The first living organisms on Earth were ancient unicellular organisms, from which modern sarco-flagella, ciliates, sporozoans and others originated.
Ancient multicellular animals originated from the ancient colonial flagellates with an animal type of food.
The formation of the first multicellular animals was a major event in the history of the development of the animal world. Multicellular animals have received great advantages over unicellular ones: they have a division of cells according to their functions, a more complex body structure, an increase in size, etc.
Multicellular organisms have become larger than unicellular organisms. In this regard, they faced several important problems that they had to solve:
- the need for more food, which led to a complete transition to a heterotrophic diet and the formation of a digestive system;
- the need for an effective method of movement, which led to the appearance of a skeleton and muscles (an elongated streamlined bilaterally symmetrical body shape facilitated movement);
- the remoteness of cells from the surface of the body and an increase in the impermeability of the outer covers led to the emergence of a transport system;
- the increased complexity of the body required the formation of controlling nervous and endocrine systems;
- in order to more quickly register signals from the external environment, the senses moved to the front side of the body, as a result of which the head became detached.
From the first multicellular organisms there were two-layer animals with an intestinal cavity and radial symmetry – coelenterates.
Ancient primary worms with a body formed from three germ layers and with bilateral symmetry also originated from ancient multicellular organisms.
The first three-layered animals were probably similar to the most primitive flatworms – the intestinal planarians. Their body was covered on the outside with a layer of ciliary cells, and inside it had digestive and intermediate cells that developed from the middle germ layer (mesoderm).
In the process of historical development, three-layer animals (flatworms) acquired progressive structural features: muscles (which ensured faster and more perfect movement of animals) and basic tissue, or parenchyma (thanks to which the internal environment of the body was formed, providing a more perfect metabolism).
In roundworms, a primary body cavity (filled with fluid) has formed as a result of partial destruction of the parenchyma. This was the first animal transport system.
The next stage in the historical development of the animal world is associated with the appearance of three-layer animals with a secondary body cavity in which the internal organs are located. This body cavity has its own wall of one layer of cells from the remnants of the primary body cavity.
The circulatory system was formed in secondary cavity animals. Thus, in secondary cavity animals, two transport systems arose: the secondary body cavity and the circulatory system. The latter began to perform mainly the function of transporting oxygen and carbon dioxide throughout the body.
In secondary cavity animals, more perfect excretory organs and a complex nervous system have been formed.
Mollusks and annelids originated from primitive secondary cavity animals, and arthropods from annelids. This group of types of secondary cavity animals is called primary stoma, since their mouth is formed from the primary mouth of the embryo.
The group of types of deuterostomes originates from primitive three-layered animals. They are called secondary stomas because the mouth of adult animals does not arise from the mouth of the embryo, but is formed anew (secondarily) at the other end of the body.
Deuterostomes include such types of animals as Echinoderms (starfish, etc.) and Chordates.