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Download PDF. LIU View author publications. View author publications. Photo credit: Hydra oligactis , Hydra viridissima. 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. Their bodies are basically just a hollow cylinder, which is solid at the base, with an opening at the mouth end.
The mouth leads into the hollow interior of the cylinder which acts as the digestive cavity. The mouth is surrounded by a variable number of tentacles usually somewhere between 6 and Because the digestive cavity has only one opening, food and subsequently, digested waste, both come in and go out through the same opening.
A structure called a basal disc on the bottom of the tube-like body secretes a sticky substance which helps hydras to attach themselves to a solid base. This hydraulic pressure is created inside their digestive cavity. The cells lining the digestive cavity have little flagellae minute hair-like structures on their surface. By beating these flagellae like miniature oars, currents can be created which draw water into the digestive cavity.
This raises the water pressure inside, which causes the hydra's body to increase in length. The body of a single hydra can change from being 20mm long when it is relaxed and extended, to being as little 0. Hydra are not always attached to the substrate and can move from one spot to another, either by gliding along on the basal disc or by somersaulting along. When somersaulting, they detach the basal disc and then bend over and place the tentacles down on the substrate.
This is followed by reattaching the basal disc further along, before repeating the whole process again. They may also float about in the water upside down. When they are floating, it is because the basal disc produces a gas bubble which carries the animal up to the water surface.
The single-celled algae Chlorella which give Chlorohydra its green colour, live inside the cells lining the digestive cavity. Interestingly, these algae can also be found living freely outside of hydras. One side of tree trunks often the north side in woods is sometimes covered in a smooth green coating almost as if it has been painted. This green covering usually advertises the presence of a whole mass of free-living Chlorella algae although other species of green algae will also grow on tree trunks.
In contrast to sea anemones, most hydras live in freshwater, although one colonial species found in Britain Cordylophora lacustris has spread into freshwater habitats from brackish water water which is partly salty. Hydra are carnivorous and feed mainly on small crustaceans like water fleas Daphnia and small worms. Although hydra are fairly simple animals, the stinging cells which they use to catch their prey are quite complex structures.
They have a number of different types of stinging cells, called nematocysts, on their tentacles. Coiled tubes are shot out at any passing prey which has had the misfortune to touch a tentacle.
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