Structure of hydra of freshwater

From this article you will learn everything about the structure of the freshwater hydra, its way of life, nutrition, reproduction.

External structure of hydra

The polyp (which means "the multi-legged") hydra is a tiny semi-transparent creature that lives in the clear transparent waters of slow-flowing rivers, lakes, ponds. This coelenterate animal leads a sedentary or attached lifestyle. The external structure of freshwater hydra is very simple. The body has an almost regular cylindrical shape. On one of its ends is a mouth that is surrounded by a crown of many long thin tentacles (five to twelve). On the other end of the body is the sole, with which the animal is able to attach to various objects under water. The body length of the freshwater hydra is up to 7 mm, but the tentacles can be stretched and reach a length of several centimeters.

Beam symmetry

Let us consider in more detail the external structure of the hydra. The table will help memorize body parts and their purpose.

Hydra symmetry is inherent in the body of a hydra, like many other animals leading an attached lifestyle . What it is? If you imagine a hydra and carry an imaginary axis along the trunk, then the tentacles of the animal will diverge from the axis in all directions, like the rays of the sun.

The structure of the body of a hydra is dictated by its way of life. It attaches to the underwater object with a sole, hangs down and begins to rock, exploring the surrounding space with the help of tentacles. The animal hunts. Since the hydra lies in wait for prey, which can appear from either side, the symmetrical ray-shaped arrangement of the tentacles is optimal.

Intestinal cavity

The internal structure of the hydra will be considered in more detail. The body of a hydra is like an oblong sac. Its walls consist of two layers of cells, between which is located intercellular substance (mesogloe). Thus, inside the body there is an intestinal (gastral) cavity. The food penetrates into it through the mouth opening. It is interesting that the hydra, which at the moment does not eat, the mouth is practically absent. Cells of the ectoderm are closed and fused in the same way as on the rest of the body surface. Therefore, every time before eating, the hydra has to break through again.

The structure of freshwater hydra allows it to change its place of residence. On the sole of the animal there is a narrow opening - the aboral pore. Through it from the intestinal cavity liquid and a small gas bubble can be released. With the help of this mechanism, the hydra is able to detach from the substrate and float to the surface of the water. In such a simple way, with the help of currents, it settles on the pond.

Ectoderm

The internal structure of the hydra is represented by ectoderm and endoderm. The ectoderm is the outer layer of cells that form the body of the hydra. If you look at the animal in a microscope, you can see that several types of cells are related to the ectoderm: stinging, intermediate and epithelial-muscular.

The most numerous group is the skin-muscular cells. They touch each other with their sides and form the surface of the animal's body. Each such cell has a base - contractible muscular fiber. This mechanism provides the ability to move.

When all the fibers are shortened, the animal's body contracts, lengthens, bends. And if the contraction occurs only on one side of the body, then the hydra is tilted. Thanks to this work of cells, the animal can move in two ways - "tumbling" and "walking."

Also in the outer layer are star-shaped nerve cells. They have long processes, with which they touch each other, forming a single network - a nerve plexus that braids the whole body of a hydra. Connect the nerve cells and skin-muscular.

Between epithelial-muscular cells there are groups of small, rounded forms of intermediate cells with large nuclei and a small amount of cytoplasm. If the body of the hydra is damaged, then the intermediate cells begin to grow and divide. They are able to turn into any type of cells.

Streptococcal cells

The structure of the hydra cells is very interesting, special mention deserves the stinging (nettle) cells, which are strewn all over the animal's body, especially the tentacles. Straggly cells have a complex structure. In addition to the nucleus and cytoplasm, a bubble-shaped stinging chamber is located in the cell, inside of which is a thin, stinging filament folded into the tube.

A sensitive hair emerges from the cell. If the prey or enemy touches this hairs, the stinging thread is sharply straightened and it is thrown outward. A sharp tip pierces the victim's body, and a poison passes through the thread's channel, which can kill a small animal.

As a rule, a lot of stinging cells are triggered. Hydra captures prey tentacles, attracts to the mouth and swallows. The poison, released by stinging cells, serves also for protection. Larger predators do not touch painfully stinging hydra. The poison of the hydra is similar in its action to the poison of the nettle.

Straggling cells can also be divided into several types. Some threads inject poison, others - they take offense around the victim, and others stick to it. After triggering, the stinging cell dies, and a new one forms from the intermediate cell.

Endoderma

The structure of the hydra also implies the presence of such a structure as the inner cell layer, the endoderm. These cells also have muscular contractile fibers. Their main purpose - the digestion of food. Cells of the endoderm produce digestive juice directly into the intestinal cavity. Under its influence, mining is split into particles. Some cells of the endoderm have long flagella, constantly in motion. Their role is to pull food particles to the cells, which in turn release the pseudopods and seize food.

Digestion continues inside the cell, therefore it is called intracellular. Food is processed in vacuoles, and undigested residues are ejected through the mouth opening. Breathing and excretion occurs through the entire surface of the body. Consider once again the cellular structure of the hydra. The table will help to do this visually.

Reflexes

The structure of the hydra is such that it is able to feel the temperature change, the chemical composition of the water, as well as touch and other stimuli. Nerve cells of the animal can be excited. For example, if you touch it with the tip of a needle, then the signal from the nerve cells felt by touch will be transmitted to the rest, and from the nerve cells to the epithelial-muscular. Skin-muscle cells react and shrink, the hydra shrinks into a lump.

Such a reaction is a vivid example of a reflex. This is a complex phenomenon, consisting of successive stages - perception of the stimulus, transmission of excitation and response. The structure of the hydra is very simple, so the reflexes are monotonous.

Regeneration

The cell structure of the hydra allows this tiny animal to regenerate. As mentioned above, the intermediate cells located on the surface of the body can be transformed into any other type.

With any damage to the body, the intermediate cells begin to divide very quickly, grow and replace the missing parts. The wound is overgrown. The regenerative abilities of the hydra are so high that if you cut it in half, one part will grow new tentacles and mouth, and the other - the stem and sole.

Asexual reproduction

The hydra can reproduce both asexually and sexually. Under favorable conditions in summer, a small tubercle appears on the animal's body, the wall protrudes. Over time, the tubercle grows, stretches. Tentacles appear on its end, mouth bursts out.

Thus, a young hydra appears, connected with the mother's body by a stem. This process is called budding, as it is similar to the development of a new shoot in plants. When a young hydra is ready to live on its own, it buds. The daughter and maternal organisms are attached to the substrate by tentacles and stretch in different directions until they separate.

Sexual reproduction

When it starts to get cold and unfavorable conditions are created, it's the turn of sexual reproduction. In autumn hydrides from the intermediate begin to form sex cells, male and female, that is, egg cells and spermatozoa. Egg cells hydra are similar to amoebae. They are large, covered with pseudopods. Spermatozoa are similar to the simplest flagellates, they are able to swim with the flagellum and leave the body of the hydra.

After the spermatozoon penetrates into the egg cell, their nuclei merge and fertilization takes place. The foot pods of the fertilized egg cell are retracted, it is rounded, and the membrane becomes thicker. An egg is formed.

All the hydras in autumn, with the onset of cold weather, perish. The maternal organism disintegrates, but the egg remains alive and hibernates. In the spring, it begins to actively divide, the cells are arranged in two layers. With the onset of warm weather, a small hydra breaks through the shell of the egg and begins an independent life.