Despite the variety of single-celled organisms, more complex organisms are much better known to man. They represent the most numerous group, which includes more than one and a half million species. All multicellular organisms have certain common characteristics, but at the same time they differ greatly. Therefore, it is worth considering individual kingdoms, and in the case of animals, classes.
General Properties
The main feature that separates unicellular and multicellular organisms is the functional difference. It came about through evolution. As a result, the cells of the complex body began to specialize, uniting into tissues. The simplest use only one for all the necessary functions. In this case, plants and fungi are traditionally considered separately, since animal and plant cells also have significant differences. But they, too, should be taken into account in the study of this topic. Unlike protozoa, they always consist of many cells, many of which have their own functions.
Mammalian class
Of course, the most famous multicellular organisms are animals. Of these, in turn, mammals stand out. This is a highly organized classchordates, which includes four and a half thousand species. Its representatives are found in any environment - on land, in soil, in fresh and s alt water, in the air. The advantages of multicellular organisms of this type over others in the complex structure of the body. It is divided into the head, neck and torso, pairs of fore and hind limbs, as well as the tail. Due to the special arrangement of the legs, the body is raised above the ground, which ensures the speed of movement. All of them are distinguished by rather thick and elastic skin with sweat, sebaceous, odorous and mammary glands located in it. Animals have a large skull and complex muscles. There is a special thoracic septum called the diaphragm. Animal modes of locomotion include activities ranging from walking to climbing. The heart consists of four chambers and supplies arterial blood to all organs and tissues. The lungs are used for respiration and the kidneys are used for excretion. The brain consists of five regions with several cerebral hemispheres and the cerebellum.
Bird class
Answering which organisms are multicellular, one cannot fail to mention birds. These are highly organized warm-blooded creatures that can fly. There are more than nine thousand modern species. The value of a multicellular organism of this class is incredibly great, since they are the most common, which means they take part in the economic activities of people and play an important role in nature. Birds are distinguished from other creatures by several basic properties. They have streamlined torsos with frontlimbs transformed into wings, and hind limbs, which are used as a support. Birds are distinguished by dry skin without glands, with horny formations known as feathers. The skeleton is thin and strong, with air cavities that ensure its lightness. The muscular system provides the ability to walk, run, jump, swim, climb and two types of flight - soaring and flapping. Most species are able to move long distances. Birds have no teeth and there is a goiter, as well as a muscular section that grinds food. The structure of the tongue and beak depends on the specialization of food.
Reptile class
It is worth mentioning this type of creatures representing multicellular organisms. Animals of this class were the first to become terrestrial vertebrates. At the moment, about six thousand species are known. The skin of reptiles is dry and devoid of glands, it is covered with a stratum corneum, which periodically descends in the process of molting. A strong ossified skeleton is distinguished by reinforced shoulder and pelvic girdle, as well as developed ribs and chest. The digestive tract is quite long and clearly differentiated; food is captured using jaws with sharp teeth. The respiratory organs are represented by lungs with a large surface, bronchi and trachea. The heart consists of three chambers. Body temperature is determined by the environment. The excretory organs are the kidneys and bladder. Fertilization is internal, eggs are laid on land and are protected by a leathery or shell membrane.
Amphibian class
Listing multicellular organisms, it is worth mentioning amphibians. This group of animals is ubiquitous, especially common in warm and humid climates. They have mastered the terrestrial environment, but have a direct connection with water. Amphibians descended from lobe-finned fish. The body of an amphibian is distinguished by a flat shape and division into a head, torso and two pairs of limbs with five fingers. Some also have a tail. Thin skin is distinguished by many mucous glands. The skeleton is made up of many cartilages. Muscles allow you to make a variety of movements. Amphibians are predators; they digest their food with their stomachs. The respiratory organs are the skin and lungs. The larvae use gills. The heart is three-chambered, with two circles of blood circulation - multicellular organisms often differ in such a system. Kidneys are used for excretion. Fertilization is external, takes place in water, development takes place with metamorphoses.
Insect class
Single-celled and multicellular organisms are not least distinguished by their amazing diversity. Insects also belong to this category. This is the most numerous class - it includes more than a million species. Insects are distinguished by the ability to fly and great mobility, which is provided by well-developed muscles with jointed limbs. The body is covered with a chitinous cuticle, the outer layer of which contains fatty substances that protect the body from drying out, ultraviolet radiation and damage. Different mouthparts reduce species competition, which allowsconstantly maintain a high number of individuals. Small size becomes an additional advantage for survival, as well as a wide range of reproduction methods - parthenogenetic, bisexual, larval. Some also differ in polyembryony. The respiratory organs provide intensive gas exchange, and the nervous system with perfect sensory organs creates complex forms of behavior due to instincts.
Plant Kingdom
By far, animals are the most common. But it is worth mentioning other multicellular organisms - plants. There are about three hundred and fifty thousand species of them. Their difference from other organisms lies in the ability to carry out photosynthesis. Plants act as food for many other organisms. Their cells have solid walls of cellulose, and contain chlorophyll inside. Most are unable to carry out active movements. Lower plants do not have a division into leaves, stem and root. Green algae live in water and can be with a different structure and methods of reproduction. Browns carry out photosynthesis with the help of fucoxanthin. Red algae are found even at a depth of 200 meters. Lichens are the next sub-kingdom. They are most significant in soil formation, and are also used in medicine, perfumery and the chemical industry. Higher plants are distinguished by the presence of leaves, root system and stems. The most primitive are mosses. The most developed are trees that can be flowering, bi- or monocotyledonous, as well as coniferous.
Mushroom Kingdom
It should go to the last type, which can be multicellular organisms. Mushrooms combine features of both plants and animals. More than one hundred thousand species are known. The diversity of cells of multicellular organisms is most pronounced in fungi - they are able to reproduce by spores, synthesize vitamins and remain immobile, but at the same time, like animals, they can feed heterotrophically, do not carry out photosynthesis and have chitin, which is also found in arthropods.
