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    Hydrozoa hydra

    hydrozoa hydra

    Hydrozoa hydra

    Hydrozoa hydra тор браузер 2011 скачать hyrda hydrozoa hydra

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    Other characteristics: Hydra are exceptionally interesting due to their regenerative ability and that they appear not to age or die of old age. However, they can still die from means other than aging senescence , such as through injury, disease, starvation, or unsuitable conditions in the habitat. Introduction: Hydra are inconspicuous freshwater relatives of corals, sea anemones and jellyfish.

    All are members of the phylum Cnidaria, characterized by radially symmetrical bodies, presence of stinging tentacles and a simple gut with only one opening gastrovascular cavity. The body is a hollow tube consisting of two layers of cells, separated by an unstructured gelatinous layer mesoglea.

    The inner layer endoderm , lining the gastrovascular cavity, produces the enzymes for digesting food. The outer layer of cells ectoderm generates the tiny stinging organelles called nematocysts. Tentacles are extensions of the body layers and circle the mouth opening. Due to simple construction, body column and tentacles are highly extensible.

    When hunting, hydra spread the tentacles, move them slowly around and wait for a contact with some suitable prey. Small animals, which come across the tentacles, are paralyzed by neurotoxins released from the stinging nematocysts. Tentacles coil around the struggling prey and drag it into expanded mouth opening. When the victim is enclosed within the body cavity, digestion may starts. Cuticles and other undigested remains are later expelled by contractions through the mouth.

    Hydra mostly reproduce asexually by producing buds on the body wall. Buds are genetically identical clones, which grow and simply break free when they are mature. Hydra has a tubular body with radial symmetry. The radial symmetry is also a shared feature of the phylum Cnidaria. You can see the body wall of a Hydra consisting of two layers of cells.

    Hydra has two main body layers. The outer layer is the epidermis like our skin which can secret a protective layer hydramacin against bacterial infection. Respiration and excretion occur by diffusion through the surface of the epidermis. The inner layer is called the gastrodermis , which lines the stomach.

    Between the two layers is a gel-like substance called mesoglea. Modified from Vogg MC. The bottom end of a hydra is secured on the surface by a simple adhesive foot called the basal disc. Gland cells in the basal disc secrete a sticky fluid that accounts for its adhesive properties. At the free end of the body is a mouth opening hypostome surrounded by tentacles. The mouth is connected to the body cavity. The prey captured by the tentacles will then be engulfed within the body cavity for digestion.

    Since hydra has only one opening, the waste after digestion will be discharged through the same mouth opening. Tentacles from 6 to 10 in number radiate outward from the rim of the mouth. These tentacles can be greatly extended at the time of feeding or locomotion.

    Each tentacle is covered with highly specialized stinging cells called cnidocytes. Cnidocytes contain specialized structures called nematocysts , which look like miniature light bulbs with a coiled thread inside. Near the outer edge of the cnidocyte is a short hair, called a cnidocil. The cnidocil functions like a trigger to control the firing of a nematocyst.

    Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release. These toxins can paralyze the prey. A cnidocyte fires a structure that contains the toxin from a characteristic subcellular organelle called a nematocyst. Left: a cnidocyte at the resting stage.

    Right: a fired cnidocyte. Hydra does not have a recognizable brain or true muscles, but hydra has a primitive nervous system called the nerve net. Nerve nets connect sensory photoreceptors light-sensitive and touch-sensitive nerve cells located in the body wall and tentacles. This simple nervous system helps hydra quickly respond to all kinds of stimuli.

    You can see the hydra has a network-like nerve system. Photo credit: ck Photo credit: U Heidelberg. Hydras mainly feed on aquatic invertebrates such as Daphnia. While feeding, Hydras extend their body to maximum length and then slowly extend their tentacles. Once fully extended can be four to five times the length of the body , the tentacles are slowly maneuvered around, waiting for contact with suitable prey.

    Upon contact with prey, hundreds of nematocysts can fire their stings and neurotoxins at the same time. This can paralyze the prey. Within minutes, the tentacles can surround the prey and move the prey into the opened mouth aperture. The prey will then be engulfed within the body cavity for digestion. Hydra can stretch its body wall considerably in order to digest prey more than twice its size.

    Since hydra has no anal pore, the indigestible remains of the prey will be discharged through the mouth aperture via contractions. The mouth disappears after it eats, meaning a hydra regrow a new mouth for each bite. Is that amazing? Some Hydra species like Hydra viridissima can form a symbiotic relationship with green algae Chlorella living within its body.

    Hydra provides a safe place to live, and green algae synthesize food by photosynthesis in return. This symbiosis gives a significant advantage to hydras when environmental conditions change food is scarce. In the absence of prey, a green hydra may still be able to survive for about 3 months using nutrients produced through photosynthesis. This allows the hydra to tolerate starvation. Green hydra Hydra viridissima and its symbiotic algae Chlorella sp.

    Modified from Marinespecies. Typically, a hydra remains attached by the basal disc to some suitable object in the water. However, occasionally hydra can move quite readily, especially when hunting. Looping is the most common type of walking similar to the movement of a caterpillar. They do this by bending over and attaching themselves to the surface with their mouth and tentacles and then relocating their foot.

    In somersaulting meaning rolling forward , the hydra extends its body and bends to one side to place its tentacles on the surface. The basal disc is free from its attachment, and the animal stands on its tentacles. The body is then extended and bent to place the basal disc on the surface, the tentacles loosen their hold, and the animal regains an upright position. These movements are repeated, and the hydra moves from place to place. Hydra can occasionally move in other ways see image below.

    Interestingly, a hydra can generate a gas bubble from its foot and then anchor itself on the bubble for floating, like riding a hot air balloon! Hydra can reproduce both asexually and sexually. The choice of reproduction depends on the conditions where they live.

    Asexual reproduction occurs under favorable environmental conditions to allow a rapid expansion of the hydra population. Asexual reproduction only requires one parent. During budding, a small bud develops near the basal part of the parent hydra through repeated mitotic cell division. These new cells grow into a mini version of hydra with smaller body, mouth, and tentacles.

    Once the bud is fully developed, it ultimately detaches from the parent and becomes an independent organism. Given that the offspring is produced through mitosis division, it is genetically identical to its parent. This budding process may take about 3 days from start to end. On the contrary, sexual reproduction occurs when environmental conditions become unfavorable.

    Such conditions could be the low temperature and the lack of sufficient food sources. To increase the chance of survival, hydras will prepare for sexual reproduction. To initiate the process of sexual reproduction, reproductive stem cells on the body wall will become ovaries and testes. Hydra could have three sexes: female has ovaries , male has testes , and hermaphrodite has both ovaries and testes. Hermaphrodite is common in hydra. Ovaries develop near the bottom of the hydra while testes form near the top.

    Each ovary hosts one egg cell. Sperm enter the ovaries of another hydra by swimming.

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