Unicellular animals

Single-celled animals are a category of organisms that stands outside the system. This means that they can not be completely attributed to any particular kingdom. Unicellular organisms are distinguished by the absence of highly organized tissues. All animals belonging to this group have no common signs among themselves. The only thing that unites them is a simple structure.

Single-celled animals are usually so tiny that they can be seen only under a microscope. Their habitat is wet. This is soil and water, as well as the body of man, animal. All of them in one way or another, with the help of various adaptations, adapt to different conditions. First of all, it is a form of the body. It may not have clear boundaries, constantly change, or may, on the contrary, be streamlined, similar to spindle or elongated. Symmetry types are also different: radial, translational-rotational, two-sided. Some unicellular animals have a shell outside, others, those that live deep under water, are unusual growths.

The cell, of which the body of these organisms consists, can contain from one to several nuclei. The membrane is either only a membrane or a more dense, more stretchable, pellicle.

The unicellular organism is moved by means of various cilia, pseudopods, flagella. They also react to the influence of external factors such as changes in the temperature regime, lighting, the presence of chemicals.

Food unicellular animals are obtained in different ways. Thus, during phagocytosis, cytoplasmal outgrowths capture solid particles of food. Pinocytosis occurs in several stages: first the surface of the whole cell captures the liquid, and then absorbs the substances contained in it, processes them with the help of digestive enzymes, which fill the vacuoles. In some protozoa (chlorella) there are chloroplasts, which, using photosynthesis, can produce organic substances from inorganic substances.

Also, the whole surface of the protozoan body participates in gas exchange: through it out products of decay and excess water.

Unicellular animals reproduce both sexually and asexually. It depends on what conditions they exist in. Asexual reproduction occurs as follows. First, the nucleus is divided into several parts, then the cytoplasm is divided into the same number of parts. Thus, from one simple organism , several (at least two) are obtained.

In sexual reproduction , female and male individuals participate. Their structure and dimensions may differ, or they may be the same. As a result of their fusion, a zygote is formed, which later multiplies independently by asexual pathway. It happens that the individuals, in contact, exchange particles of nuclei. In this case, the zygote is not formed.

When the conditions do not favor normal functioning of the protozoa, their body becomes round, covered with a dense shell. So the cyst is formed. Once the conditions improve, the body is freed from the thick film and begins to lead the same lifestyle as before.

It is generally believed that unicellular animals were the first to evolve on Earth in the course of evolution. The most ancient are the archaea and bacteria. They are similar in many respects (for example, the absence of a nucleus, the presence of a ring chromosome), for this reason they were previously assigned to one group. But modern science has proved that archaeans have their own characteristics in the structure, and evolved a little differently. Although they are also not easy to classify, as before. The matter is that in the laboratory conditions the archeas were never grown, but were found during the analysis of samples taken from the places where they live.

Unicellular organisms are a link, without which it is impossible to imagine a complete biocenosis. After all, they are eaten by many animals, which themselves serve as food for a number of other inhabitants of our planet.