Freshwater Hydra Structure

From this article you will learn all about the structure of freshwater hydra, its lifestyle, nutrition, and reproduction.

The external structure of the hydra

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

Beam symmetry

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

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

The body structure of the hydra is dictated by its lifestyle. She attaches to the underwater object with the sole, hangs down and begins to sway, exploring the surrounding space with the help of tentacles. The animal is hunting. Since the hydra lies in wait for prey that can appear on either side, the symmetrical, ray-shaped arrangement of the tentacles is optimal.

Intestinal cavity

We will consider the internal structure of hydra in more detail. The hydra's body is like an oblong pouch. Its walls consist of two layers of cells, between which an intercellular substance (mesogley) is located. Thus, inside the body there is an intestinal (gastric) cavity. Food enters it through the mouth opening. Interestingly, the hydra, which is not eating at the moment, has practically no mouth. The ectoderm cells are closed and fused in the same way as on the rest of the body. Therefore, each time before eating, the hydra has to re-open its mouth.

The structure of freshwater hydra allows her to change her place of residence. On the sole of the animal there is a narrow hole - the aboral pore. Through it, fluid and a small bubble of gas can be released from the intestinal cavity. Using 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 in a pond.

Ectoderm

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

The largest group is skin-muscle cells. They touch each other on the sides and form the surface of the animal’s body. Each such cell has a base - contractile muscle fiber. This mechanism provides the ability to move.

With the reduction of all fibrils, the animal’s body contracts, lengthens, and bends. And if the contraction occurred only on one side of the body, then the hydra tilts. Thanks to such work of the cells, the animal can move in two ways - “somersaulting” and “walking”.

Also in the outer layer are star-shaped nerve cells. They have long processes, with the help of which they are in contact with each other, forming a single network - the nerve plexus, braiding the entire body of the hydra. Nerve cells are connected with skin and muscle.

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

Stinging cells

The structure of hydra cells is very interesting, stinging (nettle) cells, which are strewn with the whole body of the animal, especially the tentacles, deserve special mention. Stinging cells have a complex structure. In addition to the nucleus and cytoplasm, the cell contains a vesicle-like stinging chamber, inside of which there is a finest stinging thread folded into a tube.

Sensitive hair comes out of the cell. If the prey or the enemy touches this hair, then the stinging thread is sharply straightened, and it is thrown out. A sharp tip sticks into the victim’s body, and poison passes through the channel passing inside the thread, which can kill a small animal.

As a rule, many stinging cells are triggered. Hydra captures prey with tentacles, attracts to the mouth and swallows. The poison secreted by stinging cells also serves as a defense. Larger predators do not touch painfully stinging hydras. The poison of hydra in its action resembles the poison of nettle.

Stinging cells can also be divided into several types. Some threads inject poison, others thread around the victim, and others stick to it. After actuation, the stinging cell dies, and a new one is formed from the intermediate.

Endoderm

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

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

Reflexes

The structure of the hydra is such that it is able to feel the change in temperature, the chemical composition of water, as well as touch and other irritants. The animal’s nerve cells can be excited. For example, if you touch it with the tip of a needle, then the signal from those who feel the touch of nerve cells will be transmitted to the rest, and from nerve cells to the epithelial-muscular. Skin and muscle cells will react and contract, the hydra will shrink 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 hydra is very simple, therefore the reflexes are monotonous.

Regeneration

The cellular structure of hydra allows this tiny animal to regenerate. As mentioned above, intermediate cells located on the surface of the body can transform 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 hydra are so high that if cut in half, one part will grow new tentacles and a mouth, and the other will grow a stem and a sole.

Asexual reproduction

Hydra can breed both asexually and sexually. Under favorable conditions in the summer, a small tubercle appears on the body of the animal, the wall protrudes. Over time, the tubercle grows, stretches. At its end tentacles appear, a mouth breaks out.

Thus, a young hydra appears, connected with the stalk by the maternal organism. This process is called budding, since 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 subsidiary and maternal organisms attach to the substrate with tentacles and stretch in different directions until they separate.

Sexual reproduction

When it starts to get colder and unfavorable conditions are created, the turn of sexual reproduction begins. In the fall, at hydras from the intermediate, germ cells, male and female, that is, egg cells and sperm cells begin to form. Hydra egg cells are similar to amoebas. They are large, studded with pseudopods. Sperm are similar to the simplest flagella, they are able to swim with the help of a flagellum and leave the body of the hydra.

After the sperm penetrates the egg cell, their nuclei fuse and fertilization occurs. The legs of the fertilized egg cell are drawn in, it is rounded, and the membrane becomes thicker. An egg is formed.

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