1. Touch: Pricking the hydra with a needle causes the body to contract. Leaves, aquatic plants, objects, etc. are stuck by hydra treads.
2. Hunger: A hungry hydra contracts and expands its muscles. Moves the body at a very fast speed to search for food. If they are not hungry, they slow down.
3. Temperature: Hydra always prefers cool water with a temperature of 20 degrees Celsius. When the surface temperature of the reservoir rises, the hydra slowly moves down. Dr. Siddiq Publications
4. Light: Hydra does not like either too much light or darkness. Always like moderate light. Dr. Siddiq Publications
5. Current: A continuous current causes the hydride body to bend towards the anode. Later, the whole body contracts. Dr. Siddiq Publications

Hydra is a suborder of animals. Their bodies have a weak nervous system. In the animal world, hydra or nidarians developed the first nervous system. Nervous system emerges from their afferent neurons and joins to form neural network. Neurons do not have axons or dendrites and never form synapses. Mesoglia has one nerve plexus on either side. Nerves are attached to the epidermis and gastrodermis. Nerves are densely located in the mouth and foot. Neurons are connected to each other and to sensory cells and muscle cells. Sensory cells receive light, touch and chemical stimuli from the environment. Then through the nerve network sent to the muscle cells. Dr. Siddiq Publications

Hydra has no specific excretory organs. Metabolism in cells produces nitrogenous waste products. The waste material generated is released into the water during the diffusion process.

Hydra has no specific respiratory organs. It causes the exchange of gaseous substances through the epidermis and gastrodermis.
1. Epidermis: Dissolved oxygen from the water around the body enters the epidermal cells by diffusion. The absorbed oxygen reaches the various cells of the body and completes the respiration process. Carbon dioxide produced as a result of respiration is released outside the body in the process of diffusion. Dr. Siddiq Publications
2. Gastrodermis: Gastrodermis consists of flagellated cells. Flagellar movement causes a constant flow of water into the stomata. Dissolved oxygen from water enters the gastrodermal cells by the process of diffusion. The absorbed oxygen reaches the various cells of the body and completes the respiration process. Carbon dioxide produced as a result of respiration is released outside the body in the process of diffusion.

If bubbles are not created in the body, the body becomes heavy. The body sinks faster due to its heaviness. It is called submerged movement. Dr. Siddiq Publications

Hydra relaxes the body and causes contraction and expansion of muscle cells. In this the body size becomes shorter and longer. As a result, a kind of movement is created and the place changes.

Hydra clings to branches of submerged plants by tentacles. Releases and shrinks the soles. Then put the floor in a new place. Thus changes the space through contraction.

Hydra moves upside down by freeing the soles. That is, the foot is on the top and the oral floor is on the bottom. Places the entire weight of the body on the Karshika. Using Karshika as a foot moves slowly. Dr. Siddiq Publications

The hydra frees the body and positions itself horizontally. Makes the karshikas move like waves. At the same time, it also moves the body. In this way, it swims by creating a wave-like movement.

The hydra moves upside down by freeing the feet. That is, the foot is on the top and the oral floor is on the bottom. Gas and mucus are secreted from the glands of the soles of the feet. The released gas and mucus combine to form bubbles. Hydra floats in water with the help of bubbles. During this time, it floats from one place to another due to the pull of the current or the impact of the waves.