· Abraham Trembly, a Swiss biologist discovered Hydra in 1744 AD.
· Linnaeus gave the name Hydra, after the great serpentine nine-headed dragon from the ancient
· Greek mythology is based on its ability to regenerate the lost parts in 1758 AD.
· The name Hydra is an old Greek word for "water-serpent".
· Hydra is a solitary polyp found in freshwater (stagnant).
· Hydra is diploblastic and radially symmetrical.
· Separate coelom is absent in Hydra, so can be called acoelomate. Among coelenterates, Hydra is one of the smallest polyps.
· Hydra has tissue grade of organization.
· Body of Hydra has no segmentation.
· Hydra has a cylindrical body with 6‑10 hollow tentacles.
· The tentacles of Hydra help in locomotion and food.
· The basal part of Hydra has a pedal or basal disc.
· Mouth is situated on a manubrium or hypostome. It is the most sensitive region in the body.
· Hydra has no anus.
· The body wall of Hydra consists of ectoderm and endoderm, in between a thin, delicate, transparent and non-cellular mesoglea.
· Ectoderm consists of epithelio‑muscular cells, sensory cells, nerve cells, interstitial cells (totipotent) and stinging cells or cnidocytes having nematocysts.
· Inner gastrodermis has nutritive muscular cells, gland cells, nerve cells, sensory and interstitial cells.
· Nutritive muscular cells bear both flagella and pseudopodia.
· The contraction of muscle fibres in the epithelio‑muscular cells shortens the body and tentacles.
· The contraction of muscle fibres in endotheliomuscular cells or nutritive muscle cells reduces the diameter of the body and works like circular muscles.
· Myofibrils of Hydra are all smooth.
· Mesogloea is thin and acellular consisting of a protein­ matrix and it can be crossed by interstitial cells. It is neither cellular nor fibrous.
· The body cavity of Hydra is called coelenteron or gastrovascular cavity.
· Coelenteron serves the double purpose of digestion and circulation.
· Cnidoblasts or nematocysts are derived from interstitial cells of the epidermis.
· Nematocysts are found only in the epidermis mainly on tentacles.
· Nematocysts are absent on the basal disc.
· Nematocysts are also known as 'independent effectors'.
· Hydra has four types of nematocysts: penetrants or stenoteles (largest), volvents (smallest), stereoline glutinants (small, atrichous) and streptoline glutinants (large holotrichous).
· Penetrant is the most complex type of nematocyst. When discharged, it releases a threaded tube by which a poisonous fluid, hypnotoxin is injected to paralyze the prey.
· Hypnotoxin is chemically protein and phenols.
· Volvents when discharged tightly coil thread around small projections such as hair or bristles of prey.
· Glutinants secrete a sticky substance used in locomotion by fastening the tentacles to solid objects.
· Nematocysts are mainly present on tentacles; a group of nematocysts is known as cnidom.
· Nematocyst or stinging capsule is a part of the cell called cnidoblast or cnidocyte.
· The trigger needle of cnidoblast is called cnidocil.
· Both chemical and mechanical stimuli are involved in the discharge of cnidoblast.
· On stimulation, the nematocyst shoots out within 0.3‑0.5 seconds.
· The action of nematocyst depends on enzymes, which bring about increased osmotic pressure inside the capsule of cnidoblast.
· Nematocyst plays an important role in locomotion, food capture both offence and defence.
· Hydra paralyses its prey by nematocyst.
· If all nematocysts of a Hydra are removed it would affect its capacity to capture prey.
· Various types of locomotion in Hydra are gliding, floating, walking (erect movement with all tentacles directed downwards using them as legs, looping and somersaulting.
· When Hydra wants to move fast it performs somersaulting.
· Locomotion in Hydra is not related to mating.
· Hydra is carnivorous; it feeds on small crustaceans.
· The most favourite food of Hydra is water fleas (Daphnia and Cyclops).
· Hydra recognizes its food because of chemical stimulus by the presence of glutathione in the tissue fluid of the prey.
· Hydra engulfs only those animals which have glutathione in their body.
· Digestion in Hydra is first extracellular (in the gastrovascular cavity) and then intracellular (in endoderm cells).
· Hydra can digest proteins, fats and some carbohydrates
· Hydra has no enzymes (No Amylase) to digest starch.
· Undigested residues are egested from coelenteron through the mouth.
· Hydra has no specialized cells for respiration. It respires by means of the general body surface.
· Oxygen is carried to tissues by diffusion.
· Nitrogenous excretory product in Hydra is ammonia and it is removed through the general body surface.
· Hydra has a nervous system, but no brain.
· Hydra has nerve cells but no nerves.
· Hydra possesses a very primitive nervous system consisting of a synaptic network of bipolar and multipolar nerve cells.
· Nerve net of Hydra is more or less non-polarized.
· Body wall of Hydra has both sensory and nerve cells.
· Hydra receives impulses and stimuli through its sensory cells.
· Hydra responds, negatively to both strong as well as very weak light. It prefers the region of moderate light intensity (dim light).
· Hydra prefers cool or cold water. It disappears from the surface at a temperature of 20‑25ÂșC.
· In an electric field, the tentacular end of Hydra bends towards the anode and the pedal disc to the cathode.
· Hydra is negatively chemotropic. It avoids chlorinated water.
· In Hydra reproduction is both asexual and sexual.
· Hydra regenerates by Morphollaxis.
· Hydra reproduces asexually by exogenous budding, a type of vegetative propagation, and sexually by the formation of gametes.
· Hydra reproduces by budding when plenty of food is available.
· Hydra may be both monoecious or dioecious. Most species (e.g. Pelmatohydra ofigactis) are dioecious or unisexual.
· Bisexual species of Hydra are protandrous, so avoids self‑fertilization.
· Hydra normally possesses a single ovary (in the aboral region) and many testes (in the oral region).
· Fertilization occurs externally on the body by the entry of sperm into the ovum.
· If sperm enters the coelenteron in Hydra it will be digested.
· Gonads of Hydra arise from interstitial cells.
· Cleavage is total and holoblastic. Gastrulation is by delamination.
· The developing embryo in Hydra drops down from the body of the parent after the formation of the gastrula.
· No free larval stage (only a planula‑like stage) occurs in Hydra.
· Emergence of a Hydra from the embryonated egg is called 'hatching'.
· In the development of Hydra, there is no moulting or ecdysis.
· Hydra has great regenerating capacities. A piece of Hydra will regenerate into a full Hydra if it contains a part of the epidermis and gastrodermis and size is not less than 1/6 mm in diameter.


1. Sedentary and colonial Free swimming and solitary
2. They are hollow conical sac and has distal hypostome or manubrium with a circular mouth They are umbrella-like manubrium and mouth lie ventrally
3. Statocyst absent present
4. Mesogloea thin Mesogloea thick
5. Coelenteron is wide and does not have a radial or circular canal Coelenteron is short and has 4 radial canals and a circular canal.
6. Nervous system poorly developed Well developed nervous system
7. Gonads absent Gonads present one in the middle of 2 radial canals.
8. They reproduce asexually by budding They reproduce sexually by gametes formation.
9. Velum is absent Velum is present around the margin of the umbrella

Taxonomic Summary

Subkingdom – Eumetazoa
Phylum Cnidaria – Hydrozoans, jellyfishes, sea wasps, sea anemones and corals

1. Class Hydrozoa
Obelia – Sea fur
Millepora – Fire coral
Physalia – Portuguese man of war
Porpita – Blue button
Velella – Little sail

2. Class Cubozoa – Sea wasps and box jellyfish
Carybdea – Sea wasp

3. Class Scyphozoa – True Jellyfish
Aurelia – Jellyfish
Cyanea – Lion's mane
Cassiopea – Upside-down jellyfish

4. Class Anthozoa – Anemones, corals, sea pens
Metridium – Sea anemone
Adamsia – Sea anemone
Corafflurn – Precious red coral
Meandrina – Brain coral
Tubipora – Organ-pipe coral
Heliopora – Blue coral
Fungia – Mushroom coral
Gorgonia – Sea fans
Alcyonium – Dead man's finger
Pennatula – Sea pen
Renifla – Sea pansies


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