Study Of Hydra

Classification of Hydra

Phylum- Coelenterata

Class- Hydrozoa

Order- Hydroidea

Suborder – Anthomedusae

Genus – Hydra

Study Of Hydra

General Study of Hydra

Hydra belongs to the most primitive Class Hydrozoa of Phylum coelenterata . It is simple in form and structure and serves as a good example for the study of coelenterate organization .

Habit and Habitat

Hydra one solitary, sessile, fresh water animals. They are cosmopolitan in distribution. They occur in lakes, ponds streams and seasonal ditches. They may be found attached to and hanging downward from underside of soiled object in water as leaves, sticks, stones weeds etc.

Characterstics of Hydra

Hydra is a polyp like or polyploid coelentrate with a tubular or cylindrical body. Body symmetry is typically radial comprising an oral and aboral axis. Aboral end of the body is closed flattered called pedal disc or basal disc used for attachment to substratum. Distal or free end of the body is produced into hypostome having circular mouth. Hypostome bears 6-10 slender contractile and tubular thread like tentaclels that helps in feeding and locomotion. Other structure like testes occurs near the oral end while rounded ovary near aboral end.

Internally, Hydra shows a central cavity called coelenteron, often referred as gastrovascular cavity surrounded by a body wall. There is no anus and excretory pore. Body of Hydra is diploblastic; formed of ectoderm and endoplasm between them is mesoglea which is delicate and transparent.

The outer layer called epidermis is enveloped by a thin layer of cuticle – It is of following types like epithelio- muscle cell which assist in muscular contraction, next to it are the gland cells which are found on pedal disc and around mouth region.

These cells secrete mucus like substance serves for attachment, protection and enlargement of prey. Then there are interstitial cells between narrow and basal end of epithelio muscle cells .They are small rounded and undiffertiated cells and are capable of developing into any other kind of cells such as reproductive, glandular, stinging and buds etc.

Study Of Hydra

Many of these interstitial cells becomes specialized to the form stinging cells called cnidoblasts (Gr: knide, nettle to blastos, germ).These form organs of offence and defense of Hydra and help Hydra in food capture locomotion and anchorage. During summer interstitials cells in certain restricted region of the body divide and proliferate like reproductive cells forming gonads, which later differentiate into either testes or ovaries. About 30 different kinds of nematocysts are found among different cnidarians. These kinds are constant for the species and are of taxonomic value.

Hydra has four basic types of nematocysts, as pentrant or stenotele is the largest (16µ India) volvent or desmoneme is small (9µ long) streptoline glutivant or holotrichous isorhizas 9µ are oval or cylindrical. Hydra is sessile animals as they are attached by their pedal disc to objects in water. It shows feeling moments as expansion and contraction which is initiated by pacemaker located near the base of hypostome.

Hydra can also move from place to place in search of food it extends and then bends over so the enlaces attach to the substratum with the help of adhesive glutinant nematocysts.It somersault like an acrobat. It glides due to creeping amoeboid movement of cells of pedal disc. Sometimes Hydra can move in an invalid condition using its tentacles as legs.

It climbs while changing location by attaching its long tentacles to some objects. Floats on leaf by pedal disc. Swimming is performed by undulating wave like movements of tentacles and body.

Digestion in Hydra

Hydra is exclusively carnivorous. It feeds on insect larvae crustaceans ( Cyclops Daphnia) and annelid worm. It can feed through nematocysts when an organism touches a tentacle; dozens of nematocysts are discharge into it at once. The penetrant puncture the victim & reject the paralyzing hyno toxin Digestion is both extracellular and intracellular.

first the prey is killed by the action of digestive juices ,churning is caused by the expansion and contraction of body wall and lashing movements of flagella, mixed up with digestive juices which is then broken into smaller particles.

While during intracellular digestive smaller fragments are engulfed by means of Pseudopodia & digested within food vacuoles. Some gastro dermal cells distribute digested food to all parts of body.

Egestions occur by sudden squirt due to muscular contraction of body so that the debris is thrown at distance.

Circulation of Hydra

Hydra has neither blood and blood vessel nor any organ for excretion & respiration. Exchange of oxygen and carbon dioxide and excretion of nitrogenous waste matter occurs directly by diffusion.

Hydra react to various kinds of internal as well as external stimuli. It shows negative reaction to strong and weak light. Hydra prefer mostly cool or cold water, It shows negative reaction to the water current and also avoids strong and injurious chemicals.

Reproduction in Hydra

Study Of Hydra

Hydra reproduces asexually by budding and sexually by formation of gametes. Hydra has great power of regeneration (Abraham Trembley in 1745). If a living Hydra is cut into two, three or more pieces, each missing part grows and becomes a complete animal.

Even it retains it polarity. Tremble observed that if head end of a Hydra is split into two it results into a Y-Shaped specimen or” two headed individual ”By further “splitting” he succeeded in producing seven – headed Hydra.

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