animal kingdom has quite a few types and classes, including worms and mollusks, crustaceans and echinoderms, arachnids and insects. And the following are classified as vertebrates: fish, amphibians, reptiles, birds, mammals. The variety of animals that live on the planet is represented quite widely by species sets. But animals so different in structure, united in one kingdom, differ in many ways. How do animals breed? For different groups, the process is carried out in different ways. Let's look at the main types.
Asexual reproduction
Some lower animals (invertebrates) are characterized by asexual reproduction, as the easiest way to create and increase offspring. Reproduction and development of animals of this kind is carried out using direct division or budding. Such animals include worms, sponges, jellyfish, lancelet, and many others. In fact, there are not so few of them. But nature in the process of evolution has created another, more laborious and complex process of reproduction.
Sexual reproduction
This type assumes the presence of two sexes to create offspring: male and female. The reproduction and development of animals here occurs by laying eggs or eggs, the birth of cubs (mainly mammals). For example, insects and arachnids, as well as snakes and birds, lay eggs. Spawn: amphibians and fish. But some fish species are viviparous (for example, guppies). All mammals (including humans) give birth to cubs and feed them milk. In some animal species, reproduction occurs without the participation of males. This phenomenon is called parthenogenesis. Common in lizards and bees, for example.
Animal development
It also proceeds differently. So in insects, for example, there are as many as three stages: an egg, a larva, a pupa, an adult individual that can lay eggs. And in some fish, the reproduction and development of animals of this species occurs in the femaleโs mouth, where she carries eggs until offspring appear. In amphibians, an intermediate form develops from eggs - a tadpole, from which an adult frog is already formed. In turtles and snakes, offspring are hatched directly from eggs, as in birds. All species of mammals are viviparous, even dolphins, which in ancient times were considered fish. They feed offspring with milk, hence the name of the class. The maturation of offspring in various animal species occurs at different times: from several minutes to tens of years. The most โfastโ is for one-day butterflies and some other insects. The youngest generation is the slowest growing in the highly organized class of mammals.