The amazing world of jellyfish!

The amazing world of jellyfish!

Jellyfish are animals that everyone associates with something formless and infinitely primitive, but their lifestyle and physiology are not as simple as it seems at first glance. The word “jellyfish” usually means animals from the Scyphoid class and representatives of the Trachylid order from the Hydroid class of the Coelenterate type. At the same time, in the scientific community this word has a broader interpretation - zoologists use this term to designate any mobile forms of coelenterates. Thus, jellyfish are closely related to mobile species of coelenterates (siphonophores, sea ships) and sessile species - corals, sea anemones, hydras. In total, there are over 200 species of jellyfish in the world.

Because of their primitiveness, jellyfish are characterized by uniformity of physiology and internal structure, but at the same time they are distinguished by an amazing variety of colors and appearance, unexpected for such simple animals. One of the main distinguishing features of jellyfish is radial symmetry. This type of symmetry is characteristic of some marine animals, but in general it is not very common in the animal world. Due to radial symmetry, the number of paired organs in the body of jellyfish is always a multiple of 4.

Jellyfish are so primitive that their body does not have any differentiated organs, and the tissues of the body consist of only two layers: the outer (ectoderm) and the inner (endoderm), connected by an adhesive substance - mesoglea. However, the cells of these layers specialize in performing different functions. For example, ectoderm cells perform an integumentary function (analogous to skin), motor (analogous to muscles), special sensitive cells are also located here, which are the rudiments of the nervous system and special germ cells that form the reproductive organs in adult jellyfish. But endoderm cells are only involved in digesting food; for this they secrete enzymes that digest prey.

The body of jellyfish is shaped like an umbrella, disk or dome. The upper part of the body (it can be called the outer part) is smooth and more or less convex, and the lower part (it can be called the inner part) is shaped like a bag. The internal cavity of this sac is both the engine and the stomach. In the middle of the lower part of the dome, jellyfish have a mouth. Its structure is very different in different species: in some jellyfish, the mouth has the shape of an elongated proboscis or tube, sometimes very long, in others there are short and wide oral lobes on the sides of the mouth, in others, instead of lobes there are short club-shaped oral tentacles.

Along the edges of the umbrella there are hunting tentacles; in some species they can be relatively short and thick, in others they can be thin, long, and thread-like. The number of tentacles can vary from four to several hundred.

In some species of jellyfish, these tentacles are modified and turned into balance organs. Such organs look like a tube-stalk, at the end of which there is a bag or vesicle with a calcareous stone - a statolith. When the jellyfish changes direction of movement, the statolite moves and touches the sensitive hairs, from which the signal is transmitted to the nervous system. The nervous system of jellyfish is extremely primitive, these animals have neither a brain nor sensory organs, but there are groups of light-sensitive cells - eyes, so jellyfish distinguish between light and darkness, but, of course, they cannot see objects.