Hydra - Class 11 PDF Download

SYSTEMATIC POSITION

(1) Division - Radiata

(A) Radial symmetry

(B) Tissue grade

(C) Common digestive and body cavity. (Gastrovascular cavity)

Hydra

(2) Phylum - Cnidaria or Coelenterata

(A) Mouth encircled by tentacles.

(B) Stinging cells present

(3) Class - Hydrozoa

(A) Life cycle includes alternating polyp and medusa stages.

(B) Mesogloea non-cellular

(4) Order - Hydroidea

(A) Polyp phase dominant

(B) Medusa phase less developed or absent

(5) Genus - Hydra

(A) Tentacles 6 – 10

(B) Stalk less developed

HISTORY OF DISCOVERY

(a) Hydra was discovered by Leeuwenhoek (1703)

(b) Full description of Hydra by Trembley (1744)

(c) Hydra name was proposed by Linnaeus (1758)

DIFFERENT SPECIES OF HYDRA

(1) Green Hydra →Chlorohydra viridissimma or Hydra viridis →(Bisexual)

(2) Brown Hydra →Pelmatohydra oligactis or Hydra fusca →(Unisexual)

(3) Pink or white Hydra →Hydra gangetica →(Bisexual)

(4) Colourless Hydra →Hydra vulgaris →(Bisexual)

HABIT AND HABITAT

(a) Fresh water/Brackish water
(b) Solitary
(c) Free Living
(d) Carnivorous
(e) Regeneration & division of labour capacity

Hydra's body is like a thin, cylindrical, hollow tube, contractile & Radial symmetry.

Hydra is cylindrical Polyp coelenterate.

The body of Hydra is 3-4 mm to 1 cm in length and diameter 1 mm.

In hydra mouth is present on distal end of free end. In Hydra mouth act as anus therefore Hydra ingestion & egestion will takes place through the one opening. It is also called as blind sac.

Hydra's body divided into two parts: 

(a) Somewhat thinner basal or proximal part, call stalk and,

(b) The remaining thicker and darker distal part called gastric region.

The closed basal or proximal end of body is flattened forming an adhesive basal disc or pedal disc for attachment ot a Substratum.

Conical part of gastric region is known as hypostome. A circlet of 6 to 10 hollow slender, thread like unbranched tentacles arise from the circular base line of hypostome. Tentacles help in feeding, locomotion and protection.

In the center through the basal disc there is a pore which is known as aboral pore. This aboral pore helps to affix the Hydra to the base.

INTERNAL STRUCTURE

Coelenteron or gastrovascular cavity : -

A cavity is present inside the body of Hydra which is known as coelenteron or gastrovascular cavity. It is surrounded by body wall. It opens out through mouth.

HISTOLOGY OF HYDRA

The body wall of Hydra is made up of two cellular layers.

(1) Outer layer-Epidermis-It is made by ectoderm

(2) Inner layer-Gastrodermis-It is made by endoderm.

A non-cellular layer is present between epidermis and gastrodermis which is known as ‘mesoglea’. Mesoglea is a jelly like fibrous layer. It is thin and non living.

Mesoglea is made up of mucopolysaccharide and it has a net of collagen and elastin fibres.

Mesoglea is formed by secretion of epidermis and gastrodermis.

The thinnest layer of mesoglea is found in the tentacles and the thickest layer of mesoglea is found in the lateral part of basal disc of Hydra.

Body of Hydra is surrounded by a thin and transparent layer of cuticle. Cuticle is absent on basal disc.

CELL OF EPIDERMIS

Its large number of cells are cuboidal in shape and forms the 1/3^rd part of the thickness of body wall.

(1) Epithelio muscular cells -

They are cuboidal in shape.

Found maximum in epidermis

Absent in basal disc.

Epithelio muscular cells has 2 parts:

(i) The part which is present towards cuticle is called as epithelial part. This part has cuticle secreting granules.

(ii) The part which is present inside (towards coelenteron) is called as muscular part. Muscular part is present in the form of muscular process, parallel to the body. Muscular process is made up of myofibrils or myoneme. Myofibrils are made up of actin (Diameter-20Å) and myosin (Diameter–200Å) and works together by forming actinomyosin.

In this way, these muscular process form a longitudinal muscle layer just out side the mesoglea. By their contraction Hydra becomes short and thick.

Function : -

(1) Formation of cuticle.

(2) Help in contraction

(2) Glandulomuscular cells -

These are modified epithelio muscular cells

They are present in the epidermis of basal disc.

Their structure is similar to epithelio muscular cells

Functions : -

(i) They secretes mucous which helps Hydra in the adhesion with substratum.

(ii) Formation of Pseudopodia-Help in gliding movement.

(iii) It helps in floating movement by forming gas bubbles.

(3) Interstitial cells -

They are spherical or oval.

They are present between the epithelio muscular cells.

They are abundantly found in the gastric region. It is called as growth zone.

They are absent in tentacles and basal disc.

Its nucleus is large and cytoplasm is dense.

They are also called as embroyonic reserve cells or totipotent cells or wandering cells.

Function

(i) Help in Regeneration

(ii) Formation of germ cells

(iii) They have ability to modified into new cells → By amoeboidal movement these cells travels from epidermis to Mesogloea GastrodermisCoelenteron, and finally reaches to their required place. They gets modified in their journey.

(iv) Help in bud formation.

According to ‘‘Brien’’, each cell of Hydra renewed after 45 days.

(v) Formation of nematoblast. 

(4) Sensory cells -

They are present between epithelio muscular cells.

They are narrow and spindle shaped.

Sensory cells are abundant in hypostome therefore hypostome is highly sensitive part.

They have a sensory cilium at their free end. It accepts external stimuli.

The inner part of sensory cilium is in the form of nodulated process. This process attached.

Sensory cilia receive impulse and transmit them into nodulated process. From their, the impulses goes to the nerve cell and then to the whole body.

Function 

Receive chemical, thermal, photo & touch impulses.

(5) Nerve cells -

Nerve cells are present at the lower part of epidermis.

They are abundant in the basal part of tentacles, hypostome and basal disc.

Nerve cells have two parts- (1) Cyton (2) Nerve fibres

Cyton : - This is the main part of nerve cell, many fibres (branched) arises from it, which are called as nerve fibres or neurites.

In Hydra nerve fibres are not differnetiated into dendrites and axone. So that nerve cells of Hydra are non polar.

The fibres of all nerve cells combine to form nerve net, which is called as Ectodermal Nerve Net. It is present just outside the mesoglea

In Hydra some nerve cells joint with each other and form a ring like structure around the mouth this is known as circum centric Ring. It is believed that, this ring is the preliminary structure of brain ring of higher animals

– Hydra doesn't have brain, it has only Nerve Net.

– Hydra has Nerve cells, but no Nerves.

Note : Microtubules are absent in nerve cells, which are present on basal disc.

(6) Nematoblast or cnidoblast : -

Presence of Nematoblast is the main character of coelenterata.

Hydra has mature nematoblast at two places – (1) Tentacles (2) Stalk part

They are abundant in Tentacles and absent in basal disc.

Some invaginations are present in the epithelio muscular cells of tentacles. Nematoblasts are found in groups in these invaginations. These groups are called as battery or cnidom. 10 – 12 nematoblasts are present in one battery.

Structure : -

It is highly complicated cell. It is spherical or oval in shape.

A pointed outgrowth is present at its free ends, which is called as cnidocil. This is the receptive (sensitive) structure of Nematoblast. Cnidocil accepts stimulus directly and doesn't depends on nerve net, therefore nematoblast is known as independent receptive organ. Cnidocil has one cilium to receive stimulus.

20 – 21 protein fibers surrounds the cilium of cnidocil, they are called as supporting fibres of cnidocil. Besides 20 – 25 fibres are also present which joints cnidocil to nematocyst, they are called as the supporting fibres of nematocyst. These fibres transmit stimulus from cilium to nematocyst.

A bag like structmatoblast, which is called as nematocyst.

NEMATOCYST

It is double capsule like structure. Both layers are made up of collagen like protein. A fluid is filled inside nematocyst at high pressure, which is called as capsular fluid. Capsular fluid contains a poisonous substance which is known as hypnotoxin. Hypnotoxin is a mixture of protein + phenol.

A lid like structure is present in nematocyst which is formed by the outer layer of nematocyst/capsule, which is called as operculum. Inner layer infolds and forms thread. The anterior end of thread is keep like, which is called as butt.

Thread is hollow from inside, and a triangular cavity is present inside it.

Spines are present at the inner layer of thread and these are arranged in the 3 spiral.

Spines are present at the inner layer of thread and these are arranged in the 3 spiral rows. The first spine of each row is large, which is called as barb or stylet. Remaining spines are smaller and are called as barbules.

Numerous myofibrils are present on the outer surface of nematocyst. Besides these contractile myofibrils, a group of fibres is also present which is called as Lasso. At the time of discharge lasso does not allow nematoblast to separate nematocyst.

Nematoblast with developed nematocyst is known as cnidocyte. Cnidocytes are present on tentacles as well as on stalk region and remaining part of Hydra is having ill developed nematocyst known as cnidoblast.

– Its main function is to help in food intake and locomotion.

Type of Nematocyst 

(On the basis of shape, structure of thread and function.) (1)(2)(3)(4)

StenotelesLarge glutinantsSmall glutinants Desmonemes or  Penetrants Streptoline Stereoline Volvents

Note : First three types of nematocyst discharged immediately on getting physical stimuli while the last nematocyst required both physical and chemical stimulus for their discharge. The chemical stimulus will be required through the glutathione present in food 

Desmonemes or Volvents

(1) Size

(2) Butt

(3) Terminal end of thread

(4) Spines

(5) Barb

(6) Barbules

(7) Capsular fluid

Note : Ewer studied function of different types of nematocyst in Hydra.

DISCHARGE OF NEMATOCYST

When nematocyst is stimulate, pressure increases inside the capsule or nematocyst. Due to increase of pressure nematocyst get discharge. Its thread push the operculum and comes out from nematocyst by turning completely inside to out side like the sleeve of shirt.

Due to turning of thread spines also comes out side. When these spines comes in contact with animal, they injured the animal and hypnotoxin is entered in animals from these injured places.

For the discharge of nematocyst both physical (provide by food contact) and chemical stimulus (Provide by glutathione) are necessary.

Once nematocyst is discharged then it became useless. It is therefore discarded from tips of tentacles and aboral pore. Discharge nematocyst is replaced in 48 hours.

Time for discharge – 0.3 to 0.5 sec.

DIFFERENT VIEW ABOUT DISCHARGE OF NEMATOCYST

According to Iwanzoff (1895), Yanagita (1943) water enters into the capsule from surrounding cytoplasm. A nematocyst is discharged due to increase in hydrostatic Pressure of capsular fluid.

1. According to Will (1914), Pickin (1953), Robson : - Water diffusing into the capsule from cytoplasm, enormously swells a Jelly like intra capsular Substance causing discharge of the thread.

According to jones (1974) : The nematocyst are always in a state of tension. Stimulation of Cnidocil causing sudden opening of operculum. Hence the thread is discharged.

This is the innermost layer of body wall.

It forms the 2/3^rd part of body wall.

Its maximum cells are columnar shaped.

It originates from embryonic endoderm.

Five types of cells are present in gastrodermis : -

1. Nutritive – Muscular cells : - 

Club shaped.

These cells forms the main part of gastrodermis.

They also have muscular part and epithelial part like that of epithelial cell. Muscular part is towards mesogloea and epithelial part is towards coelenteron.

Muscular layer forms circular muscle layer towards the inner surface of mesoglea. By the contraction of which Hydra becomes long and thin

Epithelial part is towards the coelenteron. It has 1 or 2 flagella. It helps in the grinding of food.

A pseudopodia like out growth arises from their free end. These pseudopodia helps in in takes of half digested food through coelenteron.

The cytoplasm of these cells have food vacuoles, the digestion of food takes place in them,which is known as intra cellular digestion.

These cells from sphincters around the mouth and at the base of tentacles, which helps in opening and closing of mouth pore and cavity of tentacles.

Function : 

(i) Help in grinding of food into pieces.

(ii) Help in ingestion of food by phagocytosis.

Note : Food vacuoles are absent in the nutritive muscular cells of tentacles because they do not intake food.

(2) Gland cells 

Structurally these are similar to nutritive muscular cells but muscular part is absent in them.

These are smaller, club shaped and found singly between nutritive muscular cells.

These are most numerous in hypostome, fewer in gastric region and rather scanty or even absent in tentacles, stalk and basal disc.

These cells are two types 

Mucous cells – Present in hypostome region and secrete protein digestive enzyme (Proteolytic)

Function : 

(i) Secretion of mucous lubricates the passage of food.

(ii) Digestive of food. Gland cells are independent effector.

(3) Nerve cells 

Like epidermis, the gastrodermis also contains nerve cells which are non polar.

They form internal nerve net at the inner side of mesoglea.

These are less developed as compare to epidermal nerve cells.

(4) Sensory cells 

Same as epidermal Interstitial cells.

These cells are capable of modifying into all other types of cells except germinal cells & nematoblast.

NUTRITION

A. Food : -

Hydra is a carnivorous animal.

It feeds upon small aquatic animals like larva, crustaceouns larva, Cyclops, annelid and mollusca.

Its favorite food is Cyclops and Daphnia. These are water flea.

'Glutathione' secreted by prey, helps the ingestion process of Hydra.

B. Ingestion & Digestion : -

It keeps its body extended and waves its extended tentacles in all direction in search of a prey.

Nematcysts are discharged, when any appropriate prey gives the stimulus by touch.

As the food comes in contact with cniodocil, Penetrants and Streptoline discharge . Theyunconscious the prey by secreating hypnotoxin.

Now stereoline discharge and makes the food slimy.

Now desmoneme discharge and coiled up around.

After this, the food is taken in along with desmoneme. This food is now reached in mouth.

After food reaches in mouth flagella of nutritive muscular cell crushes the food into numeroussmall pieces.

Now the food proceeds in hypostome, where the mucous cells secretes mucous, that makes the food slimy.

Now the food further proceeds to the gastric region, where zymogen cells secretes theproteolytic enzyme. This enzyme partially digest the proteins present in food. This digestion takes place in cavity or coelenteron, therefore it is known as extra cellular digestion.

(Time duration for extra cellular digestion 3-4 hours.)

Now this half digested food is intake through phagocytosis by the formation of pseudopodiaformed by nutritive muscular cells. In this way food vacuole is formed in nutritive muscular cells.

The remaining digestion of food takes place in food vacuole (intracellular). The medium of foodvacuole is first acidic and then alkaline.

C. Absorption of Digested food : -

 The absorption of digestion protein is done by the microvilli of nutritive muscular cells from coelentron.

The digested food in food vacuole goes from one cell to another cell by diffusion.

D. Egestion : -

The undigested food is thrown out by mouth. The extra food is stored in nutritive muscular cells.

Note : 

Glutathione helps in ingestion of food but not in digestion.

Hydra can digest protein, fat , carbohydrates but it has no enzyme to digest starch.

Note : 

Fat & glycogen are stored in nutritive muscular cells.

In Hydra, non-polarized nerve cells of epidermis and gastrodermis form two nerve nets.

Nerve net is connected to sensory cells.

Glandular cells, nematoblast and sensory cells are function as free receptor.

In Hydra nerve net is represented as nervous system.

The contractile basal processes of epithelio muscular cells of epidermis and nutritive muscular cells of gastrodermis respectively from Longitudinal and Circular Muscle layers upon outer and inner surface of Mesoglea. Due to this reason the mesoglea functions like a skeletal axis.

By filling the gastrovascular cavity with water through mouth, Hydra uses it also as a Hydraulic Skeleton for relaxation and extension of body.

Through body surface.

Gaseous exchange by diffusion.

Storage of energy in form of ATP.

EXCRETION

Excretory Substance : - NH₃

Ammonotelic 

Excrete Ammonia by Diffusion.

Locomotion in Hydra is due to following reasons :

In search of food.

Protection

Response to stimuli.

Note : Locomotion in Hydra is not for reproduction.

(1) Contraction & Expansion of body

When Hydra is attached to a Substratum by basal disc then Hydra exhibits contraction & expansion movement of body.

Its body remains flexed due to contraction of circular muscle processes.

Periodically the body suddenly contracts due to maximum contraction of longitudinal muscleProcesses.

Contraction and expansion of tentacles depend upon the coelomic fluid because muscle process absent in it.

(2) Gliding – (Slowest movement)

The glandulo-muscular cells of the epidermis of basal disc sometimes from pseudopodia.

Consequently, Hydra starts slowly gliding or crawling upon its substratum.

Note : Gliding is called as Snail like movement.

(3) Floating

Sometimes, glandulo-muscular cells of basal disc secrete a gas bubble.

Consequently Hydra lose contact with its substratum, turns upside down and Passively floats at water surface.

(4) Walking 

This Locomotion is cuttle fish like movement. In this locomotion Hydra looks maximum hard.

Occasionally, Hydra stand upside down and walks upon its Substratum using the tentacles as legs. Stereoline helps in walking.

During walking, Coelentron fluid reaches in tentacles and function as a hydrostatic skeleton.

(5) Swimming 

Tentacles or Hydra help in Swimming.

Peristaltic movement in muscles and lashing of tentacles help in Swimming.

(6) Looping – (Most common method)

The body first elongates, bends over to one side end fixes its tentacles to the substratum with the help of Stereoline.

Now the pedal disc glides close to the tentacles.

Next, the tentacles are disengaged and the normal upright posture is regained.

This is also called as caterpillar or leech like movement.

Distance covered in one looping action = Half length of body

During one looping action Hydra make 180° angle

(7) Somersaulting – (Fastest movement)

The body elongates, bends on a side and fixes its tentacles to the substratum.

Then the aboral end is released from the substratum and lifted up, so that the body now stands upside down on its tentacles.

Next, the body again bends fixes its pedal disc to the substratum, release the tentacles and lifts up its oral end standing upright in normal position.

Thus, the Hydra moves fast like an acrobat.

Distance covered in one Somersaulting = Total length of body

During one Somersaulting Hydra make 360° angle

(8) Climbing or Dragging

Hydra climbs an aquatic plant with the help of tentacles.

In this types of movement Hydra its body and hang from branches of aquatic plant

It is observed in brown Hydra 

In Hydra, sensory neuro muscular system is found. Hydra has sensory cells. These cells receive stimulus and transfers them to nerve net. Nerve net transfer these stimulus to longitudinal and circular muscle layer. So Hydra shows stimulation by contraction and relaxation.

Behavior OF HYDRA

1. Response against external stimuli :

(A) Touch : -

Thigmotaxis. - When touches by a rod or needle Hydra's body contract. i.e. Negative movement.

(B) Light : -

Phototaxis

Negative movement for both strong & dim light.

Like to stay in moderate light.

(C) Temperature : -

Hydra like 20-25 temperature.

Variation in temperature (high and low) → Hydra show negative movements.

(D) Chemicals : -

Hydra show negative response to harmful and strong chemicals

Show positive response to food.

(E) Chemicals : -

No response.

(F) Electric current : -

When a moving Hydra contact to a week electric currents, it bends towards the cathode.

Tentacles turn toward anode and basal disc towards cathode. When weak electric current continuously flow. (When Hydra attached with substratum).

Hydra die in a strong current.

2. Response against Internal stimuli : 

Hunger act as internal stimuli in Hydra.

Hunger : - A special type of response is shown by Hydra against hunger. Hunger act as a internal stimuli in Hydra. Hydra becomes elongated when it is hungry, and spreads its tentacles in water as long as it can, and starts rotating its tentacles. If it doesn't finds the food, than it will change its position.

Hungry hydra is longest.

If  Hydra doesn't finds food for a prolonged period, than it comes under depression and collapses at bottom. In this condition, if it is feed by force than it wouldn't take food since it doesn't have so much energy. In this condition, Hydra will die.

REPRODUCTION IN HYDRA

In Hydra, reproduction is of two type

(1) Asexual reproduction (by budding) →  During favourable conditions (October to January)

(2) Sexual reproduction → During unfavourable condition (Feb. to March)

Reproduction takes place in favourable condition by bud formation.

Budding takes during Oct. to Jan. maximum in Oct. & Nov. Because plenty of food (Micro organism) is present between Oct to Jan.

In Hydra, the formation of bud takes place at the junction of gastric region & Stalk region (sometimes more than one buds are also formed in it.)

Bud forms as an out growth of body wall.

The formation of bud takes place by the rapid division of epitheliomuscular cells of epidermis and nutritive-muscular cells of gastrodermis and then due to the transformation of interstitial cells other cells are formed.

All the four layers (cuticle, ectoderm, mesoglea, endoderm) of body wall are found in bud. Now a cavity is formed due to the advanced spread of gastrovascular cavity in bud.

In the later stage some cells of bud are dissolved and mouth is developed in bud. Tentacles are also formed by the spreading of body wall and then complete Hydra is formed. This newly formed Hydragets separated from mother Hydra by contraction in the basal part.

If conditions are more favorable then another bud is formed on that bud. i.e. secondary bud is formed on primary bud and the number of Hydra increases rapidly.

Note : Complete development of bud requires 2-3 days and a bud is changed into adult Hydra after attachment from parent Hydra with in 1-2 days.

Sexual reproduction takes place during unfavourable conditions.

Unfavourable condition starts from Feb. or March. And completed in April-May.

Mostly species of Hydra are unisexual. (In unisexual Hydra – male is small and female is large).

Testis and ovaries and formed in gastric region. In unisexual Hydra testis and ovaries are spread on gastric region. But in bisexual Hydra, testis are formed towards the mouth and ovary towards basal part.

The number of tests in Hydra may be up to 8. But the number of ovaries remains 1 (normally) or 2.

Development of Testis : -

Testis are conical in shape. First of all numerous interstitial cells collects beneath the epidermis and modified to form spermatogonia.

Sperm reformed by spermatogenesis.

Sperms are formed beneath the epidermis, as a result epidermis spreads outward and form testis.

Nipple like structure is present at the apical part of testis in bisexual Hydra (eg. Hydra litoris).The Nipple are absent in brown Hydra.

Now some cells are dissolved at the apical part of testis, due to which a pore is formed. Sperms comes out by this pore and starts swimming in water.

Note : Testis are round in shape Hydra oligactis.

Development of Ovary : -

Ovaries are round or oval in shape.

By the modification of interstitial cells at the place of formation of ovary, Oogonia are formed. And out of these, only one Oogonium develops into mature Oogonium by absorbing nutrition. This is called asprimary Oocyte. 

Notes : - Eggs are formed by Oogenesis. Normally one egg is formed but in chlorohydra viridissimma 2 and in Hydra diocia more than two eggs are formed.

Fertilization takes place in water, therefore it is called as external fertilization

In Hydra the fertilization of egg takes place at the place of its formation. i.e. egg (ovum) is not released from the ovary, so the fertilization takes place in the ovary.

In Hydra, testis are formed before the formation of ovary. Therefore, sperms are formed and released in water before the formation of ovum, This condition is called as Protandrous condition .

Since the sperms are formed earlier than ovum, therefore fertilization always takes place between the sperm and ovum of different Hydra i.e. In Hydra, fertilization is always cross type.

Only one sperm can fertilized the ovum, as a result of which, only one zygote is formed.

The development of zygote takes place in mother Hydra.

Development of Embryo : -

First two cleavage in zygote are vertical and right angle to each other and third cleavage is horizontal.As a result of these three cleavages, one 8-celled embryo is formed. In it four cells (i.e. blastomeres) are present at upper side and four cells are present at lower side.

Now cleavage takes place in blastomere as a result of which a solid ball like embryo is formed, which is known as Morula.

Now cohesion or separation takes place in the blastomeres of morula. Cohesion (i.e. separation of cells) starts and as result of which a cavity is formed inside embryo, This cavity is called blastocoel And this hollow embryo is called blastula.

Blastocoel is formed by the separation of cells. Therefore this cavity is called as schiogenous cavity.

After this gastrulation takes place. The process of gastrulation in Hydra is delamination (ingression or infilteration) . In this process, division takes place in all the blastomeres of blastula, resulting many new blastomeres are formed which fall in the blastocoel. Due to which blastocoel completely filled with blastomeres. Blastomeres that fills the blastocoel, form the endoderm and those blastomeres that are present in outer layer form the ectoderm.

[In this way 2 layers (Geminal layers)] ectoderm and endoderm are formed and this diploblastic embryo now called as gastrula.]

The gastrula of Hydra is solid, there is no cavity inside it. Therefore it is called as stereogastrula. Unfavourable conditions arrives after the formation of gastrula.

This gastrula secretes a thick, spiny and chitinous sheath around it, to protect themselves fromunfavourable conditions, this sheath is called as theca or shell. Which is round or spindle shaped.

Embryo protects itself by forming theca. But mother Hydra doesn't have any such arrangement, so mother Hydra destroys by drying and the embryo becomes free.

Some central cells of endoderm dissolves and a cavity is formed in embryo. This cavity is called as archenteron. This is the coelenteron of future. Coelenteron is formed by archenteron. Embryo spends its unfavourable condition in this form and when favourable conditions return it absorbs water due to which theca bursts and the young Hydra comes out.

In Hydra, development is direct so larval stage is absent in it .

Archenteron is formed by the dissolution of cells, so that it called lysigenous cavity.

In Hydra, cleavage is radial type and division is holoblastic, equal and indeterminate.

Hydra reproduces by budding in winter and increase their number. But at the end of winter they form embryo by sexual reproduction. In this way. Hydra is a seasonal animal.

SYMBIOSIS IN HYDRA

Symbiosis is found in two species of Hydra.

(1) Green Hydra : - Lives symbiotically with zoochlorella, which is unicellular green algae.

(2) Brown Hydra : - Lives symbiotically with zooxanthelle, which is unicellular brown algae.

In Hydra algae are found in the vacuoles of nutritive muscular cells of gastrodermis,

The type of symbiosis in Hydra is -  Mutualism i.e. both the organisms are benefited to each other.

Hydra and Algae exchange gases. Hydra provides CO₂  to algae and takes O₂ from it.

Algae provides food to the Hydra by photosynthesis while Hydra provides habitat an protection toalgae.

The symbiotic association of Hydra and algae is obligate type i.e. Symbiosis is necessary. BothHydra and Algae can not live separately (their symbiosis is permanent). They lives together in one generation to next generation.

Algae migrates from one generation to next generation through egg and budding in Hydra. When reproduction takes place in Hydra, at same time reproduction also occurs in algae and algae comes out through exocytosis from the vacuole of nutritive muscular cell of gastrodermis, now this algae reaches epidermis through mesoglea and then enter into egg through epidermis during egg formation. In this way algae migrates from one generation to next generation through egg.

PARASITISM IN HYDRA 

Parasitism is found in Hydra, Kerona and Trichodena (protozoa) are ectoparasite of Hydra. They are found as parasite on the body of Hydra. The nematoblast of Hydra are not sensitive towards these parasites.

Hydramoeba hydroxena (amoeba) eats the cells of Hydra by separating them.

REGENERATION IN HYDRA

If we cut Hydra into several pieces then each piece having size greater than 1.6 mm and has both epidermis and gastrodermis layer of body wall, can develops in to a new Hydra.

The piece of Hydra can be obtained by two ways.

(1) Transverse : - The possibility of formation of new Hydra in transverse section is high. Because in transverse part the cells are arranged in circular form. Due to which cells are rearranged easily and new cells are formed easily.

(2) Longitudinal : - The possibility of formation of new Hydra in longitudinal section is very less. Because in longitudinal part coelenteron become open from one side. Due to which cells can not rearranged easily and it die until.

Interstitial cells are maximum in the growth zone near hyposome. If the pieces cut from that region then the possibility of formation of Hydra is more and quick, because interstitial cells transformed quickly to form new Hyda.

Hydra cannot be regenerated from the part of tentacles because interstitial cells are absent in tentacles.

The type of regeneration in Hydra is morphallaxis.

Hydra maintains it polarity during regeneration.

Study of Regeneration in Hydra was done by A . Trembley.

Trembley prepared seven headed Hydra by regeneration. This seven headed Hydra was similar to hyder (Nine headed) of Greek. So on the basis of Hyder, Linnaeus proprosed the name Hydra of the seven headed Hydra of Trembley.

GRAFTING

(1) Hydra has been extensively used in grafting experiments.

(2) Zoologists have been able to graft pieces of different Hydra of the same species together Producing Peculiar specimens.

Note : Conclusion find out by the scientists on the basis of grafting experiments, highest power of regeneration is present near the hypostome.

IMMORTALITY IN HYDRA

(1) According to Brien (1955), Jansen (1951) and Lentz (1960) etc have suggested that Hydra is Potentially Immortal. According to them, Hydra has a growth Zone just below the level at which tentacles arise

(2) Interstitial cells acting as an ''Undifferentiated embryonic reserve cells. These cells continuously multiply and transform into other types of cells in the growth zone.''

(3) Consequently, older cells are progressively pushed towards the pedal disc and tips of tentacles from where these are continuously shed off. In this way, cells of the whole body of Hydra are replaced or renewed in about 45 days and there is no natural death of the Animal.

PHYSIOLOGICAL DIVISION OF LABOUR

(1) Henry Miline Edwards introduced the concept of ''Physiological Division of labour'' in multicellular organisms.

(2) It implies that, instead of all cells of body participating in all its vital activities, the cells become differently specialized for limited specific functions. In Hydra different cells performs different functions, this is called as division of labour

(3) Energy is conserve in Multicellular animals by division of Labour.