1. Defense: Nematocysts contain a poisonous substance called hypnotoxin. Hydra defends itself with this toxic substance.
2. Prey capture: Hydra captures food by means of nematocysts. Then paralyze the victim. Dr. Siddiq Publications
3. Movement: Nematocyst cells help the hydra to move. Different types of movements occur in hydra. Dr. Siddiq Publications
4. Clogging the body: Nematocysts secrete sticky substances. By this, the hydra is stuck to an object. Dr. Siddiq Publications

The word Cnidoblast is derived from the Greek words knide meaning nettle and blastos meaning germ. The goblet or flax-shaped cells on the exterior of hydra are called nidoblast cells. It can be round, oval, pear-shaped, cup-shaped or oval-shaped. Nidoblast cells are found everywhere in the body except the soles of the feet. But its number is more in Karshika. Sometimes the cells are arranged in clusters. A group of them is called a battery.
1. Integument: Each nidoblast cell is covered by a bilayered envelope. Outer cover and inner cover. It is composed of proteins and lipids. Between the two envelopes are granular cytoplasm, a nucleus, mitochondria, lysosomes, ribosomes, etc.
2. Nematocyst: Small cysts with twisted secretions present in nidoblast cells are called nematocysts. The sac of nematocyst is called capsule. Capsules contain the poisonous liquid hypnotoxin. Hypnotoxin is composed of proteins and phenols. Hypnotoxin poison is neurotoxic in nature. That is, it acts on the victim’s nervous system. The level of this venom is 75% similar to that of a gokra or cobra snake. The nematocyst has a long and hollow filament at its tip. The broad part of the base of the sutraka is called the butt or shaft. The butt has three large spines called barbs. The butt consists of spirally arranged small barbule spines. Normally, the nematocyst is inserted into the sac with butts and spines. A nematocyst once shed never reenters.Dr. Siddiq Publications
3. Operculum: The mouth of nematocyst has a lid like part. It is called operculum. It moves sideways when exposed.
4. Nidocile: The free end of the nidocyte cell consists of a tough, firm, tiny and hypersensitive hollow spine. It is called nidosil. It is a transformed cilium. It works like a trigger. As a result, patched sutraka is thrown out. Dr. Siddiq Publications
5. Muscle fibers and lasso: Several muscle fibers emerge from the lower end of the nematocyst. Besides, there is a twisted thread called lasso at the lower end.

The word Cnidoblast is derived from the Greek words knide meaning nettle and blastos meaning germ. The goblet or flax-shaped cells on the exterior of hydra are called nidoblast cells. It can be round, oval, pear-shaped, cup-shaped or oval-shaped. Nidoblast cells are found everywhere in the body except the soles of the feet. But its number is more in Karshika. Sometimes the cells are arranged in clusters. A group of them is called a battery.
1. Integument: Each nidoblast cell is covered by a bilayered envelope. Outer cover and inner cover. It is composed of proteins and lipids. Between the two envelopes are granular cytoplasm, a nucleus, mitochondria, lysosomes, ribosomes, etc.
2. Nematocyst: Small cysts with twisted secretions present in nidoblast cells are called nematocysts. The sac of nematocyst is called capsule. Capsules contain the poisonous liquid hypnotoxin. Hypnotoxin is composed of proteins and phenols. Hypnotoxin poison is neurotoxic in nature. That is, it acts on the victim’s nervous system. The level of this venom is 75% similar to that of a gokra or cobra snake. The nematocyst has a long and hollow filament at its tip. The broad part of the base of the sutraka is called the butt or shaft. The butt has three large spines called barbs. The butt consists of spirally arranged small barbule spines. Normally, the nematocyst is inserted into the sac with butts and spines. A nematocyst once shed never reenters.Dr. Siddiq Publications
3. Operculum: The mouth of nematocyst has a lid like part. It is called operculum. It moves sideways when exposed.
4. Nidocile: The free end of the nidocyte cell consists of a tough, firm, tiny and hypersensitive hollow spine. It is called nidosil. It is a transformed cilium. It works like a trigger. As a result, patched sutraka is thrown out. Dr. Siddiq Publications
5. Muscle fibers and lasso: Several muscle fibers emerge from the lower end of the nematocyst. Besides, there is a twisted thread called lasso at the lower end.
Nidoblast function
1. Defense: Nematocysts contain a poisonous substance called hypnotoxin. Hydra defends itself with this toxic substance.
2. Prey capture: Hydra captures food by means of nematocysts. Then paralyze the victim. Dr. Siddiq Publications
3. Movement: Nematocyst cells help the hydra to move. Different types of movements occur in hydra. Dr. Siddiq Publications
4. Clogging the body: Nematocysts secrete sticky substances. By this, the hydra is stuck to an object. Dr. Siddiq Publications

The jelly-like sticky extracellular layer between the epidermis and gastrodermis of Nidaria is called mesoglia. It is thin, colorless and elastic. Its diameter is 0.1 micron. Dr. Siddiq Publications
Function of mesoglia
1. Mesoglia serve as the foundation of the epidermis and endoderm.
2. It acts as an attachment point.
3. It makes the body contract and expand.
4. Acts as a flexible skeleton of the body.
5. It contains myofibrils.

The innermost cell layer of the body wall is called the gastrodermis. The thickness of the gastrodermis is two-thirds that of the body wall. It contains 5 types of cells. Nutrient cells, glandular cells, interstitial cells, nerve cells and sensory cells.
1. Nutritive cells: Nutritive cells are located throughout most of the gastrodermis. It looks stunning. Each cell has a large nucleus and cavity. It forms contractile fibrous fascicles. Nutrient cells can be divided into two types. Flagellar cells and transient cells. Dr. Siddiq Publications
(i) Flagellar cells: These cells have 1-4 thread-like flagella at the free end.
(ii) Transient cells: The free end of this cell has transient cells.
Nutrients are cell functions
(i) It thins and thickens the body by contraction-expansion.
(ii) Flagella transform the food material into particles.
(iii) Acts like a sphincter to open and close the mouth.
(iv) It allows water to enter the mouth.
(v) Digests food.

During the reproductive season, spermatozoa and ovum are formed from interstitial cells in the hydra body stem. The testes are above and the ovaries are below. The testicles produce sperm and the ovaries produce eggs. Sperm are very small and nucleated. It consists of head, midsection and movable tail. Eggs are large, round and have three polar bodies.
Function of reproductive cells: Sperm and egg participate in reproductive function.

Gland cells are located in the peduncle, hypostome and peduncle. Cells may appear cylindrical, granular, or oval. It contains mucous glands, enzyme glands and glands that secrete mucus.
Function of glandular cells
(i) A cell helps in digestion of food.
(ii) The secreted sticky juice keeps the hydra attached to an object.
(iii) It helps in locomotion by creating momentum.
(iv) Helps hydra to float in water by creating bubbles.

Nerve cells are located under the epidermis near the mesoglia. The cells appear maculate or polygonal. Cytoplasm and nucleus are present in the cell. Each cell has two or more branched nervous systems. Nervous systems combine to form the neural network. Dr. Siddiq Publications
Nerve cell function
(i) It receives nerve impulses from sensory cells and generates reports.
(ii) It facilitates coordination between different cells.

Sensory cells are scattered in the spaces between the muscle covering cells. However, it is numerous in karshika, hypostome, feet. The cells appear narrow, elongated and spindle-shaped. That is, the middle part is wide and both ends are narrow. Each cell has cytoplasm and a swollen nucleus. It has contractile membranes on the outside and sensory nerves on the inside. It is sensitive to light, heat, touch, chemicals etc. Dr. Siddiq Publications
Function of sensory cells
(i) It receives different types of stimuli from the environment.
(ii) It participates in self-defense.
(iii) It helps in selection of accommodation.
(iv) It selects food.

Interstitial cells are located in clusters between muscle cells. It is called Reserve cell or Stem cell or multipotent cell. The cells appear round, oval or triangular in shape. Each cell contains a nucleus, numerous mitochondria, endoplasmic reticulum, lysosomes, ribosomes etc. It has totipotency capability. So it transforms into any other cell as needed. The diameter of the cells is 5 μm.
Function of interstitial cells
(i) A cell can transform into any other cell.
(ii) It participates in hydra regeneration, growth, gonad and bud formation.
(iiii) After 45 days, when body cells are destroyed, interstitial cells fill the space.