There are many different types of animals that have been preserved from ancient times to the present day. Among them, there are primitive organisms that for more than six hundred million years continue to exist and propagate - hydra.
Description and lifestyle
A common inhabitant of water bodies, a freshwater polyp called hydra belongs to the intestinal animals. It is a gelatinous translucent tube up to 1 cm long. One end, on which a peculiar sole is located, is attached to aquatic plants. On the other side of the body there is a corolla with many (from 6 to 12) tentacles. They are able to extend up to several centimeters in length and serve to search for prey, which the hydra paralyzes with a stinging injection, pulls tentacles to the oral cavity and swallows.
The basis of nutrition is Daphnia, mosquito larvae, fish fry, cyclops. Depending on the color of the food eaten, the coloring of the translucent body of the hydra also changes.
Due to the contraction and relaxation of integumentary-muscle cells, this organism can narrow and thicken, stretch to the sides and move slowly. Simply put, freshwater hydra is most like a stomach moving and living an independent life. Reproduction of it, in spite of this, occurs at a fairly high pace and in various ways.
Types of hydras
Zoologists distinguish four genera of these freshwater polyps. They are very slightly different from each other. Large species with filamentous tentacles several times the body length are called Pelmatohydra oligactis (long-leaved hydra). Another species, with a body tapering to the sole, is called Hydra vulgaris or brown (ordinary). Hydra attennata (thin or gray) is similar in appearance to an even tube along the entire length with slightly longer tentacles compared to the body. The green hydra, called Chlorohydra viridissima, is so named for its grassy color, which is given to it by unicellular algae that supply this body with oxygen.
Propagation Features
This simplest creature can reproduce both sexually and asexually. In the summer, when the water warms up, the propagation of hydra occurs mainly by budding. Sex cells are formed in the hydra ectoderm only in the fall, with the onset of cold weather. By winter, adults die, leaving eggs, from which a new generation appears in the spring.
Asexual reproduction
Under favorable conditions, hydra is usually propagated by budding. Initially, a slight protrusion occurs on the wall of the body, which slowly turns into a small tubercle (kidney). Gradually, it increases in size, stretching, and tentacles are formed on it, between which you can see the mouth opening. First, the young hydra connects to the mother's body with the help of a thin stalk.
After some time, this young shoot is separated and begins an independent life. This process is very reminiscent of how a plant develops an escape from a kidney, so asexual reproduction of hydra is called budding.
Sexual reproduction
When colds occur or conditions become not entirely favorable for the life of the hydra (drying out of the reservoir or prolonged starvation), the formation of germ cells occurs in the ectoderm. In the outer layer of the lower body, eggs are formed, and in special tubercles (male gonads), which are located closer to the oral cavity, spermatozoa develop. Each of them has a long flagellum. With it, the sperm can move in the water to reach the egg and fertilize it. Since the sexual reproduction of hydra occurs in the fall, the resulting embryo is covered with a protective shell and lies at the bottom of the reservoir for the whole winter, and only begins to develop with the onset of spring.
Germ cells
These freshwater polyps in most cases are dioecious (sperm and eggs are formed on different individuals), hermaphroditism in hydra is extremely rare. With cooling in the ectoderm, the laying of the sex glands (gonads) occurs. Sex cells are formed in the hydra body from intermediate cells and are divided into female (eggs) and male (sperm). The egg looks like an amoeba and has pseudopods. It grows very rapidly, while absorbing intermediate cells located in the neighborhood. By the time of ripening, its diameter is from 0.5 to 1 mm. Propagation of hydra with the help of eggs is called sexual.
Sperm cells are similar to flagellate protozoa. Separating from the body of the hydra and swimming in the water with the help of the available flagellum, they go in search of other individuals.
Fertilization
When a sperm swims to an individual with an egg and penetrates inside, the nuclei of these two cells merge. After this process, the cell becomes more rounded due to the fact that the pseudopods are retracted. On its surface a thick shell forms with outgrowths in the form of spikes. Before the onset of winter, the hydra dies. The egg remains alive and falls into suspended animation, remaining at the bottom of the reservoir until spring. When the weather becomes warm, the overwintered cell under the protective membrane continues its development and begins to divide, forming first the rudiments of the intestinal cavity, then the tentacles. Then the shell of the egg breaks, and a young hydra appears.
Regeneration
Propagation features of hydra also include an amazing ability to restore, as a result of which a new individual is regenerated. From a separate piece of the body, sometimes making up less than one hundredth of the total volume, an entire organism can be formed.
It is worth cutting the hydra into parts, as the regeneration process starts right away, in which each piece acquires its mouth, tentacles and sole. Back in the seventeenth century, scientists conducted experiments when even seven-headed organisms were obtained by the method of fusion of different halves of hydra. Since then, this freshwater polyp got its name. This ability can be regarded as another way of propagating hydra.
