<P> If Hydra are alarmed or attacked, the tentacles can be retracted to small buds, and the body column itself can be retracted to a small gelatinous sphere . Hydra generally react in the same way regardless of the direction of the stimulus, and this may be due to the simplicity of the nerve nets . </P> <P> Hydra are generally sedentary or sessile, but do occasionally move quite readily, especially when hunting . They have two distinct methods for moving --' looping' and' somersaulting' . They do this by bending over and attaching themselves to the substrate with the mouth and tentacles and then relocate the foot, which provides the usual attachment, this process is called looping . In somersaulting, the body then bends over and makes a new place of attachment with the foot . By this process of "looping" or "somersaulting", a Hydra can move several inches (c. 100 mm) in a day . Hydra may also move by amoeboid motion of their bases or by detaching from the substrate and floating away in the current . </P> <P> When food is plentiful, many Hydra reproduce asexually by producing buds in the body wall, which grow to be miniature adults and break away when they are mature . When a hydra is well fed, a new bud can form every two days . When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some Hydra . Swellings in the body wall develop into either an ovary or testes . The testes release free - swimming gametes into the water, and these can fertilize the egg in the ovary of another individual . The fertilized eggs secrete a tough outer coating, and, as the adult dies (due to starvation and / or cold), these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into nymph Hydra . Some, like Hydra circumcincta and Hydra viridissima, are hermaphrodites and may produce both testes and an ovary at the same time . </P> <P> Many members of the Hydrozoa go through a body change from a polyp to an adult form called a medusa . However, all Hydra, despite being hydrozoans, remain as polyps throughout their lives . </P>

Which structures are found in the simple animal called the hydra