Unicellular animals are a category of organisms that is outside the system. This means that they cannot be fully attributed to any particular kingdom. Unicellular organisms are characterized by the absence of highly organized tissues. All animals belonging to this group have no common features among themselves. The only thing that unites them is a simple structure.
Unicellular animals are usually so tiny that they can only be seen under a microscope. Their habitat is humid. This is soil and water, as well as the body of a person, an animal. All of them, one way or another, with the help of various devices adapt to different conditions. First of all, this is the shape of the body. It may not have clear boundaries, constantly changing, or it may, on the contrary, be streamlined, like a spindle or elongated. Symmetry types also differ: radial, translational-rotational, bilateral. Some unicellular animals have a shell outside, others, those that live deep underwater, are extraordinary growths.
The cell that makes up the body of these organisms can contain from one to several nuclei. The shell is either only a membrane or a denser, more stretchable, pellicle.
The unicellular organism moves with the help of various cilia, pseudopods, flagella. They also respond to the influence of external factors such as changes in temperature, lighting, and the presence of chemicals.
Unicellular animals get food in different ways. So, during phagocytosis, outgrowths of the cytoplasm capture solid particles of food. Pinocytosis proceeds in several stages: first, the surface of the entire cell is captured by the liquid, and then absorbs the substances contained in it, processes them using digestive enzymes that fill the vacuoles. Inside some protozoa (chlorella) there are chloroplasts, which, using photosynthesis, can produce organic substances from inorganic substances.
Also, the entire surface of the body of the protozoa is involved in gas exchange: decay products and excess water go out through it.
Unicellular animals breed both sexually and asexually. It depends on the conditions in which they exist. 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, several (at least two) are obtained from one simple organism .
Female and male individuals participate in sexual reproduction . Their structure and size may vary, but may be the same. As a result of their merger, a zygote is formed, which further reproduces independently already asexually. It happens that individuals, in contact, exchange particles of nuclei. In this case, the zygote is not formed.
When conditions do not favor the normal functioning of protozoa, their body becomes round, covered with a dense shell. So a cyst is formed. As soon as the conditions improve, the body is freed from the thick film and begins to lead the same lifestyle as before.
It is believed that unicellular animals were the first in the process of evolution to appear on Earth. The oldest include archaea and bacteria. They are very 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 proven that archaea have their own structural features, and have evolved in a slightly different way. Although they are not as easy to classify as before. The fact is that under laboratory conditions, archaea were never grown, but were discovered 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 full-fledged biocenosis. After all, many animals eat them, which themselves serve as food for a whole series of inhabitants of our planet.