In this article we will discuss about:- 1. Habitat of Hydra 2. External Feature of Hydra 3. Histology of the Body Wall 4. Feeding and Digestion Process 5. Locomotion 6. Respiration and Excretion Process 7. Reproduction 8. Life-History 9. Hydra in Relation to Higher Metazoan.
Contents:
- Habitat of Hydra
- External Feature of Hydra
- Histology of the Body Wall in Hydra
- Feeding and Digestion Process in Hydra
- Locomotion in Hydra
- Respiration and Excretion Process in Hydra
- Reproduction in Hydra
- Life-History of Hydra
- Hydra in Relation to Higher Metazoan
1. Habitat of Hydra:
Hydra is one of the simplest of the metazoa. It is a solitary polyp of microscopic size which lives in clean fresh water ponds, attached to stones or water weeds.
When the surface water becomes foul and hot, the animal retreats to the comparatively clean and cool bottom, where it lives in a peculiar depressed state. Depression is associated with lowered metabolism, shortening of the body and gradual disintegration. Recovery may occur if circumstances become favourable.
There are several species of Hydra. The grayish-brown Hydra vulgaris (formerly called H. grisea) is the common Indian species. Pelmatohydra oligactis (formerly Hydra fused) is another common species. Its tentacles are longer than those of H. vulgaris. The green-coloured Chlorohydra viridissima (formerly H. viridis) is not commonly found in India.
2. External Feature of Hydra:
In appearance, Hydra resembles a small needle-like cylinder, about 10 mm. to 30 mm. in length, with a varying number of fine threads radiating from its open end.
The hollow tubular body is closed at one end by the basal disc which is used for attachment and locomotion, and is, therefore, known as the foot. The free end of the tube bears a small opening called mouth, upon a raised conical hypostome, the base of which is surrounded by a circlet of about half a dozen hollow threads, the tentacles.
The mouth serves for the ingestion and egestion of food. It leads through a narrow passage directly into the hollow of the tubular body which is continuous with the slender cavities of the tentacles.
The enclosed cavity is known as the enteron or more precisely coelenteron and serves as the digestive tract. The animal is elastic and flexible. It may extend fully as a slender tube and bend in any direction. When contracted, Hydra assumes a spherical shape with the tentacles appearing as knobs at the free end.
In well-fed and fairly grown specimens the solitary polyp may bear lateral buds (Fig. 55 C). In the breeding season, gonads may be found as projections from the surface of the body.
When present, the male gonads or testes are found at the oral end, whereas the single female gonad or ovary occurs near the basal disc. Hydra is radially symmetrical, that is, the parts of its body are arranged in a circle around a central median axis passing through the mouth.
3. Histology of the Body Wall in Hydra:
The body wall is composed of two layers of cells, an outer ectoderm and an inner endoderm. Between the two cellular layers, there is a thin non-cellular mesogloea of jelly-like consistency. The ectoderm forms the thin outer epidermis which is chiefly concerned with protection and sensory function.
The endoderm, which is two or three times as thick as the ectoderm, forms the inner gastro-dermis lining the coelenteron. It is mainly concerned with nutrition. The jelly-like non-cellular mesogloea serves as an elastic supporting framework and connects the two cellular layers.
Being composed of the same layers, the tentacles are structurally identical with the body wall. The cellular layers of the body wall are composed of several types of cells for carrying out different functions.
Various types of cells in the ectoderm and the endoderm are:
(1) Epitheliomuscular cells,
(2) Gland cells,
(3) Interstitial cells,
(4) Cnidoblasts or stinging cells with nematocysts,
(5) Sensory cells,
(6) Nerve cells, and
(7) Nutritive-muscular cells.
Ectoderm or Epidermis:
The ectoderm is chiefly composed of large epitheliomuscular cells. These are roughly conical in shape with their broad cuboidal surface directed outwards and fused to form a layer of protective cuticle over the outer surface of the body. The narrow ends of the cone touch the mesoglea and are produced into two or more contractile muscular processes which run along the length of the animal.
These processes act as longitudinal muscles, the contraction of which shorten the length of the body and tentacles of Hydra. Each cell has a prominent nucleus and several small vacuoles.
The protective cuticle is absent from the basal disc and the epitheliomuscular cells are replaced here by narrow and tall gland cells which secrete a sticky material for fixing the animal firmly to its substratum. Sometimes, the gland cells secrete bubbles of gas which help the animal to float.
In between the narrow ends of the epitheliomuscular cells there are intercellular spaces filled with groups of small, rounded interstitial cells. These cells are undifferentiated embryonic structures which may be modified to produce other kinds of cells, such as cnidoblasts, germ cells, etc.
Embedded in between the epitheliomuscular cells there are cnidoblasts (cnidos=nettle) or stinging cells containing nematocysts. These are particularly numerous on the tentacles and at the free end of the body, but none are found on the basal disc. A fully formed cnidoblast is a pear-shaped cell from the outer end of which a short trigger-like cnidocil projects out.
Within the cell is a fluid-filled sac called nematocyst, the narrow outer end of which is produced into a long hollow thread. This thread is inverted and lies coiled up inside the sac.
Surrounding the nematocyst there is a layer of special contractile protoplasm containing the nucleus of the cnidoblast. When the cnidocil is stimulated, the contractile layer of protoplasm squeezes the sac, and by compressing the liquid causes the long thread to be everted out and vigorously expelled.
It has been demonstrated experimentally that direct stimulation of the cnidocil by contact with a glass rod does not provoke the discharge of the thread. Chemicals, such as, acetic acid, or substances released in the water by small animals on which Hydra feeds, effectively stimulate the cnidocil and the eversion of the thread is caused by increased pressure of fluid within the sac.
There are three main types of nematocysts:
(1) The large penetrant type with a long straight thread bearing three barbs at the base and rows of small spines or nettles;
(2) The small volvent type , containing a short thick thread which is coiled spirally into two or more loops;
(3) The small glutinant type having a straight but somewhat sticky thread devoid of barbs or nettles.
In the tentacles the stinging cells occur in groups to form small swellings on the outer body wall. Such a group or battery of nematocysts usually consists of one or two of the large penetrant type in the centre surrounded by several of the other two kinds.
The three kinds of nematocysts carry on different functions. The penetrant type is used for offence and defence as well as for capturing and paralyzing the prey. When discharged, it penetrates into the tissue of the victim.
The fluid within the sac contains a poison called hypnotoxin which has a paralyzing effect when injected into the body of the victim through the hollow tube. The prey is thus held the easier, while the Hydra swallows it.
The volvent or spiral type of nematocyst coils round the victim and the sticky thread of the glutinant type adheres to its body. These are therefore used for immobilizing and capturing food. Glutinant nematocysts which produce sticky secretion are also used in locomotion.
Nematocysts that are once shot out, cannot be withdrawn into their cnidoblasts and are lost. They are replaced by new cnidoblasts formed by modification of interstitial cells in the epidermis of the body wall. From here, they migrate to their final position upon the epidermis of the tentacles.
Sensory cells are scattered throughout the superficial part of the epidermis and are abundantly present upon the tentacles, hypostome and basal disc. A sensory cell has an elongated slender cell body with delicate tips, one end of which is connected to a nerve cell.
Nerve cells are spider-like in appearance due to the presence of fine processes which originate from the cell body. The processes are joined directly to the sensory cells and to the muscular processes of the epitheliomuscular cells. Nerve cells are scattered in the deeper part of the epidermis adjacent to the mesogloea.
The processes of neighbouring nerve cells join freely to form a network in this part of the body, thus forming a very simple apparatus for effecting nervous co-ordination. The sensory cells receive stimuli from outside, the nerve cells conduct the impulse and the epitheliomuscular cells react to the latter.
Endoderm or Gastro-Dermis:
The endoderm is chiefly composed of the nutritive-muscular cells. They are tall and columnar, with their broad ends towards the coelenteron and narrow ends touching the mesogloea. Their narrow ends are produced into contractile muscular processes which extend transversely and, therefore, encircle the body.
When contracted, they act as a band of circular muscle to reduce the diameter and extend the length of the polyp. The broad ends of the columnar cells project into the coelenteron. In some, the broad ends are amoeboid and engulf particles of food by pseudopodia, digesting them like an Amoeba. Others bear whip-like flagella which are used for cutting food into bits.
In the green species, Chlorohydra viridissima, the nutritive muscular cells harbour a kind of rounded unicellular algae called Zoo chlorella. The association is mutually beneficial to both.
Hydra affords protection to the Zoo chlorella and supplies it with CO2 and nitrogenous waste products, which are utilised by the plant as raw materials for manufacturing food. The plant, on the other hand, supplies the animal with carbohydrate food manufactured by photosynthesis.
This kind of ‘living together’, where the association is mutually beneficial, is known as symbiosis (Fig. 61). Scattered in between the nutritive- muscular cells there are narrow gland cells which secrete digestive juice into the coelenteron.
The endodermal gland cells are larger in comparison to those on the epidermis of the basal disc and are mostly present in the hypostome near about the mouth. They are coarsely granular and devoid of vacuoles. Spider-like nerve cells, narrow sensory cells, and a few interstitial cells are also found scattered throughout the gastro dermis, in between the tall nutritive-muscular cells.
In Hydra, the cellular differentiation is associated with physiological division of labour. Each kind of cell is destined to perform a definite function in co-operation with other cells for welfare of the individual as a whole.
Thus, the ectodermal cells together form a protective epidermis, whereas the endodermal cells comprise the nutritive gastro-dermis. The nerve cells and the sensory cells produce a delicate nerve-net which enable the Hydra to respond to stimuli. The stinging cells or cnidoblasts are useful for securing food and in defending the animal from its enemies.
The germ cells, when formed, are meant for reproduction. Interstitial cells are the embryonic structures which can change into any other kind of cells. They produce buds, repair tissues, and manufacture germ cells. The epitheliomuscular cells are not only protective but their contractile processes are used for shortening the body.
The contractile processes of the nutritive-muscular cells lengthen the body; the flagellate cells cut food into small bits, and the amoeboid cells engulf and digest food intracellularly. The gland cells secrete mucous and digestive enzymes.
It has now been established that sub-hypostomal zone of Hydra serves as growth region. It is believed that in this region cells are produced and from here cells flow in two directions—towards tentacles and towards basal disc.
The older cells are thrown out of the body from the tip of the tentacle and from near the basal disc. Thus the Hydra is unique in the animal world in having a hidden system of replacing older cells which causes no visible change in external morphology of the animal.
Feeding and Digestion Process in Hydra:
The food of Hydra consists of small aquatic creatures such as water-fleas like Daphnia, and larvae of insects. The animal, when hungry, expands its body and tentacles to the fullest extent in search of prey. If food is not found, the animal contracts to a spherical knob, slightly bends its body and again expands fully in a different direction. Such repeated expansion and contraction of the body in search of food is known as hunger movement.
When a Daphnia, by accident, comes near a battery of cnidoblasts, it is captured. The volvent and glutinant nematocysts adhere to the body of the victim preventing its escape, and the penetrant type paralyses it by injecting hypnotoxin into its tissues.
Other tentacles bend down upon the paralyzed Daphnia and carry it to the mouth. The latter expands to receive the food which is now pushed in. The food enters the coelenteron where it is cut into bits by the beating of the whip-like flagella of the flagellate cells.
The gland cells now pour alkaline digestive juice containing proteolytic enzymes. Proteins are thus broken down into amino acids, which pass by diffusion into the endodermal cells and thence transferred to the ectodermal cells.
Small bits of food which escape digestion in the coelenteron are engulfed by the amoeboid endodermal cells and digested within food vacuoles in typical amoeboid fashion.
The contents of the food vacuoles are at first acid, then alkaline. Fat-splitting, protein-splitting, and sometimes carbohydrate-splitting enzymes are secreted into the vacuoles. Food is thus completely digested and the excess is stored within the endodermal cells chiefly as fat.
Digestion of food in Hydra is partly extracellular occurring in the digestive tract, and partly intracellular occurring within the amoeboid endodermal cells. Extracellular digestion is purely proteolytic.
Fats, and sometimes carbohydrates, are digested intracellularly. Proteins are quickly digested in the coelenteron within a few hours but intracellular digestion requires a longer time, usually a few days. Indigestible matter, such as the shell of a water-flea, is egested through the mouth.
4. Locomotion in Hydra:
Although attached to the substratum by the basal disc and usually standing erect, the Hydra has several methods of locomotion. Usually it bends down and attaches the tentacles to the substratum with the help of the glutinant nematocysts.
It now releases and removes the basal disc to a new position and again stands up by disengaging its tentacles. This type of locomotion is known as walking. It may bend its body and perform looping movement like that of a leech (Fig. 62 A). When in a hurry, the animal runs by performing a series of somersaults (Fig. 62 B).
Sometimes it walks inverted by using the tentacles as legs. Again it may glide considerably by dragging the basal disc—the cells of which have special power of throwing out pseudopodia like an Amoeba.
Occasionally Hydra secretes a bubble of gas under its basal disc and floats about by the hydrostatic action of the gas bubble. A Hydra may climb by attaching its long tentacles to a distant object. Then releasing the basal disc and contracting the tentacles the body is drawn up, and it resumes its normal position on the new substratum.
It is to be noted that:
(1) Fixation is effected by the mucus- secreting gland cells in the epidermis of the basal disc and the glutinant nematocysts with sticky threads on the tentacles.
(2) Shortening the length of the body is due to the contraction of the muscular processes of the ectodermal epitheliomuscular cells.
(3) Increasing the length of the body is produced by contracting the muscular processes of the endodermal nutritive-muscular cells.
5. Respiration and Excretion Process in Hydra:
Respiration and excretion are presumably carried out through the surface of the ectoderm and endoderm. The necessary oxygen passes by diffusion from the surrounding water directly into all the cells of the body. Carbon dioxide and nitrogenous waste products are similarly passed out by cell-to-cell osmosis.
6. Reproduction in Hydra:
Hydra reproduces by asexual and sexual methods. Asexual reproduction takes place either by budding or by fission. Budding appears to be the normal method of reproduction in Hydra and occurs at all times of the year. A bud is usually formed in the middle of the body by rapid multiplication of interstitial cells.
These are soon converted into other necessary cell- types, thus producing a bulging of the body wall into which the coelenteron extends. The bud elongates and develops a circlet of tentacles at its free end in the midst of which a mouth is perforated.
A fully formed bud breaks away from the parent by constricting its base. Then attaching itself to a new substratum it starts an independent life. More than one bud may form simultaneously upon the same parent.
Occasionally a Hydra will reproduce asexually by fission. The animal splits, either -transversely or longitudinally, into two halves. Each half grows the wanting parts, through the activity of the interstitial cells. Sexual reproduction occurs ordinarily in autumn.
Most of the Indian species of Hydra are hermaphrodite or monoecious, and bear both male and female gonads upon the same individual. Dioecious species, in which sexes are separate, commonly occur in the United States. The gonads are temporary structures, developing on the sides of the body during the breeding season.
There is only one ovary in each individual at the proximal end near the middle of the body. The same individual bears several testes at its distal end. Both kinds of gonads are formed by rapid proliferation of interstitial cells which are later modified into the germ cells.
The male gonad or testis is a blunt conical swelling of the ectoderm. It is composed of a heap of rapidly multiplying interstitial cells covered over by a protective capsule of musculo epithelial cells.
The interstitial cells thus enclosed are converted into spermatocytes, each of which then divides twice to produce four spermatozoa. The sperm has a conical head, a short neck and a wavy tail.’ The covering capsule splits when the testis matures. Spermatozoa, thus liberated, swim about in the water where they remain active for a day or two.
Development of the female gonad or ovary is similar to that of a testis at the initial stage. The interstitial cells multiply and push out the musculo-epithelial cells to form a slight bulge near the proximal end of the animal.
One of the interstitial cells, the future ovum, becomes amoeboid. It is the oocyte. It increases in size rapidly by ingesting the neighbouring interstitial cells with its pseudopodia. After finishing off the other interstitial cells, the oocyte becomes spherical.
Its cytoplasm is now loaded with dark yolk granules. Two maturation divisions quickly follow, resulting in three small polar bodies, and the ripe ovum. The ovum is the female germ cell and the polar bodies are without any function. The capsule of the ovary ruptures and shrinks away, thus exposing the mature ovum, which, however, remains attached to the ovarian wall.
The ovum secretes a gelatinous substance by which a swarm of sperms are attracted to it. One of them fertilises the ovum. If not fertilised within a short time it perishes. During fertilisation the nucleus of the sperm fuses with the nucleus of the ovum. The product of fertilisation is known as the oosperm or zygote which is destined to give rise to a young Hydra.
Normally, testes of an individual mature before the ovary. Therefore the sperms of one have to find the ovum of some other, who is older than itself. Self-fertilisation is prevented in this manner. It must, however, be admitted that inbreeding occurs occasionally.
7. Life-History of Hydra:
The fertilised egg or zygote soon begins to divide. The cleavage is total and equal. Eventually a hollow ball, bordered by a single layer of cells, is formed. This is the blastula stage and the cavity of the blastula is the blastocoel. New cells are cut off from the inner ends of the older cells. The new cells migrate into and fill up the cavity of the blastula which in this manner, is converted into two layers of cell.
This is the gastrula or strictly speaking stereo-gastrula stage. Its outer cell layer is destined to produce the ectoderm and the solid inner layer the endoderm. A jelly-like mesogloea is soon formed separating the two cellular layers, and in course of this, the embryo secretes a horny capsule or cyst with spiny outer surface. Thus enclosed it drops away from the body of the parent to the bottom of the pond.
In about two months, the capsule bursts and a young Hydra hatches out, with short tentacles and a mouth hidden amongst them. The coelenteron is formed by a split in the middle of the solid endodermal layer. The young Hydra now settles down on a water weed and begins to feed and grow. There is no larval stage.
It is evident that the sexual phase in the life-history of Hydra, with production of a resistant cyst, is primarily necessary for survival. It is a means to tide over difficulty, and possibly has an invigorating effect on individuals which are tired by repeated budding.
8. Hydra in Relation to Higher Metazoan:
It has been said that Hydra is the simplest of the metazoa.
This is proved by the following facts:
(a) Hydra has developed cellular differentiation and physiological division of labour. But still it is chiefly a tissue-animal, having no organ-systems like those of a leech or a frog.
(b) It is diploblastic, having only two cellular layers—an outer ectoderm and an inner endoderm with an intervening non-cellular mesogioae; higher metazoa are all triploblastic, having three cellular layers.
(c) Hydra possesses a number of undifferentiated interstitial cells which can be changed into any other kind of cells in the body. In higher metazoa, the body cells are highly specialised and such undifferentiated cells are usually absent.
(d) The sac-like body has a single gastro-vascular cavity or coelentcron which serves as a body cavity as well as a digestive tract in higher metazoa, the digestive tract is surrounded by a separate body cavity or coelom.
(e) Digestive tract communicates with the exterior by only one opening—the mouth; in higher metazoa, there are two openings—a mouth in front and an anus behind.
(f) Digestion is both extracellular, as in higher metazoa, and intracellular, as in some protozoa.
(g) Hydra reproduces asexually by budding or by fission; higher metazoa reproduce by the sexual method.
(h) Respiration and excretion occur by cell-to-cell osmosis from the body-surface; higher metazoa have definite respiratory and excretory organs.
(i) Nervous co-ordination is effected by a simple-nerve-net formed by a few scattered nerve cells and sensory cells; higher metazoa possess well-developed nervous system.