An Example of Phylum Cnidaria: Hydra

In this article we will discuss about Hydra:- 1. Habit and Habitat of Hydra 2. Structure of Hydra 3. Structure and Function of the Different Cellular Units 4. Locomotion 5. Nutrition 6. Respiration and Excretion 7. Reproduction 8. Life History.

Contents:

1. Habit and Habitat of Hydra
2. Structure of Hydra
3. Structure and Function of the Different Cellular Units of Hydra
4. Locomotion of Hydra
5. Nutrition in Hydra
6. Respiration and Excretion in Hydra
7. Reproduction in Hydra
8. Life History of Hydra

1. Habit and Habitat of Hydra:

One of the smallest solitary polyps amongst the cnidarians is represented by Hydra. Almost all the hydras inhabit clear, transparent freshwater ponds and lakes ex­cepting Protohydra which is marine. Hydra owes its name from Greek mythology where the sea-serpent named Hydra had the inher­ent potentiality to regenerate its head, if extirpated.

Several species of hydra are recorded. The common Indian species is the Hydra vulgaris (formerly known as Hydra grisea) and the Pelmatohydra oligacitis (formerly called as Hydra fusca). The phase orientalis of Hydra vulgaris is the commonest variety present in the neighbouring ponds of Kolkata.

Pelmatohydra oligactis has a comparatively slender body and the length of the tentacles exceeds three to four times the length of the body column. Another form of hydra which needs mentioning here is Chlorohydra viridissima (formerly known as Hydra viridis). This species harbours symbiotic algae Zoochlorella in the endodermal cells which render the colour of the body green.

Certain parasites are recorded in hydra, namely Hydramoeba hydroxena and few ciliates, Kerona pediculus and Trichodina pediculus. All the forms of hydra are built on same basic pat­tern and the following description will give an idea on the biology of hydra in general. Hydra exists only in polyp form (monomorphic) and the phenomenon of metagenesis or alternation of generation is totally absent.

2. Structure of Hydra:

Hydra has a slender tubular body and exhibits distinct radial symmetry (Fig. 12.2). The body is extremely contractile and the length varies from 10 to 30 mm. The lower end of the tubular body is closed and this side is designated as the aboral or proximal end. This end of the body is named as the foot or basal disc which is used as a struc­ture for attachment to the substratum and it aids in locomotion as well.

The opposite end (the oral or distal end) is free and possesses the opening of the mouth, situated at the summit of a conical elevation called the hypostome or manubrium. The base of the hypostome is surrounded by a number of tentacles. The usual number of tentacles is six but it may vary from four to eleven in some cases. Tentacles are totally absent in Protohydra.

The mouth leads into the coelenteron which occupies the interior of the body and is also continuous with the slender cavities of the tentacles.

In most cases, the fairly grown individuals bear buds located at the specific budding zone of the body, i.e., the region of the body column just below the middle of the body. During breeding season the mature individuals possess many male gonads or testes and usually one female gonad or ovary.

3. Structure and Function of the Different Cellular Units of Hydra:

The body of Hydra is composed of two cellular layers with a thin non-cellular mesoglea (Fig. 12.3) in between. The outer layer is known as the ectoderm and the inner layer is known as the endoderm. In addition to the power of contractility common to both the layers, the ectoderm is mainly protective and the endoderm is primarily nutritive in function.

Both the layers are constituted of several cell-types, destined to perform di­verse physiological functions (Fig. 12.4). In Hydra cellular differentiation is associated with the physiological division of labour. Each kind of cell performs definite and spe­cific functions in co-operation with each other for the individual as a whole.

I. Ectodermal cell-types:

The ectoderm is composed of epitheliomuscular cells, inter­stitial cells, cnidoblasts, sensory cells, nerve cells and basal disc cells.

A. Epitheliomuscular cells:

These cellu­lar units constitute the main bulk of the ectoderm and are comparatively larger in size. The compound name of this type is due to its dual functioning capacities, protection and the power of contraction. The cells are roughly conical in shape having their broad ends directed outwards.

These ends fuse and give rise to a continuous cuticles along the entire length of the body. The narrower ends are directed to the mesogleal side and are produced into contractile processes which are oriented along the length of the animal to act as longitudinal muscles. The contrac­tion of these processes shortens the body- column and tentacles. This cell-type has a prominent nucleus and several small vacuoles.

B. Interstitial cells:

In the intercellular spaces between the narrower ends of the epitheliomuscular cells there are small cells called the interstitial or sub-epithelial cells. These are comparatively small in size and oval in shape. These cells usually occur in clusters. Each cell has a prominent nucleus with one or more distinct nucleoli and the cytoplasm is very inconspicuous.

Interstitial cells can give rise to other cell-types, such as the cnidoblasts or nematoblasts and sex- cells. For this reason these cells are known as perennially undifferentiated embryonic cells.

C. Cnidoblasts:

This type of cellular unit constitutes a very important component of the ectoderm. The cell contains a sac-like nematocyst and a layer of specially contrac­tile cytoplasm with distinct nucleus. From the outer end of cnidoblast projects a trig­ger-like pointed process called the cnidocil. When touched, it conveys the sensation to the inner part of the cell.

Each nematocyst or cnida is a fluid-filled rounded capsule, the narrow end of which is drawn into a long, coiled, thread-like tube. It remains immersed within the cavity of the nematocyst (Fig. 12.5). It can be everted to aid in capturing the prey or in locomotion.

Cnidoblasts may oc­cur singly in the body column, but in the tentacles several smaller ones are grouped around a central one (penetrant type) to form small surface tubercle or ‘battery’. Cnidoblasts have a characteristic regional distribution. They are quite numerous in the tentacles and anterior region of the body but gradually slide down towards the aboral side and are totally absent in the basal disc.

Usually four types of nematocysts are encountered in Hydra (Fig. 12.6).

They are as follows:

Penetrant type or Stenotele:

This type has a large spherical cell-body of about 16 micra in diameter. It has a long thread-like tube which remains coiled in transverse plane. The base of the thread contains three long barbs and bears rows of small thorns or nettles. When the cniodcil is stimulated, the thread tube shoots out. It pierces the body of the prey and injects a fluid containing a toxic protein called hypnotoxin to paralyse the victim.

Volvent type or Desmoneme:

They are pear-shaped forms having an average length of about 9 micra. Each nematocyst of this kind contains a thick short thread, which usually forms a single loop. After discharge the thread coils round any part of the prey.

Glutinant Streptoline type or Holotrichous isorhiza:

They are oval in shape and are about 9 micra in length. Each nematocyst possesses a long thread with three or four transverse coils and bears small nettles. The thread may coil after discharge.

Glutinant Stereoline type or Atrichous isorhiza:

These types of nematocysts are comparatively smaller in size and are about 7 micra in length. Upon discharge the thread remains straight. The thread is unarmed.

The penetrant and volvent types of nematocysts help in capturing the prey and the streptoline and stereoline glutinant types, in addition to the capture of food, help in locomotion by producing sticky secretion.

Discharge of nematocysts:

Cnidoblasts possess an independent effector. Any stimulus affecting the cnidocil directly causes the discharge of the thread. Opinions differ as regards the actual mechanism of discharge. Mechanical stimuli are not always found to be effective.

Substances are extracted from the crustacean larvae on which Hydra feeds provoke discharge of nematocysts. Acetic acid is seen to cause discharge of the nematocysts. The discharge of thread is caused by the increased osmotic pressure of the fluid within the nematocyst capsule.

Nematocysts once shot out are not incor­porated in the body system. New cnido­blasts develop out of interstitial cells. Usu­ally one cnidoblast develops from one inter­stitial cell, but often two or more may arise.

D. Basal disc cells:

In the basal disc region the epitheliomuscular cells are modi­fied into tall cells. These cells have deeply stained granules in the cytoplasm and have faint striations. These cells produce a sticky secretion which helps the animal to adhere to the substratum under water. Sometimes these cells also produce gas bubble which helps the animal to float in water.

E. Sensory cells:

Scattered in the ectoderm there are numerous slender sen­sory cells. Several types of sensory cells are seen in Hydra. Sensory cells have delicate tips with sensory hairs. Sensory cells are quite abundant in the basal disc region, hypostome and tentacles. The bases of these sensory cells are connected to the nerve cells.

F. Nerve cells:

Existence of nerve cells in the ectoderm of Hydra is a highly controver­sial issue. By special histochemical tech­niques, some workers on this line have dem­onstrated the presence of spider-like cell- bodies, designated as the nerve cells. Deli­cate processes originating from the cell-bod­ies form a network in the ectoderm adjacent to the mesoglea.

These processes are con­nected to the sensory cells, to the similar processes of other nerve cells and lastly, to the contractile fibrils of the epitheliomuscular cells. Such combination of nerve connection produces a sort of simplest sensory-neuromotor mechanism in animals. This system co-ordinates the movement of the body and tentacles.

II. Endodermal cell-types:

The endoderm of Hydra is primarily made up of nutritive-muscular cells and gland cells. In­terstitial cells and cnidoblasts occur very rarely in the endoderm. Sensory cells and nerve cells are also present in the endodermal layer as in the ectoderm.

A. Nutritive-muscular cells:

The nutriti­ve-muscular cells constitute the main bulk of the endoderm. They are tall columnar cells encircling the coelenteron. In the hypostome region, the cytoplasm of the nutritive-mus­cular cells is granular and homogeneous. But in the tentacles and in the basal disc regions these cells are highly vacuolated with little cytoplasmic content. In the rest of the stem body these cells are highly developed.

The nucleus is usually located towards the mesogleal side and contains one or two nucleoli. Cytoplasm is less vacuolated. Deeply stained spherical bodies are abundantly found in these cells. The mesogleal ends of the nutritive-muscular cells are produced into contractile processes oriented transversely to encircle the body.

These processes act as circular muscles. When contracted, the dia­meter of the body is reduced and conse­quently the length of the body is extended. Nutritive-muscular cells are of two types— amoeboid and flagellated. The broader end of amoeboid cells projecting in the coe­lenteron produces pseudopodia to engulf the particles of food.

The cells with whip-like flagella cut the particles of the food into pieces. Recent observations have, however, failed to record the presence of such flagel­lated cells. As stated earlier symbiotic Zoochlorella and some parasitic amoebae and coccidia are recorded amidst the endodermal cells, particularly the nutritive-muscular cells of some species of Hydra.

B. Gland cells:

Gland cells are present all through the endoderm of Hydra excepting the tentacles. They are plenty in the oral region but their number gradually reduces towards the aboral region. They are usually located in between the nutritive-muscular cells. Two types of gland cells are encoun­tered in the endoderm. The first category of the gland cells is smaller in size. The shape is oval.

The end facing the coelenteron is broader and the other end is gradually nar­rowed to a thread-like projection touching the mesoglea. The cytoplasm is intensively basophilic and vacuolated. Nucleus is dis­tinct and is situated at the narrow end of the cell. The second category of the gland cells exhibits less basophilic cytoplasm. Vacuoles of different sizes are dispersed within the cell body. Nuclei are not distinctly visible.

The above mentioned types of the gland cells also have characteristic regional distri­bution. The former category of the gland cells is quite abundant in the oral part of the body, while the other category has outnum­bered them in the aboral region.

It has been observed that the distribution of various cell-types varies along the entire length of the body. The difference in cell- population at different zones of the body column indicates the presence of functional diversity in the different regions of the body (Fig. 12.7).

Cell flow in Hydra:

It has been observed that Hydra loses its old cells through the basal disc and tip of the tentacle. New cells are believed to be formed in a region imme­diately beneath the hypostome. This subhypostomal region is called the ‘growth zone’ of Hydra. The cells produced in this region migrate in two directions—one to­wards the tentacles and the other towards the basal disc.

This phenomenon of two-direc­tional cell-flow in the body of Hydra can be experimentally shown. Mookerjee (1962) has shown that a middle piece of Hydra, when grafted in the subhypostomal region induces a basal disc. Similar result has also been obtained by Sinha (1967) by implanting mid­dle piece material at the base of any one of the tentacles.

When the host Hydra is cultured, the induced basal disc at the subhypostomal region gradually descends down and merges with the original host basal disc. While the induced basal disc at the base of the tentacle ascends up to the tip of the tentacle and is discharged through the tentacular tip.

These experiments demonstrate the existence of two directional cell flow from the subhypostomal region of Hydra. By this cell flow Hydra not only replaces the old cniodoblasts but also renews the entire cell column. The body of Hydra thus shows a dynamic state and such phenomenon of cell replacement has ena­bled this animal to be ‘immortal’.

4. Locomotion of Hydra:

Hydra normally remains attached to the substratum by basal disc and stands erect. For capturing the prey and to change the location, Hydra exhibits several types of movement (Fig. 12.8).

Looping:

While standing erect the ani­mal bends its body and fixes the tentacles to the substratum by the glutinant nematocysts. It then releases the attachment of the basal disc and moves its free end to a new site. The animal now stands up by disengaging its tentacles.

Somersaulting:

In this type of move­ment, Hydra fixes itself on the substratum by the hypostomal end and shifts the attach­ment of basal disc. The basal disc is then rotated 180° and is fixed at a new point. The hypostome is again raised.

Other types of movement:

Hydra often uses its tentacles as legs and moves in an inverted way.

Gliding:

Occasionally, Hydra may glide along the substratum by the pseudopodial action of the basal disc cells.

Floating:

Sometimes, Hydra can produce a bubble of gas, secreted by the basal disc cells, which helps the animal to float on the surface of the water and is passively carried from one place to another by water current or wind flow.

Climbing:

Hydra can climb by attaching its tentacles to some distant object and then releasing the basal disc and by contracting the tentacles, the body is drawn up to a new position.

5. Nutrition in Hydra:

Nutrition in Hydra is holozoic. The entire process may be divided into ingestion, di­gestion and egestion.

Food and ingestion:

The food of Hydra comprises mainly of minute crustaceans and their larvae. Instances of taking insect larvae and Tubifex are also common. When Hydra is hungry, it exhibits a special kind of move­ment in search of food. It extends its body to its fullest extent and the tentacles are fully stretched in search of food.

Whenever wan­dering forms like Daphnia or Artemia larvae come in contact with the tentacles, nematocysts are discharged at once. It has been demonstrated that feeding reaction may be induced by the application of glutath­ione.

The penetrant type of nematocysts penetrates the body of the prey to inject the hypnotoxin to cause paralysis and the volvent types wrap the body of the prey. After causing paralysis, the tentacles bend inward and carry the food towards the mouth (Fig. 12.9).

The mouth then opens and by muscular contraction it pushes the food into the coelenteron. It is said that the food is then cut into bits by the beating of flagella of the flagellated type of the nutritive-muscular cells.

Digestion:

The gland cells of the endo­derm become activated and produce diges­tive juices containing only proteolytic en­zymes. The proteins are broken down into amino-acids which are absorbed by endodermal cells and are then distributed among the body cells. This particular type of digestion is called extracellular digestion.

The small bits of food matters that escape digestion are engulfed by the pseudopodia of the amoeboid nutritive-muscular cells. In these amoeboid cells the food particles are taken within food vacuoles where the food matters are digested. This type of amoeba-like diges­tion is called the intracellular digestion.

In the food vacuole, carbohydrate-splitting, fat- splitting as well as protein-splitting enzymes are produced. Thus in Hydra a combination of both higher (extracellular) and lower (in­tracellular) types of digestion occur.

Egestion:

Indigestible residues are passed out to the exterior through mouth which also acts as an anus.

6. Respiration and Excretion in Hydra:

Hydra utilises the oxygen dissolved in water for respiration. Oxygen is diffused from the surrounding water directly into the

different cells. The carbon dioxide goes out mainly from the ectoderm by diffusion. The nitrogenous waste products are also removed from the body surface through diffusion.

7. Reproduction in Hydra:

Hydra reproduces asexually as well as sexually.

Asexual reproduction:

The asexual re­production includes budding and fission. Budding is most common and regarded as the normal way of propagation which occurs throughout the season in well-fed mature individuals. A bud appears as a conical protuberance of the body wall from the budding zone. The projection contains ectoderm, mesoglea, endoderm and the con­tinuation of the coelenteron of mother Hy­dra.

Gradually, this projection elongates and attains a considerable length. Rudiments of tentacles appear at the free end of the bud. The hypos-tome is conical and bears the open­ing of the mouth at the centre. After this a constriction appears at the region of junction of the developing but with the mother’s body.

Gradually, the constriction becomes deeper. By this time basal disc in the bud is formed. Finally, the bud detaches itself from the body of the mother and leads an inde­pendent life (Fig. 12.10).

Several buds may occur in a single mother Hydra at a time, although the usual number is one. When several buds are present the older one is located towards the proximal side while the fresh ones are on the distal side.

Fission:

Both longitudinal and trans­verse fissions are seen in Hydra. But these fissions generally follow accidental breakdown and are not routine modes of reproduction. In this type of reproduction, the rest of the part is grown by a process called regeneration.

Events of regeneration:

Hydra is capable of replacing any of its lost structures within a short time.

This power of regeneration is due to highly plastic nature of its cellular organisa­tion. When the hypostome or the basal disc is amputated, the lost structures are reformed by the existing mass of cells within 48 hours (Fig. 12.11). The whole phenomenon of res­titution may be divided into two phases.

At first, the closure of the wound occurs which involves extension of the endoderm cells, migration and secretion of gland cells, for­mation of a slime plug from the secreted

material and ultimately coalescence of the endoderm cells at the centre of the wound. This completes the phase of wound healing and after wound healing, the second phase, i.e., the differentiation of the lost structure takes place.

The histo-differentiation of the hypo­stome starts with the formation of a new layer of ectoderm over the cut surface from the endodermal cells, plastering the open end. Finally, during the re-organisation of the hypostome, the gland cells gradually lose their granules and take up a non-granular amorphous nature.

The gland cells and other endodermal cells at the future hypostome site change their shape to crescentic forms. Formation of tentacles is initiated by the extension of the coelenteron at specific sites. When the tentacle grows a good number of cells are seen coming out from the junction of tentacle and the hypostome.

The gland cells never enter the tentacle, and other endodermal cells undergo vacuolation while entering the tentacular lumen. During tenta­cle formation a larges cale differentiation of nematocysts takes place.

The wound healing phase in the restitu­tion of basal disc is similar to what happens in the case of hypostome regeneration. After the formation of a new layer of ectoderm from endoderm, the delaminated cells be­come flattened sidewise and assume appre­ciable tallness.

These newly formed colum­nar cells further take up fibrous nature and spread over the presumptive basal disc area. Almost all of the gland cells including some other endoderm cells which once migrated to the amputated site are now thrown out and are found in free state inside the coe­lenteron. The cellular events during hypostomal and basal disc regeneration are shown in Figure 12.11.

Sexual reproduction:

Propagation by sexual process is very rare in Hydra. Sexual reproduction does not occur throughout the year, but happens occasionally. In Hydra, experimental alteration of temperature for 2-3 weeks will induce sexuality which occurs in the early winter or summer. Others have suggested that the tem­perature plays a major role in the initiation of sexuality than the photoperiod.

Most species of Hydra are dioecius while monoecious species are also common. Phe­nomenon of sex reversal is also seen in Hydra. Hydra once producing male gonads in one part of the year is seen to produce female gonads in another period of the year.

Gonads are temporary structures and occur in dis­tinct localised zones of the body column. The female gonad (ovary) produces eggs and the male gonad (testis) produces spermatozoa (Fig. 12.12). Both the gonads develop by the modification of the interstitial cells.

Male gonads or testes:

Several testes usually occur at the upper half of the body. Each testis is a conical swelling of the ectoderm. It has a heap of rapidly multiply­ing interstitial cells covered by a protective layer of epitheliomuscular cells. These rapidly multiplying interstitial cells are called the spermatogonia. Each spermatogonium divides repeatedly to produce a large number of spermatocytes which migrate outwards and become round.

Each cell now undergoes two maturation divisions to form four nuclei in an undivided mass of cytoplasm. These are called as the spermatids which are transformed into spermatozoa. Each spermato­zoon has a conical head, short neck and a long wavy tail. When the testis matures the covering capsule ruptures and the spermato­zoa are liberated in water where they remain viable for a day or two.

Female gonad or ovary:

The develop­ment of ovary resembles the formation of testis at the earlier stages. Of the rapidly multiplying interstitial cells, one becomes amoeboid. This is the oocyte which increases in size at the expense of the others which provide nutrition for the developing oocyte.

The cytoplasm of the oocyte is heavily loaded with dark yolk granules. The oocyte then enters into two maturation divisions to form three small non-functional polar bodies and one mature ovum or egg.

Fertilization:

When the egg becomes mature the capsular wall of the ovary rup­tures to form a small opening in the ectoderm for the entry of the sperm and thus the egg is exposed. The ovum secretes a gelatinous substance by which a number of sperms are attracted to it and only one sperm fertilizes the ovum.

8. Life-History of Hydra:

The egg after fertilization begins to di­vide. The cleavage is total and equal blastomeres are formed. The blastomeres then arrange themselves to form a cellular hollow ball called the blastula (coeloblastula). The wall of the blastula is composed of a single layer of cells and the enclosed cavity is called the blastocoel.

New cells are formed and cut off from the inner end of the existing cells and migrate into the blastocoel to form the inner layer. This stage is called the gastrula. The outer layer is destined to form the ectoderm and the inner solid layer is con­verted into the endoderm. This type of solid gastrula is called stereo-gastrula.

The endo­derm is solid at first which hollows out subsequently to form the coelenteron. A jelly like mesoglea is formed in between the two cellular layers. The embryo now secretes a horny capsule or cyst with spinous outer surface. The embryo then drops away from the mother’s body and sinks to the bottom of the pond.

After some time (the period varies from about 10 to 70 days) the capsule softens and a young Hydra with small tentacles hatches out. Thus young Hydra now settles down and begins to feed and grow (Fig. 12.13).

Thus the development of Hydra is direct and a larval form which is the characteristic of other cnidarians is absent.